Climate - 2 Class 11 Geography

The Rhythm Of Seasons

The climatic conditions of India can best be described in terms of an annual cycle of seasons. Four seasons are:

The Cold Weather Season

• Found from November to February
• December and January are the coldest months in the northern plain.
• The mean daily temperature remains below 21°C, over most parts of northern India.
• The night temperature may be quite low, sometimes going below freezing point in Punjab and Rajasthan.

There are three main reasons for the excessive cold in north India during this season:

• States like Punjab, Haryana and Rajasthan being far away from the moderating influence of sea experience continental climate.
• The snowfall in the nearby Himalayan ranges creates cold wave situation; and
• Around February, the cold winds coming from the Caspian Sea and Turkmenistan bring cold wave along with frost and fog over the northwestern parts of India.

Pressure and Winds:

• By the end of December (22nd December), the sun shines vertically over the Tropic of Capricorn in the southern hemisphere.
• The weather in this season is characterized by feeble high pressure conditions over the northern plain. In south India, the air pressure is slightly lower respectively.
• As a result, winds start blowing from northwestern high pressure zone to the low air pressure zone over the Indian Ocean in the south. Due to low pressure gradient, the light winds with a low velocity of about 3-5 km per hour begin to blow outwards.
• By and large, the topography of the region influences the wind direction. They are westerly or northwesterly down the Ganga Valley. They become northerly in the Ganga-Brahmaputra delta. Free from the influence of topography, they are clearly northeasterly over the Bay of Bengal.
• During the winters, the weather in India is pleasant.
• The pleasant weather conditions, however, at intervals, get disturbed by shallow cyclonic depressions originating over the east Mediterranean Sea and travelling eastwards across West Asia, Iran, Afghanistan and Pakistan before they reach the northwestern parts of India.

Rainfall:

• Winter monsoons do not cause rainfall as they move from land to the sea.
• It is because firstly, they have little humidity;
• Secondly, due to anti cyclonic circulation on land, the possibility of rainfall from them reduces. So, most parts of India do not have rainfall in the winter season.

However, there are some exceptions to it:

In northwestern India, some weak temperate cyclones from the Mediterranean sea cause rainfall in Punjab, Haryana, Delhi and western Uttar Pradesh. Although the amount is meagre, it is highly beneficial for Rabi crops. The precipitation is in the form of snowfall in the lower Himalayas. It is this snow that sustains the flow of water in the Himalayan rivers during the summer months. The precipitation goes on decreasing from west to east in the plains and from north to south in the mountains.

• Central parts of India and northern parts of southern Peninsula also get winter rainfall occasionally.
• Arunachal Pradesh and Assam in the northeastern parts of India also have rains between 25 mm and 50 mm during these winter months.
• During October and November, northeast monsoon while crossing over the Bay of Bengal, picks up moisture and causes torrential rainfall over the Tamil Nadu coast, southern Andhra Pradesh, southeast Karnataka and south east Kerala.

The Hot Weather Season

• With the apparent northward movement of the sun towards the Tropic of Cancer in March, temperatures start rising in north India.
• April, May and June are the months of summer in north India. In most parts of India, temperatures recorded are between 30°-32°C.
• In March, the highest day temperature of about 38°C occurs in the Deccan Plateau while in April, temperature ranging between 38°C and 43°C are found in Gujarat and Madhya Pradesh.
• In May, the heat belt moves further north, and in the north-western part of India, temperatures around 48°C are not uncommon.
• Temperatures remain between 26°C and 32°C. Due to altitude, the temperatures in the hills of Western Ghats remain below 25°C.

Pressure and Winds:

• The summer months are a period of excessive heat and falling air pressure in the northern half of the country.
• Roughly, this elongated low pressure monsoon trough extends over the Thar desert in the northwest to Patna and Chotanagpur plateau in the east-southeast.
•  The location of the ITCZ attracts a surface circulation of the winds which are southwesterly on the west coast as well as along the coast of West Bengal and Bangladesh.
• They are easterly or southeasterly over north Bengal and Bihar.
• In the heart of the ITCZ in the northwest, the dry and hot winds known as ‘Loo’, blow in the afternoon, and very often, they continue to well into midnight.
• Dust storms in the evening are very common during May in Punjab, Haryana, Eastern Rajasthan and Uttar Pradesh.
• These temporary storms bring a welcome respite from the oppressing heat since they bring with them light rains and a pleasant cool breeze.
• Occasionally, the moisture-laden winds are attracted towards the periphery of the trough.
• A sudden contact between dry and moist air masses gives rise to local storms of great intensity.
• These local storms are associated with violent winds, torrential rains and even hailstorms.

