CLIMATE

• Temperature decreases with increase in height at the rate of 6.5°C per 1000 meters difference in height or 1°C for every 165m (Normal Lapse rate). The high peaks of the Himalayas are thus always covered by snow. Shimla is therefore cooler than Delhi as it lies at a higher elevation. Ooty in the Nilgiris in the Western Ghats is cool even in summer because of its height though it lies near the equator

DISTANCE FROM THESEA

The places which are in the interior experience continental climate, as they arefar away from the moderating influence of the water bodies. Thus, such placesexperience very hot summer and cold dry winters. In contrast the coastal areasenjoy almost the same type of climatic conditions throughout the years.

IMPACT OF PREVAILING WINDS: The monsoons are the prevailing winds over the whole of India. These winds are a unique feature of the climate of India. During the summer season, the monsoon wind covers the entire India and brings rainfall to practically every part of the country.

RELIEF : Relief forms an important factor that affects the climate of India. The high Himalayan Mountain in the north act as a barrier to the south-west monsoon winds forcing the winds to shed most of its rainfall in India before crossing the Himalayas. The cold winds in winter from Central Asia are mainly obstructed from coming into India by these high Himalayas. So the severity of winter in India is not that intense. The Western and Eastern Ghats on either side of the Peninsular in South India force the monsoon winds to give plenty of rainfall in the coastal regions. Again, the eastern slopes of the Western Ghats fall in the rain-shadow area as the Western Ghats obstructs the monsoon winds and get heavy rainfall on the western slopes.

SEASONS

WINTER

The winter season begins from mid-November and stays till February in northern India. December and January are the coldest months in the northernpart of India. The temperature ranges between 10°-15°C in the northern plains, while it ranges between 24°-25°C in Chennai.The northeast trade winds prevail over the country in this season. Asthese winds blow from land to sea, most parts of the countryexperience a dry season.The inflow of the cyclonic disturbances from the west and the northwest is a characteristic feature of the cold weather over the northern plains. These low-pressure systems originate over the Mediterranean Sea and Western Asia and move into India. They cause winter rains over the plains and snowfall in the mountains. The winter rainfall is in small amount but is very important for the rabi crop in the north western India . The peninsular region does not get a well-defined winter because of the moderatinginfluence of the sea.

SUMMER• The summer season is from March to May. During this period,

the global heat belt shifts towards north because of the apparent northward movement of the sun. During summer,the temperatures rise and air pressure falls in the northern part of the country.

• In March, the highest day temperatures of about 38 degree C occur in the Deccan Plateau. Therefore, Peninsular India places south of Satpuras experience temperature between 26-32 degree C. Central India, comprising of Delhi and Madhya Pradesh experience temperature between 40-45 degree C .

• North-West India, comprising mainly of Rajasthan has very high temperature more than 45 degree C, also due to features like sandy soil, direct insolation and lack of cloud cover. Towards the end of May, an elongated low-pressure area develops in the region which extends from the Thar Desert in the northwest to Patna and Chota Nagpur Plateau in the east and southeast.

LOCAL STORMS DURING SUMMER•Mango Showers (since the rain showers are good for the mango trees) occurs along the coast of Kerala.

•Kalbaisakhi in the month of Baisakh occursin Assam and West Bengal. These are thunderstorms, accompanied with strong winds are heavy rainfall. This is good for the teacrop in Assam and the jute and rice in West Bengal. In Assam these storms are called Bardoli chheerha .

•Loo is the name given to the hot, dry winds that blow inthe Northern Plains. It is very common in Punjab, Haryana,Western Uttar Pradesh (called “Aandhi") and Bihar.

•Blossom Shower - With this shower, coffee flowersblossom in Karnataka and its nearbyareas.

LOCAL STORMS DURING SUMMER

Monsoons

Atmospheric circulation

Permanent Winds

Inter Tropical Convergence Zone

• The Inter Tropical Convergence Zone (ITCZ,) is a broad trough of low pressure in equatorial latitudes. This is where the northeast and the southeast trade winds converge. This convergence zone lies more or less parallel to the equator but moves north or south with the apparent movement of the sun.

Southwest Monsoon/ Advancing monsoon(The Rainy Season)• By early June, the low-pressure condition over the northern plains

intensifies. It attracts, the trade winds of the southern hemisphere. These south-east trade winds originate over the warm subtropical areas of the southern oceans. They cross the equator and blow in a south-westerly direction entering the Indian peninsula as the south-west monsoon. As these winds blow over warm oceans, they bring abundant moisture to the subcontinent. These winds are strong and blow at an average velocity of 30km per hour.

