• Storm Surge is an abnormal rise of sea level as the cyclone crosses the coast.
  • Sea water inundates the coastal strip causing loss of life, large scale destruction to property & crop.
  • Increased salinity in the soil over affected area makes the land unfit for agricultural use for two or three seasons.
  • Storm surge depends on intensity of the cyclone (Maximum winds and lowest pressure associated with it and Coastal bathymetry (shallower coastline generates surges of greater heights).

Storm Surge-cyclone

  1. What is storm tide?
  • The storm tide is the combination of storm surge and the astronomical tide.

What are the disaster potential of Storm Surge?

  • Disaster potential due to cyclones is due to high storm surges occurring at the time of landfall. The storm surges are by far the greatest killers in a cyclone. as sea water inundates low lying areas of the coastal regions causing heavy floods, erosion of beaches and embankments, damage to vegetation and reducing soil fertility.
  • Flooding due to storm surges pollute drinking water sources resulting in shortage of drinking water and causing out-break of epidemics, mostly water borne diseases Very strong winds (Gales) may cause uprooting of trees, damage to dwellings, overhead installations, communication lines etc., resulting in loss of life and property.
  • Past records show that very heavy loss of life due to tropical cyclones have occurred in the coastal areas surrounding the Bay of Bengal. Cyclones are also often accompanied by very intense & heavy precipitation (exceeding 40-50 cm in a day or about 10cm or more per hour in some places)

Why do ‘tropical cyclones’ winds rotate counter-clockwise (clockwise) in the Northern (Southern) Hemisphere?

  • As the earth’s rotation sets up an apparent force (called the Coriolis force) that pulls the winds to the right in the Northern Hemisphere (and to the left in the Southern Hemisphere).
  • So, when a low pressure starts to form over north of the equator, the surface winds will flow inward trying to fill in the low and will be deflected to the right and a counter-clockwise rotation will be initiated. The opposite (a deflection to the left and a clockwise rotation) will occur south of the equator.
  • This Coriolis force is too tiny to effect rotation in, for example, water that is going down the drains of sinks and toilets.
  • The rotation in those will be determined by the geometry of the container and the original motion of the water.
  • Thus, one can find both clockwise and counter-clockwise flowing drains no matter what hemisphere you are located. If you don’t believe this, test it out for yourself.

cyclone - cyclonic rotation - coriolis force

Why there are fewer cyclones over the Arabian Sea as compared to the Bay of Bengal?

  • Cyclones that form over the Bay of Bengal are either those develop insitu over southeast Bay of Bengal and adjoining Andaman Sea or remnants of typhoons over Northwest Pacific and move across south China sea to Indian Seas.
  • As the frequency of typhoons over Northwest Pacific is quite high (about 35 % of the global annual average), the Bay of Bengal also gets its increased quota.
  • The cyclones over the Arabian Sea either originate insitu over southeast Arabian Sea (which includes Lakshadweep area also) or remnants of cyclones from the Bay of Bengal that move across south peninsula. As the majority of Cyclones over the Bay of Bengal weaken over land after landfall, the frequency of migration into Arabian Sea is low.
  • In addition to all the above the Arabian Sea is relatively colder (mosnsoon winds) than Bay of Bengal and hence inhibits the formation and intensification of the system.

cyclones over the Arabian Sea - Bay of Bengal

Why there are very few Tropical Cyclones during southwest monsoon season?

  • The southwest monsoon is characterized by the presence of strong westerly winds in the lower troposphere (below 5 km) and very strong easterly winds in the upper troposphere (above 9 km). This results in large vertical wind shear. Strong vertical wind shear inhibits cyclone development.
  • Also the potential zone for the development of cyclones shifts to North Bay of Bengal during southwest monsoon season.
  • During this season, the low pressure system upto the intensity of depressions form along the monsoon trough (ITCZ), which extends from northwest India to the north Bay of Bengal.
  • The Depression forming over this area crosses Orissa – West Bengal coast in a day or two. These systems have shorter oceanic stay (they make landfall very quickly) which is also one of the reasons for their non-intensification into intense cyclones.

What are the causes of disaster during cyclone?

  • The dangers associated with cyclonic storms are generally three fold.
  1. Floods
  2. Winds
  3. Storm Surge

Very heavy rains causing floods.

  • The rainfall associated with a storm vary from storm to storm even with the same intensity. Record rainfall in a cyclonic storm has been as low as trace to as high as 250 cms. It has been found that the intensity of rainfall is about 85 cms/day within a radius of 50 kms and about 35 cms/day between 50 to 100 kms from the centre of the storm. Precipitation of about 50 cm/day is quite common with a C.S. This phenomenal rain can cause flash flood.

Strong wind.

  • The strong wind speed associated with a cyclonic storm. (60-90 kmph) can result into some damage to kutcha houses and tree branches likely to break off. Winds of a severe Cyclonic storm (90-120 kmph) can cause uprooting of trees, damage to pucca houses and disruption of communications. The wind associated with a very severe Cyclonic storm and super cyclonic storm can uproot big trees, cause wide spread damages to houses and installations and total disruption of communications. The maximum wind speed associated with a very severe Cyclonic storm that hit Indian coast in the past 100 years was 260 kmph in Oct., 1999 (Paradeep Super cyclone).

Storm surge

  • Storm surge occur in places where a tropical cyclone crosses the coast (makes landfall).
  • The severest destructive feature of a tropical storm is the storm surge popularly called tidal waves.
  • The costal areas are subjected to storm surge and is accentuated if the landfall time coincides with that of high tides. This is again more if the sea bed is shallow.
  • Storm surge as high as 15 to 20 ft. may occur when all the factors contributing to storm surge are maximum. This storm tide inundates low lying coastal areas which has far reaching consequences apart from flooding.
  • The fertility of land is lost due to inundation by saline water for a few years to come.

Mains 2013: Naming of Cyclones

The recent cyclone on east coast of India was called ‘Phailin’. How are the tropical cyclones named across the world? Elaborate.
  • WMO (World meteorological organization) divided the world Oceans into Basins and assigned the responsibility of naming the Cyclones to the respective regional bodies.
  • Each regional body has its own rules in naming cyclones. In most regions pre-determined alphabetic lists of alternating male and female names are used.
  • In the north-west Pacific the majority of names used are not personal names. While there are a few male and female names, majority are names of flowers, animals, birds, trees, foods or descriptive adjectives.

How are cyclones named in Northern Indian Ocean Region

  • The names of cyclones in Indian Seas are not allocated in alphabetical order, but are arranged by the name of the country which contributed the name.
  • It is usual practice for a storm to be named when it reaches tropical storm strength (winds of 34 knots).


  • The knot (pronounced not) is a unit of speed equal to one nautical mile (1.852 km) per hour, approximately 1.151 mph
  •  Worldwide, the knot is used in meteorology, and in maritime and air navigation—for example, a vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.
  • 1 international knot = 1 nautical mile per hour (exactly) = 1.852 kilometres per hour (exactly) = 0.514 metres per second (approximately)
  • The Indian Meteorological Department (IMD) which issues cyclone advisors to eight countries has a list of names contributed by each of them.
  • Every time a cyclone occurs, a name is picked in the order of the names that are already submitted.
  • Each country gets a chance to name a cyclone. After all the countries get their turn, the next list of names is followed.

naming of cyclones indian ocean

Why is this system of uniformity in naming a cyclone in the region

  • Tropical cyclones are named to provide ease of communication between forecasters and the general public regarding forecasts and warnings.
  • Since the storms can often last a week or even longer and more than one cyclone can be occurring in the same region at the same time, names can reduce the confusion about what storm is being described
  • Naming them after a person/flower/animal etc. makes it easier for the media to report on tropical cyclones, increases community preparedness, also helps in quick information exchange between faraway stations, ships etc.

Polar or Arctic Cyclones

  • Arctic or polar cyclones occur in Antarctic regions and can reach up to 1,200 miles wide.
  • Polar cyclones differ with others because they are not seasonal. [Tropical Cyclones are seasonal]
  • They can occur at any time of the year.
  • Polar cyclones can also form quickly (sometimes less than 24 hours), and their direction or movement cannot be predicted.
  • They can last from a day up to several weeks. [Tropical Cyclones doesn’t for more than a week]
  • Most frequently, polar cyclones develop above northern Russia and Siberia.

Maximum Sustained Wind

  • India Meteorological Department (IMD) uses a 3 minutes averaging for the sustained wind.
  • Maximum sustained wind is the highest 3 minutes surface wind occurring within the circulation of the system.

