The Rhythm of Seasons

The climate of India is best understood as an annual cycle of seasons, and meteorologists divide this cycle into four distinct seasons: the cold weather season, the hot weather season, the southwest monsoon season, and the retreating monsoon season. Each has its own characteristic temperatures, pressure patterns, winds, and rainfall.

The Cold Weather Season

Temperature. The cold weather season sets in by mid-November in northern India, with December and January being the coldest months in the northern plain. The mean daily temperature remains below 21°C over most of northern India, and night temperatures can be quite low, sometimes falling below freezing point in Punjab and Rajasthan.

Three main reasons explain the excessive cold in north India during this season. First, states like Punjab, Haryana, and Rajasthan lie far from the moderating influence of the sea and therefore experience a continental climate. Second, snowfall in the nearby Himalayan ranges creates cold wave conditions. Third, around February, cold winds coming from the Caspian Sea and Turkmenistan bring cold waves along with frost and fog over the northwestern parts of India.

The Peninsular region, however, does not have a well-defined cold weather season. There is hardly any seasonal change in temperature distribution in coastal areas because of the moderating influence of the sea and the proximity to the equator. For example, the mean maximum temperature for January at Thiruvananthapuram is as high as 21°C, and for June it is 29.5°C. Temperatures at the hills of the Western Ghats remain comparatively low.

Pressure and Winds. By the end of December, around 22nd December, the sun shines vertically over the Tropic of Capricorn in the southern hemisphere. The weather in this season is characterised by feeble high pressure conditions over the northern plain, while in south India the air pressure is slightly lower. The isobars of 1019 mb and 1013 mb pass through northwest India and the far south respectively. As a result, winds blow from the northwestern high pressure zone towards the low air pressure zone over the Indian Ocean in the south.

Due to the low pressure gradient, 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. These winds are westerly or northwesterly down the Ganga Valley, become northerly in the Ganga-Brahmaputra delta, and are clearly northeasterly over the Bay of Bengal where they are free from topographic influence.

During winter, the weather in India is generally pleasant. However, these pleasant conditions get disturbed at intervals by shallow cyclonic depressions originating over the east Mediterranean Sea and travelling eastwards across West Asia, Iran, Afghanistan, and Pakistan before reaching the northwestern parts of India. On their way, their moisture content gets augmented from the Caspian Sea in the north and the Persian Gulf in the south. The westerly jet streams play a role in steering these depressions into India.

Rainfall. Winter monsoons do not cause rainfall because they move from land to sea. They have little humidity, and due to anticyclonic circulation on land, the possibility of rainfall from them is reduced. So most parts of India do not receive rainfall in the winter season. There are, however, some exceptions:

• 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 takes the form of snowfall in the lower Himalayas, and this snow sustains the flow of water in the Himalayan rivers during the summer months. The precipitation decreases from west to east in the plains and from north to south in the mountains. The average winter rainfall in Delhi is around 53 mm, while in Punjab and Bihar it remains between 25 mm and 18 mm respectively.
• Central parts of India and the northern parts of the southern Peninsula also get winter rainfall occasionally.
• Arunachal Pradesh and Assam in the northeast also have rains between 25 mm and 50 mm during these winter months.
• During October and November, the 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 southeast Kerala.

The Hot Weather Season

Temperature. 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 summer months in north India, where temperatures are recorded between 30°C and 32°C in most places. In March, the highest day temperature of about 38°C occurs in the Deccan Plateau, while in April, temperatures 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 northwestern part of India, temperatures around 48°C are not uncommon.

The hot weather season in south India is mild and not as intense as in north India. The Peninsular situation of south India, with the moderating effect of the oceans, keeps temperatures lower, so they remain between 26°C and 32°C. Due to altitude, temperatures in the hills of the Western Ghats remain below 25°C. In the coastal regions, the north-south extent of isotherms parallel to the coast confirms that temperature does not decrease from north to south; rather, it increases from the coast to the interior. The mean daily minimum temperature during the summer months also remains quite high and rarely goes below 26°C.