• With the exception of the extreme north-west, the monsoon winds cover the country in about a month. The inflow of the south-west monsoon into India brings about a total change in the weather . Early in the season, the windward side of the Western Ghats receives very heavy rainfall, more than 250 cm. The Deccan Plateau and parts of Madhya Pradesh also receive some amount of rain in spite of lying in the rain shadow area.

• The maximum rainfall of this season is received in the north-eastern part of the country. Mawsynram in the southern ranges of the Khasi Hills receives the highest average rainfall (11,872 mm) in the world. Rainfall in the Ganga valley decreases from the east to the west. Rajasthan and parts of Gujarat get scanty rainfall.

• Another phenomenon associated with the monsoon is its tendency to have ‘breaks’ in rainfall. Thus, it has wet and dry spells. In other words, the monsoon rains take place only for a few days at a time. They are interspersed with rainless intervals.

• These breaks in monsoon are related to the movement of the monsoon trough. For various reasons, the trough and its axis keep on moving northward or southward, which determines the spatial distribution of rainfall.

Retreating/Post Monsoons(The Transition Season)• During October –November , with the apparent movement of the sun towards

the south, the monsoon trough or the low-pressure trough over the northern plains becomes weaker. This is gradually replaced by a high-pressure system. The south-west monsoon winds weaken and start withdrawing gradually. By the beginning of October , the monsoon withdraws from the Northern Plains.

• The months of October -November for m a period of transition from hot rainyseason to dry winter conditions.

• The retreat of the monsoon is marked by clear skies and rise in temperature. While day temperatures are high, nights are cool and pleasant. The land is still moist. Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive during the day. This is commonly known as ‘October heat’. In the second half of October, the mercury begins to fall rapidly in northernIndia.

• The low-pressure conditions, over north-western India, get transferred to the Bay of Bengal by early November. This shift is associated with the occurrence of cyclonic depressions, which originate over the Andaman Sea. These cyclones generally cross the eastern coastsof India cause heavy and widespread rain. These tropical cyclones are often very destructive. The thickly populated deltas of the Godavari, the Krishna and the Kaveri are frequently struck by cyclones, which cause great damage to life and property.

• Sometimes, these cyclones arrive at the coasts of Orissa, West Bengal and Bangladesh. The bulk of the rainfall of the Coromandel Coast is derived from depressions and cyclones.

THE ONSET OF THE MONSOON AND WITHDRAWAL• The Monsoon, unlike the trades, are not steady winds but are pulsating

in nature, affected by different atmospheric conditions encountered by it, on its way over the warm tropical seas. The duration of the monsoon is between 100-120 days from early June to mid-September.

• Around the time of its arrival, the normal rainfall increases suddenly and continues constantly for several days. This is known as the ‘burst’of the monsoon, and can be distinguished from the pre-monsoon showers.

• The monsoon arrives at the southern tip of the Indian peninsula generally by the first week of June. Subsequently, it proceeds into two – the Arabian Sea branch and the Bay of Bengal branch.

• The Arabian Sea branch reaches Mumbai about ten days lateron approximately the 10th of June. This is a fairly rapidadvance.

• The Bay of Bengal branch also advances rapidly and arrives in Assam in the first week of June. The lofty mountains causes the monsoon winds to deflect towards the west over the Ganga plains.

• By mid-June the Arabian Sea branch of the monsoon arrives over Saurashtra-Kutch and the central part of the country. The Arabian Sea and the Bay of Bengal branches of the monsoon merge over the northwestern part of the Ganga plains. Delhi generally receives the monsoon showers from the Bay of Bengal branch by the end of June (tentative date is 29th of June).

• By the first week of July, western Uttar Pradesh, Punjab, Haryanaand eastern Rajasthan experience the monsoon. By mid-July, themonsoon reaches Himachal Pradesh and the rest of the country

• Withdrawal or the retreat of the monsoon is a more gradual process. The withdrawal of the monsoon begins in northwestern states of India by early September. By mid-October, it withdraws completely from the northern half of the peninsula. The withdrawal from the southern half of the peninsula is fairly rapid.

• By early December , the monsoon has withdrawn from the rest of the country. The islands receive the very first monsoon showers, progressively from south to north, from the last week of April to the first week of May.