Low Pressure, Depression and Cyclone

How are low pressure system classified in India? What are the differences between low, depression and cyclone?
  • The pressure criteria is used, when the system is over land and wind criteria is used, when the system is over the sea.
  • The system is called as low if there is one closed isobar in the interval of 2 hPa.
  • It is called depression, if there are two closed isobars, a deep depression, if there are three closed isobars and cyclonic storm if there are four or more closed isobars.
  • The detailed classification based on wind criteria are given in the Table below.
System Pressure deficient


Associated wind speed

Knots (Kmph)

Low pressure area 1.0 <17(<32)
Depression 1.0- 3.0 17-27 (32–50)
Deep Depression 3.0 – 4.5 28-33 (51–59)
Cyclonic Storm 4.5- 8.5 34-47 (60-90)
Severe Cyclonic Storm (SCS) 8.5-15.5 48-63 (90-119)
Very Severe Cyclonic Storm 15.5-65.6 64-119 (119-220)
Super Cyclonic Storm >65.6 >119(>220)

Central Dense Overcast (CDO)

  • “CDO” is an acronym that stands for “central dense overcast“.
  • This is the cirrus cloud shield that results from the thunderstorms in the eyewall of a tropical cyclone and its rainbands.
  • Before the tropical cyclone reaches very severe cyclonic storm (64 knots,), typically the CDO is uniformly showing the cold cloud tops of the cirrus with no eye apparent.

Mature stage tropical cyclone

Annual frequency of Cyclones over the Indian Seas

  • The average annual frequency of tropical cyclones in the north Indian Ocean (Bay of Bengal and Arabian Sea) is about 5 (about 5-6 % of the Global annual average) and about 80 cyclones form around the globe in a year.
  • The frequency is more in the Bay of Bengal than in the Arabian Sea, the ratio being 4:1.

States Vulnerable to Cyclones

States Vulnerable to CyclonesCyclone prone states india

Which sector of the cyclone experiences strongest winds?

  • In general, the strongest winds in a cyclone are found on the right side of the storm. The “right side of the storm” is defined with respect to the storm’s motion: if the cyclone is moving to the west, the right side would be to the north of the storm; if the cyclone is moving to the north, the right side would be to the east of the storm, etc.
  • The strongest wind on the right side of the storm is mainly due to the fact that the motion of the cyclone also contributes to its swirling winds.
  • A cyclone with a 145 kmph winds while stationary would have winds up to 160 kmph on the right side and only 130 kmph on the left side if it began moving (any direction) at 16 kmph.

What is the normal movement of a Tropical Cyclone?

  • The cyclones, which cross 20° N latitude generally, recurve and they are more destructive.
  • Tropical Cyclones move as a whole. They casually move west-northwestwards or northwestwards in the northern hemisphere.
  • The average speed is 15-20 kmph (360-480 km per day). They may change their direction of movement towards north. During this change their speed of movement decreases to 10 kmph or even less.
  • A larger fraction of such storms later turn towards northeast and move northeastwards very fast at a speed of 25 kmph or more.

What is the role of upper tropospheric westerly trough ?

  • An Upper tropospheric westerly trough is important for tropical cyclone forecasting as they can force large amounts of vertical wind shear over tropical disturbances and tropical cyclones which may inhibit their strengthening.
  • There are also suggestions that these troughs can assist tropical cyclone genesis and intensification by providing additional forced ascent near the storm centre and/or by allowing for an efficient outflow channel in the upper troposphere.
  • The location of this trough and its intensity can also influence the movement of the storm and hence can be used for cyclone track forecasting.

What is 4-stage warning system for Tropical Cyclones?

IMD and Cyclone Diasster Management

  • 1999, IMD introduced a 4-Stage warning system to issue cyclone warnings to the disaster managers. They are as follows:
Pre-Cyclone Watch
  • Issued when a depression forms over the Bay of Bengal irrespective of its distance from the coast and is likely to affect Indian coast in future. The pre-cyclone watch is issued by the name of Director General of Meteorology and is issued at least 72 hours in advance of the commencement of adverse weather. It is issued at least once a day.
Cyclone Alert
  • Issued atleast 48 hours before the commencement of the bad weather when the cyclone is located beyond 500 Km from the coast. It is issued every three hours.
Cyclone Warning
  • Issued at least 24 hours before the commencement of the bad weather when the cyclone is located within 500 Km from the coast. Information about time /place of landfall are indicated in the bulletin. Confidence in estimation increases as the cyclone comes closer to the coast
Post landfall outlook
  • It is issued 12 hours before the cyclone landfall, when the cyclone is located within 200 Km from the coast. More accurate & specific information about time /place of landfall and associated bad weather indicated in the bulletin. In addition, the interior distraction is likely to be affected due to the cyclone are warned in this bulletin.

Modifying cyclones?

  • Seeding with silver iodide.
  • Placing a substance on the ocean surface.
  • By nuking them.
  • By cooling the surface waters with deep ocean water.
  • By adding a water absorbing substance.

How are Tropical Cyclones monitored by IMD?

  • A good network of meteorological observatories (both surface and upper air) is operated by IMD, covering the entire coastline and islands.
  • The conventional observations are supplemented by observational data from automatic weather stations (AWS), radar and satellite systems.
  • INSAT imagery obtained at hourly intervals during cyclone situations has proved to be immensely useful in monitoring the development and movement of cyclones.



  • Urban Heat Island effect is defined as the presence of significantly higher temperatures in urban areas compared to the temperatures in surrounding rural zones mainly due to human factors
  • Usually urban heat islands have a mean temperature 8 to 10 degrees more than the surrounding rural areas
  • These can affect communities by increasing summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and mortality.


Causes of Urban Heat Island effect

  • Use of construction materials like Asphalt and concrete: Asphalt and concrete, needed for the expansion of cities, absorb huge amounts of heat, increasing the mean surface temperatures of urban areas.
  • Dark surfaces: Many buildings found in urban areas have dark surfaces, thereby decreasing albedo and increased absorption of heat.
  • Air conditioning: Buildings with dark surfaces heat up more rapidly and require more cooling from air conditioning, which requires more energy from power plants, which causes more pollution. Also air conditioners exchange heat with atmospheric air, causing further local heating. Thus there is a cascade effect that contributes to the expansion of urban heat islands.
  • Urban Architecture: Tall buildings, and often, accompanying narrow streets, hinder the circulation of air, reduce the wind speed, and thus reduce any natural cooling effects. This is called the Urban Canyon Effect.
  • Need for mass transportation system: Transportation systems and the unimpeded use of fossil fuels also add warmth to urban areas.
  • Lack of Trees and green areas: which impedes evapotranspiration, shade and removal of carbon dioxide, all the processes that help to cool the surrounding air.


Thus the increased use of manmade materials and increased anthropogenic heat production are the main causes of the UHI. To reduce UHI there is need for planned urbanization for which we don’t require only smart cities but Smart-Green cities.



Meaning of temperature inversion

  • Under normal conditions, temperature usually decreases with increase in altitude in the troposphere at a rate of 1 degree for every 165 metres. This is called normal lapse rate. 
    • But on some occasions, the situations get reversed and temperature starts increasing with height rather than decreasing. This is called temperature inversion.
  • Temperature inversion: It is a reversal of the normal behavior of temperature in the troposphere. Under this meteorological phenomenon a layer of warm air lies over the cold air layer.
    • It is caused in stac atmospheric conditions while some times, it occurs due to horizontal or vertical movement of air.
    • Temperature inversion is usually of short duration but quite common nonetheless.

Favourable Conditions for Temperature Inversion

  • Long winter nights: Loss of heat by terrestrial radiation from the ground surface during night may exceed the amount of incoming solar radiation.
  • Cloudless and clear sky: Loss of heat through terrestrial radiation proceeds more rapidly without any obstruction.
  • Dry air near the ground surface: It limits the absorption of the radiated heat from the Earth’s surface.
  • Slow movement of air: It results in no transfer or mixing of heat in the lower layers of the atmosphere.
  • Snow covered ground surface: It results in maximum loss of heat through reflection of incoming solar radiation.

Types of Temperature Inversion

  • Temperature inversion occurs in several conditions ranging from ground surface to great heights. Thus there are several kinds of temperature inversions.
  • The following are classified on the basis of relative heights from the earth’s surface at which it occurs and the type of air circulation:
  • Non-Advectional
    • Radiation Inversion (Surface Temperature Inversion)
      • Surface temperature inversion develops when air is cooled by contact with a colder surface until it becomes cooler than the overlying atmosphere; this occurs most often on clear nights, when the ground cools off rapidly by radiation. If the temperature of surface air drops below its dew point, fog may result.
      • It is very common in the higher latitudes. In lower and middle latitudes, it occurs during cold nights and gets destroyed during day time.
    • Subsidence Inversion (Upper Surface Temperature Inversion)
      • When a widespread layer of air descends, it is compressed and heated by the resulting increase in atmospheric pressure, and as a result the lapse rate of temperature is reduced.
      • The air at higher altitudes becomes warmer than at lower altitudes, producing a temperature inversion. This type of temperature inversion is called subsidence inversion.
      • It is very common over the northern continents in winter (dry atmosphere) and over the subtropical oceans; these regions generally have subsiding air because they are located under large high-pressure centers.
      • It is also called upper surface temperature inversion because it takes place in the upper parts of the atmosphere.