Some Famous Local Storms of Hot Weather Season

• Mango Shower: Towards the end of summer, there are pre-monsoon showers which are a common phenomena in Kerala and coastal areas of Karnataka. Locally, they are known as mango showers since they help in the early ripening of mangoes.
• Blossom Shower: With this shower, coffee flowers blossom in Kerala and nearby areas.
• Norwesters: These are dreaded evening thunderstorms in Bengal and Assam. Their notorious nature can be understood from the local nomenclature of ‘Kalbaisakhi’, a calamity of the month of Baisakh.
• These showers are useful for tea, jute and rice cultivation. In Assam, these storms are known as “Bardoli Chheerha”.
• Loo: Hot, dry and oppressing winds blowing in the Northern plains from Punjab to Bihar with higher intensity between Delhi and Patna.

The Southwest Monsoon Season

• Due to high temperature there is lo to attract the trade winds of Southern Hemisphere coming from the Indian Ocean.
• These southeast trade winds cross the equator and enter the Bay of Bengal and the Arabian Sea,.
• After crossing the equator, they follow a southwesterly direction. That is why they are known as southwest monsoons.
• The rain in the southwest monsoon season begins rather abruptly. One result of the first rain is that it brings down the temperature substantially.
• This sudden onset of the moisture-laden winds associated with violent thunder and lightning, is often termed as the “break” or “burst” of the monsoons.

The monsoon approaches the landmass in two branches:

• The Arabian Sea branch
• The Bay of Bengal branch

Monsoon Winds of the Arabian Sea

The monsoon winds originating over the Arabian Sea further split into three branches:

• Its one branch is obstructed by the Western Ghats. These winds climb the slopes of the Western Ghats from 900-1200 m. Soon, they become cool, and as a result, the windward side of the Sahyadris and Western Coastal Plain receive very heavy rainfall ranging between 250 cm and 400 cm. After crossing the Western Ghats, these winds descend and get heated up.
• Another branch of the Arabian sea monsoon strikes the coast north of Mumbai. Moving along the Narmada and Tapi river valleys, these winds cause rainfall in extensive areas of central India. The Chotanagpur plateau gets 15 cm rainfall from this part of the branch. Thereafter, they enter the Ganga plains and mingle with the Bay of Bengal branch.
• A third branch of this monsoon wind strikes the Saurashtra Peninsula and the Kachchh. It then passes over west Rajasthan and along the Aravalis, causing only a scanty rainfall. In Punjab and Haryana, it too joins the Bay of Bengal branch. These two branches, reinforced by each other, cause rains in the western Himalayas.

Monsoon Winds of the Bay of Bengal

• The Bay of Bengal branch strikes the coast of Myanmar and part of south east Bangladesh. But the Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent.
• The monsoon, therefore, enters West Bengal and Bangladesh from south and southeast instead of from the south-westerly direction. From here, this branch splits into two under the influence of the Himalayas and the thermal low is northwest India.
• Its one branch moves westward along the Ganga plains reaching as far as the Punjab plains. The other branch moves up the Brahmaputra valley in the north and the northeast, causing widespread rains. Its sub-branch strikes the Garo and Khasi hills of Meghalaya. Mawsynram, located on the crest of Khasi hills, receives the highest average annual rainfall in the world.

Here it is important to know why the Tamil Nadu coast remains dry during this season.

There are two factors responsible for it:

• The Tamil Nadu coast is situated parallel to the Bay of Bengal branch of southwest monsoon.
• It lies in the rain shadow area of the Arabian Sea branch of the south-west monsoon.