• The withdrawal, takes place progressively from north to south from the first week of December to the first week of January. By this time the rest of the country is already under the influence of the winter monsoon.

Mawsynram---o

• El Nino and La Nina are opposite phases of what is known as the El Niño-Southern Oscillation (ENSO) cycle. The ENSO cycle is a scientific term that describes the fluctuations in temperature between the ocean and atmosphere in the east-central Equatorial Pacific ocean.

La-Nina

• La Niña means The Little Girl in Spanish. La Niña is also sometimes called ElViejo, anti-El Niño, or simply "a cold event.“ La Niña, is the cooling of waterin the Eastern Pacific Ocean.

• The water in Eastern Pacific, which is otherwise cool; gets colder than normal. There is no reversal of the trade winds but it causes strong high pressure over the eastern equatorial Pacific. On the other hand, low pressure is caused over Western Pacific and off Asia.

This has so far caused the following major effects:

• Drought in Ecuador and Peru. Heavy floods in Australia; High Temperaturein Western Pacific, Indian Ocean, off coast Somalia and good rains in India (monsoons are helped).

El Nino Southern Oscillation(ENSO)

• El Niño means The Little Boy, or Christ Child in Spanish. El Niño was originally recognized by fishermen off the coast of South America in the 1600s, with the appearance of unusually warm water in the Pacific Ocean.

• In normal times, when the tropical south Pacific ocean experiences high pressure, alternatively the tropical Indian ocean experiences low pressure conditions.

• However, these pressure conditions are sometimes reversed, and results in low pressure in the Pacific and alternatively high pressure in the Indian ocean. This is the periodic change in pressure conditions which is referred to as the Southern Oscillation. These changes in the pressure conditions being developed in the Pacific and Indian oceans are connected with the phenomenon of El Nino. This connected phenomenon is referred to as the El Nino Southern Oscillations or the ENSO.

• El Nino weakens the trade winds of the affected region and adversely affect monsoons in India and cause drought conditions. In Australia too, it can bring about drought conditions.

OCEAN CURRENTS

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 due to violent winds (squalls), very heavy rainfall (torrential rainfall) and storm surge.

• They are irregular wind movements involving closed circulation of air around a low pressure center. This closed air circulation (whirling motion) is a result of rapid upward movement of hot air which is subjected to Coriolis force. The low pressure at the center is responsible for the wind speeds.

• The cyclonic wind movements are anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere due to Coriolis force.

• The Coriolis force is zero at the equator (no cyclones at equator because of zero Coriolis Force) but it increases with latitude. Coriolis force at 5° latitude is significant enough to create a cyclone. About 65 per cent of cyclonic activity occurs between 10° and 20° latitude.

Conditions favourable for a tropical cycloneformation

• Large sea surface with temperature higher than 26.5°C,

• Presence of the Coriolis force enough to createa cyclonic vortex,

• Small variations in the vertical wind speed,

• A pre-existing weak low-pressure area or low-level-cyclonic circulation.

Ocean waters having temperatures of 26.5° C or moreis the source of moisture which feeds the storm. The condensation of moisture releases enough latent heat of condensation to drive the storm.

• Air over the warm ocean is uplifted as it is warm and light.At certain height, due to lapse rate, the temperature of air falls and moisture in the air undergoes condensation.

• Condensation releases latent heat of condensation making the air more warmer. It becomes much lighter and is further uplifted.

• The space is filled by fresh moisture laden air. Condensation occurs in this air and the cycle is repeated as long as the moisture is supplied.

• Due to excess moisture over oceans, the thunderstorm intensifies and sucks in air at much faster rate. The air from surroundings rushes in and undergoes deflection dueto Coriolis force creating a cyclonic vortex (spiralingaircolumn).

• The air in the vortex is forced to form a region of calmness called an eye at the center of the cyclone. The innersurface of the vortex forms the eye wall, the most violent regionof the cyclone.

• All the wind that is carried upwards loses its moisture and becomes cold and dense. It descends to the surface through the cylindrical eye region and at the edges of the cyclone.

• The spiraling winds create multiple convective cells with successive calm and violent regions.

• The regions with cumulonimbus cloud (rising limbs of convective cell) formation are called rain bands below which intense rainfall occurs.

• The ascending air will lose moisture at some point and descends (subsides) back to surface through the calm regions (descending limbs of convection cell – subsiding air) that exist between two rain bands.

• Continuous supply of moisture from the sea is the majordriving force behind every cyclone. On reaching the landthe moisture supply is cut off and the stormdissipates.