  • Advectional
    • Valley inversion in intermontane valley
      • In high mountains or deep valleys, sometimes, the temperature of the lower layers of air increases instead of decreasing with elevation along a sloping surface.
      • Here, the surface radiates heat back to space rapidly and cools down at a faster rate than the upper layers. As a result the lower cold layers get condensed and become heavy.
      • The sloping surface underneath makes them move towards the bottom where the cold layer settles down as a zone of low temperature while the upper layers are relatively warmer.
      • This condition, opposite to normal vertical distribution of temperature, is known as Temperature Inversion.
    • Frontal or Cyclonic inversion
      • When the warm and cold fronts meet, then the warm front rises up and being heavier the cold front sinks down. It results in formation of Frontal Inversion.
      • It has considerable slope, whereas other inversions are nearly horizontal. It often takes place in the temperate zone and causes cyclonic conditions which result in the precipitation in different forms.
      • A frontal inversion is unstable and is destroyed as the weather changes.


  • Temperature inversion determines the precipitation, forms of clouds, and also causes frost due to condensation of warm air due to its cooling.
    • Dust particles hanging in the air: Due to inversion of temperature, air pollutants such as dust particles and smoke do not disperse on the surface.
    • Stops the movement of air: It causes the stability of the atmosphere that stops the downward and upward movement of air.
    • Less rainfall: Convection clouds can not move high upwards so there is less rainfall and no showers. So, it causes a problem for agricultural productivity.
    • Lower visibility: Fog is formed due to the situation of warm air above and cold air below, and hence visibility is reduced which causes disturbance in transportation.
    • Thunderstorms and tornadoes: Intense thunderstorms and tornadoes are also associated with inversion of temperature because of the intense energy that is released after an inversion blocks an area’s normal convention patterns.
    • Diurnal variations in temperature tend to be very small.
  •  ConclusionTo conclude, temperature inversion might be a desirable phenomena when it comes to cooler air temperatures, and comfort after an extremely hot and oppressive day, the after-effects on air quality are certainly not desirable.


  • El Nino and La Nina are complex weather patterns resulting from variations in ocean temperatures in the Equatorial Pacific Region. They are opposite phases of what is known as the El Nino-Southern Oscillation (ENSO) cycle.
    • The ENSO cycle describes the fluctuations in temperature between the ocean and atmosphere in the east-central Equatorial Pacific.
    • El Nino and La Nina episodes typically last nine to 12 months, but some prolonged events may last for years.
  • El Nino is a climate pattern that describes the unusual warming of surface waters in the eastern tropical Pacific Ocean.
    • It is the “warm phase” of a larger phenomenon called the El Nino-Southern Oscillation (ENSO).
    • It occurs more frequently than La Nina.
  • La Nina, the “cool phase” of ENSO, is a pattern that describes the unusual cooling of the tropical eastern Pacific.
    • La Nina events may last between one and three years, unlike El Nino, which usually lasts no more than a year.
    • Both phenomena tend to peak during the Northern Hemisphere winter.

El Nino

  • El Nino was first recognized by Peruvian fishermen off the coast of Peru as the appearance of unusually warm water.
    • The Spanish immigrants called it El Nino, meaning “the little boy” in Spanish.
  • El Nino soon came to describe irregular and intense climate changes rather than just the warming of coastal surface waters.
  • The El Nino event is not a regular cycle, they are not predictable and occur irregularly at two- to seven-year intervals.
    • The climatologists determined that El Nino occurs simultaneously with the Southern Oscillation.
      • The Southern Oscillation is a change in air pressure over the tropical Pacific Ocean.
  • When coastal waters become warmer in the eastern tropical Pacific (El Nino), the atmospheric pressure above the ocean decreases.
    • Climatologists define these linked phenomena as El Nino-Southern Oscillation (ENSO).

Monitoring El Nino and La Nina

  • Scientists, governments, and non-governmental organizations (NGOs) collect data about El Nino using a number of technologies such as scientific buoys.
    • A buoy is a type of an object that floats in water and is used in the middle of the seas as locators or as warning points for the ships. They are generally bright (fluorescent) in colour.
    • These buoys measure ocean and air temperatures, currents, winds, and humidity.
    • The buoys transmit data daily to researchers and forecasters around the world enabling the scientists to more accurately predict El Nino and visualize its development and impact around the globe.
  • The Oceanic Nino Index (ONI) is used to measure deviations from normal sea surface temperatures.
    • The intensity of El Nino events varies from weak temperature increases (about 4-5° F) with only moderate local effects on weather and climate to very strong increases (14-18° F) associated with worldwide climatic changes.

Oceanic Nino Index (ONI)

  • The Oceanic Niño Index (ONI), is a measure of the departure from normal sea surface temperature in the east-central Pacific Ocean, is the standard means by which each El Nino episode is determined, gauged, and forecast.

Impact of El Nino

  • In order to understand the concept of El Nino, it’s important to be familiar with non-El Nino conditions in the Pacific Ocean.
    • Normally, strong trade winds blow westward across the tropical Pacific, the region of the Pacific Ocean located between the Tropic of Cancer and the Tropic of Capricorn.
  • Impact on Ocean: El Nino also impacts ocean temperatures, the speed and strength of ocean currents, the health of coastal fisheries, and local weather from Australia to South America and beyond.
  • Increased Rainfall: Convection above warmer surface waters brings increased precipitation.
    • Rainfall increases drastically in South America, contributing to coastal flooding and erosion.
  • Diseases caused by Floods and Droughts: Diseases thrive in communities devastated by natural hazards such as flood or drought.
    • El Nino-related flooding is associated with increases in cholera, dengue, and malaria in some parts of the world, while drought can lead to wildfires that create respiratory problems.
  • Positive impact: It can sometimes have a positive impact too, for example, El Nino reduces the instances of hurricanes in the Atlantic.
  • In South America: As El Nino brings rain to South America, it brings droughts to Indonesia and Australia.
    • These droughts threaten the region’s water supplies, as reservoirs dry and rivers carry less water. Agriculture, which depends on water for irrigation, is also threatened.
  • In Western Pacific: These winds push warm surface water towards the western Pacific, where it borders Asia and Australia.
    • Due to the warm trade winds, the sea surface is normally about 0.5 meter higher and 4-5° F warmer in Indonesia than Ecuador.
    • The westward movement of warmer waters causes cooler waters to rise up towards the surface on the coasts of Ecuador, Peru, and Chile. This process is known as upwelling.
      • Upwelling elevates cold, nutrient-rich water to the euphotic zone, the upper layer of the ocean.

Previous El Nino Events:

  • El Nino events of 1982-83 and 1997-98 were the most intense of the 20th century.
  • During the 1982-83 event, sea surface temperatures in the eastern tropical Pacific were 9-18° F above normal.
  • The El Nino event of 1997-98 was the first El Nino event to be scientifically monitored from beginning to end.
  • The 1997-98 event produced drought conditions in Indonesia, Malaysia, and the Philippines. Peru and California experienced very heavy rains and severe flooding.
  • The Midwest experienced record-breaking warm temperatures during a period known as “the year without a winter.”

La Nina

  • La Nina means The Little Girl in Spanish. It is also sometimes called El Viejo, anti-El Nino, or simply “a cold event.”
  • La Nina events represent periods of below-average sea surface temperatures across the east-central Equatorial Pacific.
    • It is indicated by sea-surface temperature decreased by more than 0.9℉ for at least five successive three-month seasons.
  • La Nina event is observed when the water temperature in the Eastern Pacific gets comparatively colder than normal, as a consequence of which, there is a strong high pressure over the eastern equatorial Pacific.

The Conditions of La Nina.

  • La Nina is caused by a build-up of cooler-than-normal waters in the tropical Pacific, the area of the Pacific Ocean between the Tropic of Cancer and the Tropic of Capricorn.
  • La Nina is characterized by lower-than-normal air pressure over the western Pacific. These low-pressure zones contribute to increased rainfall.
  • La Nina events are also associated with rainier-than-normal conditions over southeastern Africa and northern Brazil.
    • However, strong La Nina events are associated with catastrophic floods in northern Australia.
  • La Nina is also characterized by higher-than-normal pressure over the central and eastern Pacific.
    • This results in decreased cloud production and rainfall in that region.
  • Drier-than-normal conditions are observed along the west coast of tropical South America, the Gulf Coast of the United States, and the pampas region of southern South America.