Characteristics of Monsoonal Rainfall

• Rainfall received from the southwest monsoons is seasonal in character, which occurs between June and September.
• Monsoonal rainfall is largely governed by relief or topography. For instance the windward side of the Western Ghats register a rainfall of over 250 cm. Again, the heavy rainfall in the north -eastern states can be attributed to their hill ranges and the Eastern Himalayas.
• The monsoon rainfall has a declining trend with increasing distance from the sea. Kolkata receives 119 cm during the southwest monsoon period, Patna 105 cm, Allahabad 76 cm and Delhi 56 cm.
• The monsoon rains occur in wet spells of few days duration at a time. The wet spells are interspersed with rainless, interval known as ‘breaks’. These breaks in rainfall are related to the cyclonic depressions mainly formed at the head of the Bay of Bengal, and their crossing into the mainland. Besides the frequency and intensity of these depressions, the passage followed by them determines the spatial distribution of rainfall.
• The summer rainfall comes in a heavy downpour leading to considerable run off and soil erosion.
• Monsoons play a pivotal role in the agrarian economy of India because over three-fourths of the total rain in the country is received during the southwest monsoon season.
• Its spatial distribution is also uneven which ranges from 12 cm to more than 250 cm.
• The beginning of the rains sometimes is considerably delayed over the whole or a part of the country.
• The rains sometimes end considerably earlier than usual, causing great damage to standing crops and making the sowing of winter crops difficult.

Season of Retreating Monsoon

• October and November are known for retreating monsoons.
• By the end of September, the southwest monsoon becomes weak as the low pressure trough of the Ganga plain starts moving southward in response to the southward march of the sun.
• The monsoon retreats from the western Rajasthan by the first week of September.
• It withdraws from Rajasthan, Gujarat, Western Ganga plain and the Central Highlands by the end of the month.
• By the beginning of October, the low pressure covers northern parts of the Bay of Bengal and by early November, it moves over Karnataka and Tamil Nadu.
• By the middle of December, the centre of low pressure is completely removed from the Peninsula.
• The retreating southwest monsoon season is marked by clear skies and rise in temperature.
• The land is still moist. Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive. This is commonly known as the ‘October heat’. Here, October and November are the rainiest months of the year.
• The widespread rain in this season is associated with the passage of cyclonic depressions which originate over the Andaman Sea and manage to cross the eastern coast of the southern Peninsula.

Traditional Indian Seasons

Distribution of Rainfall

The average annual rainfall in India is about 125 cm, but it has great spatial variations.

• Areas of High Rainfall : The highest rainfall occurs along the west coast, on the Western Ghats, as well as in the sub-Himalayan areas is the northeast and the hills of Meghalaya. Here the rainfall exceeds 200 cm. In some parts of Khasi and Jaintia hills, the rainfall exceeds 1,000 cm. In the Brahmaputra valley and the adjoining hills, the rainfall is less then 200 cm.
• Areas of Medium Rainfall : Rainfall between 100-200 cm is received in the southern parts of Gujarat, east Tamil Nadu, northeastern Peninsula covering Odisha, Jharkhand, Bihar, eastern Madhya Pradesh, northern Ganga plain along the sub-Himalayas and the Cachar Valley and Manipur.
• Areas of Low Rainfall : Western Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir, eastern Rajasthan, Gujarat and Deccan Plateau receive rainfall between 50-100 cm.
•  Areas of Inadequate Rainfall: Parts of the Peninsula, especially in Andhra Pradesh, Karnataka and Maharashtra, Ladakh and most of western Rajasthan receive rainfall below 50 cm.Snowfall is restricted to the Himalayan region.

Climatic Regions of India

Major climatic types of India based on Koeppen‘s scheme: Koeppen based his scheme of Climatic classification on monthly values of temperature and precipitation. He identified five major climatic types, namely:

• Tropical climates, where mean monthly temperature throughout the year is over 18°C.
• Dry climates, where precipitation is very low in comparison to temperature, and hence, dry. If dryness is less, it is semi arid (S); if it is more, the climate is arid(W).
• Warm temperate climates, where mean temperature of the coldest month is between 18°C and minus 3°C.
• Cool temperate climates, where mean temperature of the warmest month is over 10°C, and mean temperature of the coldest month is under minus 3°C.
• Ice climates, where mean temperature of the warmest month is under 10°C.