Bay of Bengal is more cyclone prone than the Arabian sea because

Water Temperature

• The Bay of Bengal receives higher rainfall and constant inflow of fresh water from the Ganga, Brahmaputra, Irrawady, Mahanadi, Godavari and Krishna rivers. This means that the Bay’s surface water keeps getting refreshed, making it difficult for the warm water to mix with the cooler water below, making it ideal for a depression. On the other hand, the Arabian Sea receives stronger winds thathelp dissipate the heat, and the lack of constant fresh water supply helps the warm water mix with the cool water, reducing the temperature. Much higher monsoonal rainfall along with fresh water input from the landmass over the bay decreases the surface water salinity making it one of the freshest tropical oceans ever known.

• Winds over the Bay of Bengal are sluggish unlike relatively very strong winds over the Arabian Sea during the summer. The weak wind forcing in the bay offsets vertical mixing of water allowing a well stratified sea with warmer surface water persisting for a long time producing huge amount of moisture via convective activities. In the Arabian Sea, strong wind forcing encourages high evaporation, surface cooling and hence cause overturning of water always forming a weakly stratified sea than the Bay of Bengal.

Link with Pacific oceanBay of Bengal is linked with the pacific ocean through a passage way that facilitates movement of windsystem. The western pacific ocean is a hot bed for cyclonic activities and some system are so powerfulthat it can move into Bay of Bengal region and subsequently gain power to become cyclones.

• Thus, Arabian Sea system lacks the desired features to make it suitable for cyclonic activity. Even though it may host low-pressure wind system , it does not gain enough power to turn into a full scale cyclonic activity capable of causing large-scale destruction.

• Bhola Cyclone -1970 was the deadliest cyclone that affected Indian subcontinent. It haddevastating effect on East Pakistan(present day Bangladesh) killing 5 lakh people.

• Diviseema cyclone, Andhra Pradesh – 1977 Diviseema cyclone was a devastating tropicalcyclone that hit Andhra Pradesh in November 1977, killing approx 10,000 people.

• Odisha Super cyclone – 1999 The Odisha super cyclone in the year 1999 was the strongest cyclonic storm to hit the Indian coast and also the strongest tropical cyclone that affected India killing approx 10,000 people.

• Cyclone Phailin – 2013 Cyclone Phailin was second strongest tropical cyclone in India since the 1999 Odisha cyclone, resulted heavy rainfall in Odisha, Andhra Pradesh, Jharkhand as well as other Indian states.

• Cyclone Hudhud – 2014 Cyclone Hudhud was a strong tropical cyclone that causeddamage to Visakhapatnam city of Andhra Pradesh.

• Cyclone Titli, 2018 Very severe cyclonic storm Titli was part of the 2018 North Indian Ocean cyclone season and made landfall in Odisha’s Gopalpur and Srikakulam of Andhra Pradesh.

• Cyclone Gaja, 2018 Cyclone Gaja hit the Tamil Nadu, Andhra Pradesh and some parts of coastal Puducherry with heavy rainfall.

• Cyclone Fani, 2019 was an Extremely Severe cyclone that hit Indian states of Odisha andAndhra Pradesh and the first severe cyclonic storm of the 2019.

• Cyclone Amphan, 2020 a super cyclone affected the West Bengal in India and Bangladesh.The Super Cyclonic Storm Amphan is the second super cyclone since 1999.

Naming of Tropical Cyclones

• In 2000, a group of nations called WMO/ESCAP (World Meteorological Organisation/United Nations Economic and Social Commission for Asia and the Pacific), which comprised Bangladesh, India, Maldives, Myanmar, Oman, Pakistan, Sri Lanka and Thailand, decided to start naming cyclones in the region. After each country sent in suggestions, the WMO/ESCAP Panel on Tropical Cyclones (PTC) finalized the list.

• The WMO/ESCAP expanded to include 5 more countries in 2018 —Iran, Qatar, Saudi Arabia, United Arab Emirates and Yemen.

• The recent list of 169 cyclone names were provided by thesecountries — 13 suggestions from each of the 13 countries.

• The names are assigned sequentially whenever a cyclonic stormdevelops.

• The 13 names in the recent list that have been suggested by India include: Gati, Tej, Murasu, Aag, Vyom, Jhar (pronounced Jhor), Probaho, Neer, Prabhanjan, Ghurni, Ambud, Jaladhi and Vega.