Impact of La Nina

  • Europe: In Europe, El Nino reduces the number of autumnal hurricanes.
    • La Nina tends to lead to milder winters in Northern Europe (especially UK) and colder winters in southern/western Europe leading to snow in the Mediterranean region.
  • North America: It is continental North America where most of these conditions are felt. The wider effects include:
    • Stronger winds along the equatorial region, especially in the Pacific.
    • Favourable conditions for hurricanes in the Caribbean and central Atlantic area.
    • Greater instances of tornados in various states of the US.
    • South America: La Nina causes drought in the South American countries of Peru and Ecuador.
      • It usually has a positive impact on the fishing industry of western South America.
  • Western Pacific: In the western Pacific, La Nina increases the potential for landfall in those areas most vulnerable to their effects, and especially into continental Asia and China.
    • It also leads to heavy floods in Australia.
    • There are increased temperatures in Western Pacific, Indian Ocean and off the Somalian coast.

ENSO and India

  • El Nino: Strong El Nino events contribute to weaker monsoons and even droughts in India Southeast Asia.
  • La Nina: The cold air occupies a larger part of India than the El Nino cold air.
  • In the ‘La Nina year’, rainfall associated with the summer monsoon in Southeast Asia tends to be greater than normal, especially in northwest India and Bangladesh.
    • This generally benefits the Indian economy, which depends on the monsoon for agriculture and industry.
  • It usually brings in colder than normal winters in India.
  • La Nina influences the Indian subcontinent by piping in cold air from Siberia and South China, which interacts with the tropical heating to produce a north-south low-pressure system.
  • The cold air of La Nina associated with this north-south trough tends to extend much further south into India.
    • This is remarkably different from the more northwest-southeast blast of cold air associated with El Nino.
    • The pressure pattern going north-south means lesser impact of western disturbances.
    • The cold temperature can go down as far as Tamil Nadu, but may not affect the North East that much.5.TROPICAL CYCLONES ARE LARGELY CONFINED TO SOUTH CHINA SEA, BAY OF BENGAL AND GULF OF MEXICO. WHY?
    • Introduction
      • Cyclones are rapid inward air circulation around a low-pressure area. The air circulates in an anticlockwise direction in the Northern hemisphere and clockwise in the Southern hemisphere.
      • Cyclones are usually accompanied by violent storms and bad weather.
      • The word Cyclone is derived from the Greek word Cyclos meaning the coils of a snake. It was coined by Henry Peddington because the tropical storms in the Bay of Bengal and the Arabian Sea appear like coiled serpents of the sea.


      • There are two types of cyclones:
        • Tropical cyclones; and
        • Extra Tropical cyclones (also called Temperate cyclones or middle latitude cyclones or Frontal cyclones or Wave Cyclones).
      • The World Meteorological Organisation uses the term ‘Tropical Cyclone’ to cover weather systems in which winds exceed ‘Gale Force’ (minimum of 63 km per hour).
        • Tropical cyclones develop in the region between the Tropics of Capricorn and Cancer. They are large-scale weather systems developing over tropical or subtropical waters, where they get organized into surface wind circulation.
      • Extra tropical cyclones occur in temperate zones and high latitude regions, though they are known to originate in the Polar Regions.


      • An anticyclone is the opposite of a cyclone i.e. i.e., it has an outward-spiralling air circulation around a high pressure centre.
      • An anticyclone’s winds rotate clockwise in the Northern Hemisphere around a center of high pressure.
      • In anticyclones, air comes in from above and sinks to the ground. High pressure centers generally have fair weather.

      Tropical Cyclones

      • Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas bringing about large scale destruction caused by violent winds, very heavy rainfall and storm surges.
      • Tropical Cyclones are one of the most devastating natural calamities in the world.
      • Tropical cyclones originate and intensify over warm tropical oceans. The conditions favourable for the formation and intensification of tropical storms are:
        • Large sea surface with temperature higher than 27° C.
        • Presence of the Coriolis force.
        • Small variations in the vertical wind speed.
        • A pre-existing weak low- pressure area or low-level-cyclonic circulation.
        • Upper divergence above the sea level system.

      Stages of Formation: Tropical Cyclones

      The development cycle of tropical cyclones may be divided into three stages:

      Formation and Initial Development Stage

      • The formation and initial development of a cyclonic storm depends upon the transfer of water vapour and heat from the warm ocean to the overlying air, primarily by evaporation from the sea surface.
      • It encourages formation of massive vertical cumulus clouds due to convection with condensation of rising air above the ocean surface.

      Mature Stage

      • When a tropical storm intensifies, the air rises in vigorous thunderstorms and tends to spread out horizontally at the tropopause level. Once air spreads out, a positive pressure at high levels is produced, which accelerates the downward motion of air due to convection.
      • With the inducement of subsidence, air warms up by compression and a warm ‘Eye’ (Low pressure centre) is generated. The main physical feature of a mature tropical cyclone in the Indian Ocean is a concentric pattern of highly turbulent giant cumulus thundercloud bands.

      Modification and Decay

      • A tropical cyclone begins to weaken in terms of its central low pressure, internal warmth and extremely high speeds, as soon as its source of warm moist air begins to ebb or is abruptly cut off.
      • This happens after its landfall or when it passes over cold waters.

      Nomenclature of Tropical Cyclones

      • The naming of tropical cyclones is a recent phenomenon. The process of naming cyclones involves several countries in the region and is done under the aegis of the World Meteorological Organization (WMO).
      • For the Indian Ocean region, a formula for naming cyclones was agreed upon in 2004. Eight countries in the region – Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand – all contributed a set of names which are assigned sequentially whenever a cyclonic storm develops.
      • Hudhud, Titli, Phethai, Fani, Vayu and Amphan are among the names of cyclones in the Indian Ocean region.

      Worldwide Terminology of Tropical Cyclones

      • They are given many names in different regions of the world – eg.they are known as Typhoons in the China Sea and Pacific Ocean; Hurricanes in the West Indian islands in the Caribbean Sea and Atlantic Ocean; Tornados in the Guinea lands of West Africa and southern USA.; Willy-willies in north-western Australia and Tropical Cyclones in the Indian Ocean.

      Extratropical Cyclone

      • Extratropical cyclones are referred to as mid-latitude depressions, temperate cyclones, frontal depressions and wave cyclones.
      • These are active above the mid-latitudinal region between 35° and 65° latitude in both the hemispheres. The direction of movement is from west to east and more pronounced in the winter seasons. It is in these latitude zones the polar and tropical air masses meet and form fronts

      Formation of Extratropical Cyclones

      • The origin and development of temperate cyclones is best explained by the Polar Front theory.
      • According to this theory, the warm-humid air masses from the tropics meet the dry-cold air masses from the poles and thus a polar front is formed.
      • The cold air mass is denser and heavier and due to this reason, warm air mass is pushed up.
      • This interaction of cold and warm air masses creates instability and a low pressure is created at the junction particularly in the center of interactions.
      • Thus, a void is created because of lessening of pressure. The surrounding air rushed in to occupy this void and coupled with the earth’s rotation a cyclone is formed.
      • Extratropical cyclones present a contrast to the more violent cyclones or hurricanes of the tropics, which form in regions of relatively uniform temperatures.

      Air Mass

      • Air Mass is an extremely large body of air whose properties of temperature and moisture content (humidity), at any given altitude, are fairly similar.
        • It can cover hundreds of thousands of square miles of area.
        • It may have only a little horizontal variation in temperature and moisture throughout the air mass.
      • When an air mass remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area. The homogenous regions can be the vast ocean surface or vast plains.


      • When two different air masses (having distinctly different properties) meet, the boundary zone between them is called a front.
      • There are four types of fronts:
        • Stationary front: When the front remains stationary, it is called a stationary front.
        • Cold front: When the cold air moves towards the warm air mass, its contact zone is called the cold front,
        • Warm front: If the warm air mass moves towards the cold air mass, the contact zone is a warm front.
        • Occluded front: If an air mass is fully lifted above the land surface, it is called the occluded front.
      • The fronts occur in middle latitudes and are characterised by steep gradient in temperature and pressure. They bring abrupt changes in temperature and cause the air to rise to form clouds and cause precipitation.

      Cyclones in India

      Tropical Cyclones

      • Tropical cyclones originate over the Bay of Bengal, Arabian Sea and the Indian ocean. These tropical cyclones have very high wind velocity and heavy rainfall and hit the Indian Coastal states of Tamil Nadu, Andhra Pradesh, West Bengal,Odisha and Gujarat (These five states are more vulnerable to cyclone disasters than others in India).
      • Most of these cyclones are very destructive due to high wind velocity and torrential rain that accompanies it.
      • There are three elements associated with cyclones which cause destruction during its occurrence. These are-
        • Strong Winds/Squall: It damages installations, dwellings, communications systems, trees etc., resulting in loss of life and property.
        • Torrential rains and inland flooding: Rain is a serious problem for the people who become shelter less due to the cyclone. Heavy rainfall is usually spread over a wide area and causes large scale soil erosion and weakening of embankments.
        • Storm Surge: It is an abnormal rise of sea level near the coast caused by a severe tropical cyclone. Due to storm surge sea water inundates low lying areas of coastal regions drowning human beings and livestock, causes eroding beaches and embankments, destroys vegetation and leads to reduction of soil fertility.