Pressure and Winds. The summer months are a period of excessive heat and falling air pressure in the northern half of the country. Because of the heating of the subcontinent, the ITCZ moves northwards, occupying a position centred at 25°N in July. Roughly, this elongated low pressure monsoon trough extends over the Thar desert in the northwest to Patna and the Chotanagpur plateau in the east-southeast. The location of the ITCZ attracts a surface circulation of winds which are southwesterly on the west coast as well as along the coast of West Bengal and Bangladesh, and easterly or southeasterly over north Bengal and Bihar. These currents of southwesterly monsoon are in reality 'displaced' equatorial easterlies. The influx of these winds by mid-June brings about a change in the weather towards the rainy season.

In the heart of the ITCZ in the northwest, dry and hot winds known as 'Loo' blow in the afternoon and very often continue 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 light rains and a pleasant cool breeze. Occasionally, moisture-laden winds are attracted towards the periphery of the trough, and a sudden contact between dry and moist air masses gives rise to local storms of great intensity, associated with violent winds, torrential rains, and even hailstorms.

Some Famous Local Storms of the Hot Weather Season

• Mango Shower: Towards the end of summer, pre-monsoon showers are common in Kerala and the coastal areas of Karnataka. 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.
• Nor Westers: These are dreaded evening thunderstorms in Bengal and Assam. Their notorious nature is understood from the local name '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 'Bardoisila'.
• 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

As a result of the rapid increase in temperature in May over the northwestern plains, the low pressure conditions there get further intensified. By early June, they are powerful enough to attract the trade winds of the Southern Hemisphere coming from the Indian Ocean. These southeast trade winds cross the equator and enter the Bay of Bengal and the Arabian Sea, only to be caught up in the air circulation over India. Passing over the equatorial warm currents, they bring with them moisture in abundance. After crossing the equator, they follow a southwesterly direction, which 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 moisture-laden winds, associated with violent thunder and lightning, is often termed the 'break' or 'burst' of the monsoons. The monsoon may burst in the first week of June in the coastal areas of Kerala, Karnataka, Goa, and Maharashtra, while in the interior parts of the country it may be delayed to the first week of July. The day temperature registers a decline of 5°C to 8°C between mid-June and mid-July.

As these winds approach the land, their southwesterly direction is modified by the relief and the thermal low pressure over northwest India. The monsoon approaches the landmass in two branches: the Arabian Sea branch and the Bay of Bengal branch.

Monsoon Winds of the Arabian Sea. The monsoon winds originating over the Arabian Sea further split into three branches:

• One branch is obstructed by the Western Ghats. These winds climb the slopes of the Western Ghats from 900–1200 m, become cool, and as a result, the windward side of the Sahyadris and the 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, which reduces their humidity. As a result, these winds cause little rainfall east of the Western Ghats. This region of low rainfall is known as the rain-shadow area.
• Another branch 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 of rainfall from this part of the branch. Thereafter, they enter the Ganga plains and mingle with the Bay of Bengal branch.
• A third branch strikes the Saurashtra Peninsula and Kachchh. It then passes over west Rajasthan and along the Aravalis, causing only 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 southeast 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 the south and southeast instead of from the southwesterly direction. From here, this branch splits into two under the influence of the Himalayas and the thermal low in northwest India. 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 the Khasi hills, receives the highest average annual rainfall in the world.

The Tamil Nadu coast remains dry during this season for two reasons: it is situated parallel to the Bay of Bengal branch of the southwest monsoon, and it lies in the rain-shadow area of the Arabian Sea branch.

Season of Retreating Monsoon

The months of 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 western Rajasthan by the first week of September, and withdraws from Rajasthan, Gujarat, the western Ganga plain, and the Central Highlands by the end of the month. By the beginning of October, the low pressure covers the 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 a rise in temperature. The land is still moist, and owing to the conditions of high temperature and humidity, the weather becomes rather oppressive. This is commonly known as the 'October heat'. In the second half of October, the mercury begins to fall rapidly, particularly in northern India. …