      Management of Cyclones

      There are many structural and non-structural measures for effective disaster management of cyclones.

      • The structural measures include construction of cyclone shelters, construction of cyclone resistant buildings, road links, culverts, bridges, canals, drains, saline embankments, surface water tanks, communication and power transmission networks etc.
      • Non-structural measures like early warning dissemination systems, management of coastal zones, awareness generation and disaster risk management and capacity building of all the stakeholders involved.
        • These measures are being adopted and tackled on a State to State basis under the National Cyclone Risk Mitigation Project (NCRMP) being implemented through World Bank Assistance.

      Western Disturbance

      • Western Disturbance is a common weather phenomena in India. A western disturbance is an extratropical cyclone originating in the Mediterranean region that brings sudden winter rain to the northwestern parts of the Indian subcontinent.
      • They are the cause of the most winter and pre-monsoon season rainfall across North-West India (such as Punjab, Haryana, Delhi and western Uttar Pradesh). This phenomenon is usually associated with cloudy sky, higher night temperatures and unusual rain.
      • This precipitation during the winter season has great importance in agriculture particularly for rabi crops including wheat. It is estimated that India gets close to 5-10% of its total annual rainfall from western disturbances.


  • The term monsoon has been derived from the Arabic word mausim meaning ‘season’.
  • It marks the seasonal reversal of easterly winds blowing from the northeast during cooler months and reverse direction to blow from the southwest during the warmer months of the year.
  • Indian monsoon is the most prominent of the world’s monsoonsystems, which primarily affects India and its surrounding water bodies.
  • Majority of rainfall in India is convectional in nature and falls between the months of June and September.

Changing pattern of monsoon

  • Indian monsoon is considered a ‘textbook phenomenon’ clearly defined which has not changed much in the preceding century.
  • The average rainfall has remained within the 10% of the long term average.
  • However this process has hit an erratic front, with floods in the northwest and the northeast and rainfall deficit in southern part of the nation.
  • Rainfall extremes have increased threefold over the last few years and now extend over all of central India – from Gujarat to Odisha.
  • Onset of monsoon has delayed every year since 2002 and it also lasts for shorter duration, compressing the Indian monsoon.
  • The interspersed breaks in the monsoon have increased resulting in larger drier periods in the monsoon itself.
  • Rainfall intensity, duration, frequency and spatial distribution have significantly undergone change in the past decade or two.

Possible reasons of change

  • Decreasing mean rainfall, increasing spatial variability of rainfall, and a threefold rise in rainfall extremes – are associated with a weakening monsoon wind circulation and a decrease in the number of monsoon depressions from the Bay of Bengal.
  • Subcontinent has warmed significantly in the last decade and the Indian Ocean has cooled down during the same period due to anthropogenic reasons.
  • The contrast in the temperature between land and sea might result in decreased moisture demand from land.
  • India has experienced a reduction in low clouds, due to increase in anthropogenic aerosols such as black carbon or soot, which simultaneously absorb and heat the surrounding air, and prevent clouds from forming.

 Implications of changing Indian monsoon

  • Shifting monsoon patterns of the country has resulted in acute water shortage in the nation, with drying up of wells and rivers.
  • Major Indian reservoirs runs 10% lower than their normal at any given point of time in the year
  • There has been economic loss across agriculture and industry sectors caused by water shortage.
  • Cycles of droughts and floods have become more common in many parts of India.
  • Water shortage may fuel interstate tensions in India, ex- Cauvery river dispute between Karnataka and Tamil Nadu; Krishna river dispute among Andhra Pradesh, Maharashtra, Karnataka and Telangana;
  • Variation in monsoon has also resulted in the incidence of vector borne diseases such as malaria, dengue.

Importance of Indian monsoon

  • Indian monsoon plays vital role in India’s attempt to achieve food security.
  • About 64 % Indian population depend on agriculture for their livelihood, which is based on southwest monsoon.
  • Nearly 60 percent of the country’s farms lack irrigation facilities, leaving millions of farmers dependent on the rains
  • Monsoon is critical to replenish 81 reservoirs necessary for power generation, irrigation and drinking.
  • Monsoon regime emphasizes the unity of India with the rest of Southeast Asian region.



  • Ocean currents are continuous movements of water in the ocean that follow set paths, kind of rivers in the ocean.
  • There are two type of Ocean Currents, based on depth, viz. surface currents (surface circulation- which make up about 10% of all the water in the ocean) and deep water currents (thermohaline circulation- which make up the other 90% of the ocean).
  • Based on temperature, Ocean currents are classified into two types: cold currents (Labrador Current) and warm currents (Kuroshio current)


Factors which impact the ocean current formations are:

  • Planetary winds: The planetary winds are permanent winds (Trade winds, Westerlies and Polar Easterlies) that blow from one pressure belt to the other.  The oceanic circulation pattern roughly corresponds to the earth’s atmospheric circulation pattern. E.g.: There is a change in the direction of ocean currents with a change in direction of the monsoon winds in the Indian Ocean.
  • Temperatures: The differential heating of the Sun at the equator and the poles causes a difference in the temperature of ocean water. Warm water from the equator slowly moves along the surface towards the poles, while the cold water from the poles slowly creeps along the bottom of the sea towards the equator.
  • Salinity:  Waters of low salinity have lower density enabling them to flow on the surface of waters of high salinity while waters of high salinity flow at the bottom.
  • Earth’s rotation: According to Ferrel’s law- Coriolis forces deflect winds and freely moving objects to the right in the northern hemisphere and to the left in the southern hemisphere. Therefore, the movement of ocean currents in the northern hemisphere is in the clockwise and in the southern hemisphere it is in the anti-clockwise direction.
  • Landmass: A land mass obstructs the direction of flow of ocean current and divides the ocean current to flow in a different direction.

The ocean currents  are very important in determining the climates of different regions of the world, especially those regions bordering on the ocean.

  • Local Climate: Warm and Cold currents affect the local climate of a region. E.g.: the North Atlantic Drift keeps the coasts of North Sea (western coast of Europe) warm which is unusual for such high latitudes. Similarly, the warm waters of the Kuroshio current in the North Pacific ocean keep the ports of the Alaskan coast ice-free in winter.
  • Precipitation: Warm currents flow along the east coast of continents resulting in warm and rainy climates while cold currents flow along the west coast of continents.
  • Desert Formation: Cold ocean currents have a direct effect on desert formation in west coast regions of the tropical and subtropical continents. E.g.: Peru Current, also called Humboldt Current, is a cold-water current of the southeast Pacific Ocean and a primary reason for the aridity of Atacama desert (driest desert of the world).
  • Moderating effect: They are responsible for moderate temperatures at coasts. eg: Warm North Atlantic Drift in England, Canary cold current in Spain, Portugal etc.
  • Tropical cyclones: They pile up warm waters in tropics and this warm water is the major force behind tropical cyclones.


  • Not only there are number of factors which impact the formation of ocean currents, but also the ocean current themselves play a major role in determining regional as well as global climate.



Air Masses

  • When the air remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area. The homogenous regions can be the vast ocean surface or vast plains and plateaus.
  • The air with distinctive characteristics in terms of temperature and humidity is called an air mass. It is a large body of air having little horizontal variation in temperature and moisture.
  • Air masses form an integral part of the global planetary wind system. Therefore, they are associated with one or other wind belt.
  • They extend from surface to lower stratosphere and are across thousands of kilometers.

Source regions

  • The homogenous surfaces, over which air masses form, are called the source regions.
  • The main source regions are the high pressure belts in the sub tropics (giving rise to tropical air masses) and around the poles (the source for polar air masses).
  • Source Region establishes heat and moisture equilibrium with the overlying air mass.
  • When an air mass moves away from a source region, the upper level maintains the physical characteristics for a longer period. This is possible because air masses are stable with stagnant air which do not facilitate convection. Conduction and radiation in such stagnant air is not effective.

Conditions for the formation of Air masses

  • Source region should be extensive with gentle, divergent air circulation (slightly at high pressure).

convergence - divergence - cyclonic-anticyclonic (Custom)

  • Areas with high pressure but little pressure difference or pressure gradient are ideal source regions.
  • There are no major source regions in the mid-latitudes as these regions are dominated by cyclonic and other disturbances.

Air masses based on Source Regions

  • There are five major source regions. These are:
  1. Warm tropical and subtropical oceans;
  2. The subtropical hot deserts;
  3. The relatively cold high latitude oceans;
  4. The very cold snow covered continents in high latitudes;
  5. Permanently ice covered continents in the Arctic and Antarctica.
  • Accordingly, following types of airmasses are recognised:
  1. Maritime tropical (mT);
  2. Continental tropical (cT);
  3. Maritime polar (mP);
  4. Continental polar (cP);
  5. Continental arctic (cA).
  • Tropical air masses are warm and polar air masses are cold.
  • The heat transfer processes that warms or cools the air takes place slowly.

Air masses -Source Regions

Cold Air Mass

  • A cold air mass is one which is colder than the underlying surface and is associated with instability and atmospheric turbulence.

Cold source regions (polar air masses)

  • Arctic Ocean – cold and moist
  • Siberia – cold and dry
  • Northern Canada – cold and dry
  • Southern Ocean – cold and moist

Warm Air Mass

  • A warm air mass is one which is warmer than the underlying surface and is associated with stable weather conditions.

Warm source regions (tropical air masses)

  • Sahara Desert – warm and dry
  • Tropical Oceans – warm and moist

Influence of Air Masses on World Weather

  • The properties of an air mass which influence the accompanying weather are vertical distribution temperature (indicating its stability and coldness or warmness) and the moisture content.
  • The air masses carry atmospheric moisture from oceans to continents and cause precipitation over landmasses.
  • They transport latent heat, thus removing the latitudinal heat balance.
  • Most of the migratory atmospheric disturbances such as cyclones and storms originate at the contact zone between different air masses and the weather associated with these disturbances is determined by characteristics of the air masses involved.

Classification of Air Masses

  • Broadly, the air masses are classified into polar and tropical air masses.
  • Both the polar and the continental air masses can be either of maritime or continental types.

Continental Polar Air Masses (CP)

  • Source regions of these air masses are the Arctic basin, northern North America, Eurasia and Antarctica.
  • These air masses are characterized by dry, cold and stable conditions.
  • The weather during winter is frigid, clear and stable.
  • During summer, the weather is less stable with lesser prevalence of anticyclonic winds, warmer landmasses and lesser snow.

Maritime Polar Air Masses (MP)

  • The source region of these air masses are the oceans between 40° and 60° latitudes.
  • These are actually those continental polar air masses which have moved over the warmer oceans, got heated up and have collected moisture.
  • The conditions over the source regions are cool, moist and unstable. These are the regions which cannot lie stagnant for long.
  • The weather during winters is characterized by high humidity, overcast skies and occasional fog and precipitation.
  • During summer, the weather is clear, fair and stable.

Continental Tropical Air Masses (CT)

  • The source-regions of the air masses include tropical and sub-tropical deserts of Sahara in Africa, and of West Asia and Australia.
  • These air masses are dry, hot and stable and do not extend beyond the source.
  • They are dry throughout the year.

Maritime Tropical Air Masses (MT)

  • The source regions of these air masses include the oceans in tropics and sub-tropics such as Mexican Gulf, the Pacific and the Atlantic oceans.
  • These air masses are warm, humid and unstable.
  • The weather during winter has mild temperatures, overcast skies with fog.
  • During summer, the weather is characterized by high temperatures, high humidity, cumulous clouds and convectional rainfall.



Tropical Humid Climates

  • Tropical humid climates exist between Tropic of Cancer and Tropic of Capricorn.
  • The sun being overhead throughout the year and the presence of Inter Tropical Convergence Zone (ITCZ) make the climate hot and humid.
  • Annual range of temperature is very low and annual rainfall is high.
  • The tropical group is divided into three types, namely
  1. Af- Tropical wet climate 
  2. Am – Tropical monsoon climate 
  3. Aw- Tropical wet and dry climate 

Tropical Monsoon Climate

  • Monsoons are land and sea breezes on a much larger scale.
  • Unlike equatorial wet climate, monsoon climate is characterized by distinct wet and dry seasons associated with seasonal reversal of winds.
  • Floods in wet season and droughts in dry season are common.
  • Usually there are three seasons namely summer, winter and rainy season.

Tropical Monsoon Climate

Distribution of Tropical Monsoon Climate

  • Occur within 5° to 30° N and S of the equator.
  • On-shore [sea to land] tropical monsoons occur in the summer and off-shore [land to sea] dry monsoons in the winter.
  • They are best developed in the Indian sub-continent, Burma, Thailand, Laos, Cambodia, parts of Vietnam and south China and northern Australia.

Distribution of Tropical Monsoon Climate


  • The basic cause of monsoon climates is the difference in the rate of heating and cooling of land and sea (This is old theory. New theory will be explained while studying Indian Climate).
  • In the summer, when the sun is overhead at the Tropic of Cancer, a low pressure is created in Central Asia.
  • The seas, which warm up much slower, remain comparatively at high pressure. At the same time, the southern hemisphere experiences winter, and a region of high pressure is set up in the continental interior of Australia.
  • Winds blow outwards as the South-East Monsoon, to Java, and after crossing the equator are drawn towards the continental low pressure area reaching the Indian sub-continent as the South-West Monsoon (Coriolis force).
  • In the winter, conditions are reversed.


  • Monthly mean temperatures above 18 °C.
  • Temperatures range from 30-45° C in summer. Mean summer temperature is about 30°C.
  • In winters, temperature range is 15-30° C with mean temperature around 20-25° C.


  • Annual mean rainfall ranges from 200-250 cm. In some regions it is around 350 cm.
  • Places like Cherrapunji & Mawsynram receive an annual rainfall of about 1000 cm. [They lie on the windward side of the Meghalaya hills, so the resulting orographic lift (orographic rainfall) enhances precipitation. Also, they are located between mountains which enhances cloud concentration due to funneling effect]

Monsoons - periodic winds


  • Seasons are chief characteristics of monsoon climate.

The cool, dry season (October to February)

  • Out blowing dry winds, the North-East Monsoon, bring little or no rain to the Indian sub-continent.
  • However, a small amount of rain falls in Punjab from cyclonic sources (Western Disturbances: Frontal precipitation brought by jet streams) and this is vital for the survival of winter cereals.
  • North-East Monsoons blowing over the Bay of Bengal acquires moisture and bring rains to the south-eastern tip of the peninsula at this time of the year (Nov-Dec).

The hot dry season (March to mid-June)

  • The temperature rises sharply with the sun’s northward shift to the Tropic of Cancer.
  • Day temperatures of 35° C are usual in central India and the mean temperature in Sind and south India may be as high as 44° C.
  • Coastal districts are a little relieved by sea breezes. There is practically little rain. [Hailstorms (thunderstorms with hail) occurs here and there]

The rainy season (mid-June to September)

  • With the ‘burst’ of the South-West Monsoon in mid-June, torrential downpours sweep across the country. Almost all the rain for the year falls within this rainy season.
  • This pattern of concentrated heavy rainfall in summer is a characteristic feature of the Tropical Monsoon Climate.

The Retreating Monsoon

  • The amount and frequency of rain decreases towards the end of the rainy season. It retreats gradually southwards after mid-September until it leaves the continent altogether.
  • The skies are clear again and the cool, dry season returns in October, with the out blowing North-East Monsoon.

The role of monsoons in India is vital for its economy.

Tropical Marine Climate

  • Outside the monsoon zone, the climate is modified by the influence of the on-shore Trade Winds all the year round. This type of climate is called Tropical Marine Climate. Such a climate has a more evenly distributed rainfall.
  • Such a climate is experienced in Central America, West Indies, north-eastern Australia, the Philippines, parts of East Africa, Madagascar, the Guinea Coast and eastern Brazil.
  • The rainfall is both orographic where the moist trades meet upland masses as in eastern Brazil, and convectional due to intense heating during the day and in summer.
  • Its tendency is towards a summer maximum without any distinct dry period.
  • Due to the steady influence of the trades, the Tropical Marine Climate is more Favourable for habitation, but it is prone to severe tropical cyclones, hurricanes or typhoons.

Tropical Monsoon Forests

Drought-deciduous forest; dry forest; dry-deciduous forest; tropical deciduous forest.

  • Broad-leaved hardwood trees. Well developed in southeast Asia.
  • Trees are normally deciduous, because of the marked dry period, during which they shed their leaves to withstand the drought [They shed their leaves to prevent loss water through transpiration].
  • The forests are more open and less luxuriant than the equatorial jungle and there are far fewer species.
  • Where the rainfall is heavy, e.g. in southern Burma, peninsular India, northern Australia and coastal regions with a tropical marine climate, the resultant vegetation is luxuriant.
  • With a decrease in rainfall in summer, the forests thin out into thorny scrubland or savanna with scattered trees and tall grass.
  • In parts of the Indian sub-continent, rainfall is so deficient that semi-desert conditions are found in summer. Monsoonal vegetation is thus most varied, ranging from forests to thickets, and from savanna to scrubland.

Population and Economy in Monsoon Climate

  • Monsoon climatic regions support high population density.
  • Income levels are low as most of these regions are underdeveloped or developing.
  • Subsistence farming is the main occupation. (crops grown with an intention to secure food for the season. The crops are not sold as the production is very low).
  • Intensive cultivation is common in regions with irrigational facilities.
  • Shifting cultivation is followed in North-East India and South-East countries.
  • Major crops include rice, sugar, cotton, jute, spices, etc..
  • Cattle and sheep rearing is carried out for domestic and commercial purposes. Livestock industry is not as profitable as in temperate regions.

Agricultural Development in the Monsoon Lands

  • Much of the monsoon forest has been cleared for agriculture to support the very dense population. Subsistence agriculture is the major occupation.
  • Farms are small and the people are forever ‘land hungry.’ Industrialization make things worse.
  • Tropical agriculture dependent on natural rainfall and a large labour force, reaches its greatest magnitude in the monsoon lands.
  • Farming is the dominant occupation of the Indian sub-continent, China, South- East Asia, eastern Brazil and the West Indies. The following types of agriculture are recognizable.


  • Rice is the most important staple crop.
  • Irrigation water from rivers, canals, dams or wells is extensively used in the major rice producing countries.
  • Other food crops like maize, millet, sorghum, wheat, gram and beans are of subsidiary importance. They are cultivated in the drier or cooler areas where rice cannot be grown.

Lowland cash crops

  • The most important crop in this category is cane sugar.
  • As much as two-thirds of world’s sugar production comes from tropical countries.
  • Some of the major producers include India, Java, Formosa, Cuba, Jamaica, Trinidad and Barbados.
  • Jute is confined almost entirely to the Ganges – Brahmaputra delta, in India and Bangladesh.
  • Other crops include cotton, a major commercial crop of the Indian sub-continent.

Highland plantation crops

  • The colonization of tropical lands by Europeans gave rise to a new form of cultivated landscape in the cooler monsoonal highlands.
  • Thousands of acres of tropical upland forests were cleared to make way for plantation agriculture in which tea and coffee are the most important crops.


  • Coffee originated in Ethiopia and Arabia.
  • But Brazil accounts for almost half the world’s production of coffee.
  • It is mainly grown on the eastern slopes of the Brazilian plateau.
  • The crop is also cultivated on the highland slopes in the Central American states, India and eastern Java.


  • Tea originated in China and is still an important crop there.
  • It requires moderate temperatures (about 15° C), heavy rainfall (over 150 cm) and well drained highland slopes.
  • It thrives well in the tropical monsoon zone (highlands).
  • The best regions are thus the Himalayan foothills of India and Bangladesh, the central highlands of Sri Lanka and western Java, from all of which it is exported.
  • In China tea is grown mostly for local consumption.


  • Most of the forests yield valuable timber, and are prized for their durable hardwood.
  • Lumbering is undertaken in the more accessible areas. This is particularly important in continental South-East Asia.
  • Of the tropical deciduous trees, teak, of which Burma is the leading producer, is perhaps the most sought after. It is valuable on account of its great durability, strength, immunity to shrinkage, fungus attack and insects.
  • Teak logs are so heavy that they will not float readily on water. It is therefore necessary to ‘poison’ the tree several years before actual felling, so that it is dry and light enough to be floated down the Chindwin and the Irrawaddy to reach the saw mills at Rangoon.
  • Other kinds of timber include Neem, Banyan, Mango, Teak, Sal, Acacia, Eucalyptus
  • Together with the forests are bamboo thickets, which often grow to great heights.


  • Burma alone accounts for as much as three – quarters of the world’s production.
  • It is such a durable timber that it is extensively used for ship building, furniture and other constructional purposes.

Shifting Cultivation

  • This most primitive form of farming is widely practiced.
  • Instead of rotating the crops in the same field to preserve fertility, the tribesmen move to a new clearing when their first field is exhausted.
  • Maize, dry padi, sweet potatoes and some beans are the most common crops.
  • Farming is entirely for subsistence, i.e. everything is consumed by the farmer’s family, it is not traded or sold.
  • As tropical soils are rapidly leached and easily exhausted, the first crop may be bountiful but the subsequent harvests deteriorate.
  • Shifting cultivation is so widely practiced amongst indigenous peoples that different local names are used in different countries.



Name of Shifting Cultivation

Malaysia Lacking
Burma Taungya
Thailand Tamrai
Philippines Caingin
Java Humah
Sri Lanka Chena
Africa and Central America Milpa
North-east India Jhum




The cryosphere is the frozen earth’s surface, where water is found in solid form i.e. sea ice, lake ice, snow cover, glaciers, ice sheets, and frozen ground. It is usually found in Antarctica, Arctic, and mountainous regions.



The cryosphere affect the global climate in the following way

Reflecting sheet: The cryosphere acts like a heat reflecting sheet and thus influences the heating and cooling system of Earth. The system eventually controls air temperatures, ocean currents, sea levels, and storm patterns.

Carbon sink: The polar region has the ability to capture tonnes of carbon inside its soil. And when the ice melts, it releases Methane (Greenhouse Gas), which is responsible for absorbing Sun’s heat and causes global warming.

Heat exchanger: The conversion of seawater into seawater causes highly dense and saltier surrounding water that initiates thermohaline circulation patterns across the oceans of the world. It acts as a natural heat exchanging phenomenon. It transports warm water from the equator to the poles and cold water from the poles back to the equatorial area, which directly influences climatic incidents like the El-Nino La-Nina effect.

Cascading effect: Rising sea level creates a cascade of effects which includes higher temperatures, sea-level rise, heavy rainfall, heat stress, and inundation damage.

Water cycle: The volume of water gets increased due to the melting of sea ice, glaciers. And the changes in the water cycle affect global energy and global climate.

Just a little change in the cryosphere can influence climate changes and also dramatically alter the Earth’s snow and ice-covered areas. With nearly 70% of Earth’s freshwater stored in glaciers and ice caps, we must protect the cryosphere to save the biosphere.



The term flood is generally used when the water-flows in rivers, streams and other water bodies cannot be contained. Floods occur regularly in India affecting about 12% of area.

India is dependent on the monsoons for rainfall. This leads to abundance on water (often flooding) in particular months and scarcity of water during the others. The National Disaster Management Authority (NDMA) has outlined a plan to use this flood water as a sustainable source of irrigation and for all-weather inland navigation.

    It has been suggested that the excess water can be stored in dams, reservoirs and dams during the monsoons and used later. This would provide a reliable sustainable water supply to meet the irrigation needs.
    Also, inland navigation which is often hampered because of low water level can be carried in a regular manner by releasing and maintaining water level from these storage facilities.
 The government has taken proactive steps to desilt and expand the existing storage facilities apart from planning for newer ones. When implemented on its entirety the proposal would not only help in averting disasters but contribute significantly to irrigation and inland navigation.

Floods as sustainable source of irrigation

• Reducing run-off

– Infiltration method through afforestation

– Through digging wells

– Biopore infiltration method

• Reducing flood peaks by volume reduction

– By constructing dams and detention basins

– Marshy areas, old quarries and mines can act as detention basins

• Reducing flood level

– Stream channelization (close network of canals reduces flood hazard)

• Flood diversion

– It includes diverting flood water in marshes, depressions and spreading it thinly over paddy fields

and dry lands E.g. Ghaggar Reversion Scheme.

Flood as all-weather inland navigation

• By the method of network of canals from the flood prone rivers the water in canal can also be used

for inland navigation.

• Interlinking of rivers will help in pan India development of inland navigation.

• Channel improvement can also be done by deepening, widening, straightening, lining and cleaning

out of vegetation and debris from river channel

A major portion of the territory of India experiences flood during monsoon season in June-July. And several other places like Uttarakhand experience the same due to abrupt rainfall.

The incidents eventually cause surface runoff, crop damage, wealth, and life losses. It could be avoided by the following methods

  • Inter-basin transfer: It is the moving of the water from a surplus area to a shortage area. India has been practicing the method for a long time. Some of the examples in India are the Beas-Sutlej link project, Ken-Betwa link project, Telugu Ganga Project, etc. It also helps in the development of inland navigation.
  • Barrage and Dams: During monsoon season, dams and barrages store excess water to prevent floods. Later it releases the water in a regulated manner or uses it in power generation (In the case of Dams). For example, Damodar Valley Corporation (DVC) was set up in 1948 by following the Tennessee Valley Authority in the USA. The main objective of DVC was flood control but it has been generating and transmitting power since 1953.
  • Rain-water harvesting: Rainwater harvesting is the collection and storage of rainwater to use it in the future for different purposes. The rain-water can be stored in wells, tanks. Countries like China, Argentina, and Brazil practice rooftop rainwater harvesting to use it for drinking, irrigation and to avoid the depletion of groundwater.
  • Irrigation: Some of the south Indian states like Maharashtra, Tamil Nadu are heavily drought-prone. River interlinking projects could transfer direct water to those areas or the barrage could store water for future uses. It would definitely be beneficial to maintain continuous agricultural growth.
  • All the methods stated above could have the ability to convert floods into sustainable sources of irrigation and all-weather inland navigation in India.
  • India experiences monsoons for a period of four months during which sometimes incessant rains cause floods and devastation, while for the rest of the year it remains dry for most parts, often resulting in water shortages. This excess flood water can surely be used as a valuable resource in water scarce regions for the non-monsoon months, thereby solving the twin problems of flood and water scarcity.

 The following methods may be used to achieve this objective:

  • River linking:The government has been ambitious with this project of diverting excess water from overflowing rivers to rivers in non-perennial regions, in order to solve the problems of flood and water shortage. These river linking channels could also be useful as all-weather inland navigation waterways, thereby helping in creating a cheaper and pollution free mode of transport.
  • Rain water harvesting:The excess water can be captured and stored in wells, tanks etc. during rains as was practiced in many parts of India during medieval period (in form of stepwells/baolis etc).
  • Multi-purpose projects/dams:Dams can be erected in flood areas to capture excess water which can then be released slowly over the year as per irrigation requirements.
  • Inundation canals and weirs:Flood water can also be managed by making diversions through inundation canals, small irrigation structures, and with weirs that take away excess water to the agricultural fields.

The methods stated above,can go a long way in solving various water woes of India if implemented expeditiously and on a large scale.

  • In India normally floods don’t occur to all the rives at a same time . Because of the monsoon pattern different rivers get affected with floods at different point of time.
  • When floods occur to a river , that entire water flows and finally meet with sea/ocean . Instead of allowing it into the sea if we inter link the rivers, that excess water can be diverted into the other river.
  • This water is stored and transferred to the other river, where ever is required in that link. That water can be used for the purpose of agriculture, we can also develop inland water ways also.
  • Here source of irrigation become sustainable because at one or other point of time in an year any of the part is experiencing the Monsoon, which results in floods, now we can divert that water into other deficit area. We can also use that flow for inland water ways.

The project already proposed are :

  • Ganga-Kaveri link
  • Ganga-Brahmaputra link
  • ken-betwa-son link
  • west flowing rivers of western ghats link

Most of the flood waters of Ganga, Brahmaputra wasting every year on one hand. The southern states in Krishna, Kaveri, Penna basins facing acute water shortage for agriculture. Linking is the best solution to solve this problem.


Introduction (Define / Origin / Data)

  • (Definition) Salinity is the the total amount of dissolved salts in sea water. It’s generally expressed as parts per thousand (ppt).
  • (Origin) The average salinity of the oceans is 35 parts per thousand (ppt). But the exact levels in each ocean are not same, due to the effect of following factors:

Body#1: Variation in Oceanic salinity- factors responsible

  • Evaporation rate:
    • Oceans between 20 to 30 degree North and south have high salinity because of (1) high temperature (2) low humidity .
    • Temperate oceans have lower salinity due to lower temperature.
  • Amount of Fresh water added in ocean:
    • Equatorial waters have lower salinity due to heavy rainfall and high humidity.
    • Oceans fed by large rivers like Amazon, Congo, Ganges, Mekong etc have lower salinity.
    • Melting of icebergs and resultant fresh water intake into Baltic, Arctic and Antarctic ocean waters makes them less saline. Thus, global warming and greenhouse gas effect has indirectly affects the salinity levels of the oceans.
  • Currents Mixing:
    • In open oceans, currents mix and flow freely, hence salinity is average 35ppt or lower.
    • Whereas in the Caspian sea, Mediterranean sea, Red sea and other wholly / partially enclosed seas – the fresh water doesn’t mix freely with ocean water hence salinity is higher.
  • Salinity, temperature and density of water are interrelated. Hence, any change in the temperature or density influences the salinity of an area.

Body#2: Variation in salinity: its multi-dimensional effects

Current Circulation-> temperature, rainfall affected

  • Because of density difference: (1) High salinity seawater sinks below the lower salinity water and; (2) Cold water sinks below the warm water.
  • Therefore, the cold water at the poles sinks and slowly moves towards the equator and warm-water moves from equator to poles to replace the sinking cold water.
  • Thus salinity has a great role to play in the formation and circulation of oceanic currents via the thermohaline process.
  • Since Earth’s temperature and rainfall is affected by currents, the level of salinity has indirect role in Earth’s overall climate.
  • Salt water has a much lower freezing temperature and without salt more of the oceans would freeze during winter- affecting the movements of both fish-schools and cargo ships.

Marine Biodiversity, survival of the species

  • Seawater is more than 800 times denser than air. This high density is related to salinity and temperature and means that objects that might sink in freshwater are able to float in seawater. This has a big effect on life in the sea. For example:
    • Despite having no fins, the plankton are kept perpetually afloat due to the density of seawater. Without plankton, entire marine ecosystem will collapse, the atmospheric levels of oxygen will be altered- posing threat to all forms of life even on land.
    • Whales, sharks, sea-cows and other marine species can grow to be so much bigger in size than the ordinary animals on land.
  • Salt affects water’s freezing point and salinity affects circulation of cold and warm currents. The marine organisms have accordingly adapted to a particular level of temperature, pH and salinity. Human-induced changes in oceanic salinity threatens their survival.
  • Example, certain oceanic fishes lay eggs on the grounds near estuaries. With right saline density, the eggs will remain there until the fish is born. If it changes- the egg float to surface, and become easy feed for the predators.

    Conclusion (account for = Discussion = Summary)

    Oceanic salinity is affected by factors such as temperature, ingress of fresh water and mixing of currents. Oceanic salinity plays important role in the growth of marine organisms, circulation of oceanic currents and distribution of temperature and rainfall across the globe. Thus, it’s suffice to say oceanic salinity plays a crucial role in the survival of both marine and terrestrial lifeforms on Earth.


The year 2016 was declared as International year of pulses by United Nations to create awareness of the benefits of pulses, promote utilization and encourage connections to enhance global production.
    Pulses are annual leguminous crops harvested solely for dry grain, such as beans, chick, peas, pegion pea, filed pea, summer moong bean, ctc.
    The benefits of pulse cultivation are enormous. Some are,
  • Health and nutritional benefits pulses are rich in protein and essential amino acids and vitamins and contribute significantly to the food security of the masses.
  • They also help fight obesity.
  • They present and help manage chronic disease such as diabetes, Coronary conditions and cancer.
  • Pulses are used as feed for livestock.
  • Pulses have less carbon foot print than other crops.
  • Pulses enhance soil fertility by nitrogen fixation
  • Pulses need less fertilizer because of their unique nitrogen fixation abilities.
  • Pulses promote healthy micro organizes which ‘Crowd-out’ disease causing bacteria and fungi.
  • Pulses use less water and are suitable for semi-arid conditions and can tolerate drought stress.
India needs a robust policy to promote pulses. Inclusion of pulses in public procurement can guarantee farmers stable prices and encourage production.


The UN Convention to Combat Desertification (UNCCD) states that desertification means land degradation in arid, semi-arid, and dry sub-humid areas resulting from various factors, including climatic variations and human activities. Although it’s an issue that reaches far beyond those living in and around the world’s deserts, threatening the food security and livelihoods of more than two billion people, defying climate boundaries.


There are many factors that contribute to the process of increasing desertification. Moreover, the process of desertification is not just restricted to a particular climate boundary, i.e. arid and semi-arid areas. It is a global phenomenon that can be corroborated in the following examples.

  • Climate Change: Climate change is a global issue that poses a threat to all progress done by mankind in the last two centuries. The effect of Climate change is also a significant factor in increasing desertification.
    • As the land surface is warming more quickly than the Earth’s surface as a whole, this results in smaller increases in surface ocean temperatures compared to the land surface as global temperatures rise.
    • Further, both natural variabilities in climate and global warming can also affect rainfall patterns around the world, which can contribute to desertification.
    • While this sustained, human-caused warming can by itself add to heat stress faced by vegetation, it is also linked to worsening extreme weather events like Floods, Droughts, Landslides.
  • Soil Erosion: One of the main processes for desertification is erosion. This is typically through some force of nature such as wind, rain, and waves, but can be exacerbated by man-made activities including plowing, grazing, or deforestation.
    • The World Atlas of Desertification (2018) indicated that it is not possible to deterministically map the global extent of land degradation.
    • Further, soil erosion is a global phenomenon that affects almost all major biomes in the world.
    • The occurrences of dust storms in northern India testifies to this observation.
  • Loss of Soil Fertility: A loss of soil fertility is another form of degradation. In order to increase agricultural production, whether it is a developed or developing country, soils are being exposed to the overuse of fertilizers.
    • Due to this salinization and acidification of soils is increasing.
  • Urbanization: According to several reports, urbanization is increasing at a rapid pace. Even in India, almost 50% of the population is expected to live in urban areas, by 2050.
    • As urbanization increases, the demand for resources increases, drawing more resources and leaving lands that easily succumb to desertification.


As can be seen from the above arguments, desertification and its impacts are not restricted to certain climatic boundaries. That’s why UNCCD describes Desertification as one of the greatest environmental challenges of our time and it must be tackled in a holistic manner.

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