Different Types of Seasons
The meteorologists recognize the following four seasons:
(i) The cold weather season - December to February;
(ii) The hot weather season - March to may in south and up to June in the north;
(iii) The advancing southwest monsoon season - June to September;
(iv) The retreating southwest monsoons season - October and November
(i) The Cold Weather Season
The important characteristics of cold weather season are:
- Low temperatures in the north and their gradual increase towards southern parts of India.
- Blowing of cold and dry northeast monsoon resulting in dry weather conditions in most parts. Coromandel coast receives rainfall during winter.
- Western disturbances cause light rain in northern plains and snowfall over the Himalayan ranges
Temperature-
- The cold weather season in northern India typically begins around mid-November.
- December and January are the coldest months in the northern plain.
- This is due to snowfall in the nearby Himalayan ranges, as well as cold winds from the Caspian Sea and Turkmenistan.
- The Peninsular region of India doesn't have a well-defined cold weather season.
- Coastal areas in this region don't experience much seasonal change in temperature distribution.
- This is because of the moderating influence of the sea and the proximity to the equator.
- In summary, while northern India experiences a distinct cold weather season, the Peninsular region does not.
Pressure and Winds-
- The current season's weather is identified by weak high-pressure conditions in the northern plain.
- In southern India, the air pressure is marginally lower.
- As a result, winds start to blow from the high-pressure zone in the northwest to the low-pressure zone over the Indian Ocean in the south.
- The wind direction is affected by the topography of the region. In the Ganga Valley, they are westerly or north-westerly. They become northerly in the Ganga-Brahmaputra delta.
- Over the Bay of Bengal, free from the topography's influence, they are clearly north-easterly.
Rainfall:
- Rainfall during winter monsoons doesn't occur as they move from land to sea.
- Weak temperate cyclones from the Mediterranean Sea cause rainfall in Punjab, Haryana, Delhi and western Uttar Pradesh in northwestern India.
- Precipitation decreases from west to east in the plains and from north to south in the mountains.
- Arunachal Pradesh and Assam experience rainfall between 25 mm and 50 mm during winter months.
- Northeast monsoon causes torrential rainfall over the Tamil Nadu coast, southern Andhra Pradesh, southeast Karnataka, and southeast Kerala when it crosses the Bay of Bengal in October and November.
(ii) The Hot Weather Season
- The hot weather season is characterized by hot and dry weather.
- In the northern plains, there is a hot dry wind known as Loo.
- Afternoon dust storms are common during this season.
- There are occasional drizzles and mild showers in Kerala, known as Mango showers.
- In West Bengal and Assam, there are Northwesters or Kal Baisakhi, which also bring mild showers.
Temperature:
- Temperatures in most parts of India range from 30°-32°C.
- The hot weather in south India is less intense than in the north.
- The Peninsular location of south India, coupled with the moderating effect of the oceans, keeps temperatures lower than in the north.
- Along the coast, the north-south extent of isotherms confirms that temperature increases from the coast to the interior, rather than decreasing from north to south.
Pressure and Winds:
- During summer, the northern half of the country experiences excessive heat and low air pressure.
- Due to the subcontinent's heating, the ITCZ (Intertropical Convergence Zone) moves northwards and centers around 25°N in July.
- The ITCZ location causes a surface circulation of winds, with south-westerly winds along the west coast and West Bengal/Bangladesh coast, and easterly or south-easterly winds over north Bengal and Bihar.
- The ITCZ's center in the northwest region experiences hot and dry winds called 'Loo' that blow in the afternoon and often continue till midnight.
Local Storms-
- 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:
- This refers to the pre-monsoon showers that are common in Kerala and the coastal areas of Karnataka. They are known as mango showers.
Blossom Shower:
- In Kerala and nearby areas, coffee flowers blossom with this shower.
Nor Westers:
- These are evening thunderstorms that occur in Bengal and Assam.
- Local nomenclature - ‘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”.
(iii) The Advancing Southwest Monsoon Season
- The Southwest Monsoon season is a rainy season in India.
- The monsoons usually arrive in the first week of June and hit the coast of Kerala.
- By mid-July, the monsoons cover most parts of India.
- This weather pattern lasts until September.
Entry of south west winds
- During may-June 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 south-westerly direction. That is why they are known as southwest monsoons
Burst of monsoon
- The start of the monsoon season is marked by a sudden onset of moisture-laden winds, accompanied by thunder and lightning.
- This event is commonly referred to as the "break" or "burst" of the monsoons.
- The coastal regions of Kerala, Karnataka, Goa, and Maharashtra may experience this burst in the first week of June.
- The monsoon season brings heavy rainfall to these areas and is crucial for agricultural activities.
The monsoon approaches the landmass in two branches:
A. The Arabian Sea branch
B. The Bay of Bengal branch
A. The Arabian Sea branch
- Monsoon winds from the Arabian Sea split into three branches.
(1)1st Branch-
- The first branch is blocked by the Western Ghats, causing heavy rainfall in the Sahyadris and Western Coastal Plain.
- After crossing the Western Ghats, these winds descend and heat up, causing little rainfall in the rain-shadow area east of the Western Ghats, including parts of Maharashtra, Karnataka, and Telangana.
(2) 2nd branch -
- The second branch strikes the coast north of Mumbai and moves along the Narmada and Tapi river valleys, causing rainfall in central India and 15 cm of rainfall in the Chotanagpur plateau.
- This branch then enters the Ganga plains and merges with the Bay of Bengal branch.
(3)3rd Branch
- The third branch then joins the Bay of Bengal branch in Punjab and Haryana.
- The two branches, reinforced by each other, cause rainfall in the western Himalayas.
B. Monsoon Winds of the Bay of Bengal –
- The Bay of Bengal branch splits into two sub-branches when it encounters the eastern Himalayas.
- One sub-branch moves towards the east-northeast direction, causing heavy rains in the Brahmaputra valley and northeast hills of India.
- The other sub-branch moves towards the northwest along the Ganga valley and the Himalayan ranges, resulting in heavy and widespread rainfall over vast areas.
- The amount of rainfall in this region decreases from east to west due to the progressive decrease in humidity of these winds.
- During this season, the Tamil Nadu coast remains dry.
- The Tamil Nadu coast runs parallel to the Bay of Bengal branch of the southwest monsoon and lies in the rain shadow area of the Arabian Sea branch of the southwest monsoon.
IN NUTSHELL
- Low pressure conditions occur over northwestern parts of India while high pressure conditions occur over seas during the advancing Southwest monsoon season.
- The general wind direction in the Arabian Sea and the Bay of Bengal is southwest to northeast, leading to widespread rain with dry spells.
- The monsoons start in the first week of June and end by the last week of September.
- The weather is generally hot and humid during the Southwest monsoon season.
(iv) The retreating southwest monsoons season - October and November
- 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.
October heat -
- The retreat of the southwest monsoon season is characterized by clear skies and higher temperatures.
- The land remains moist, leading to a hot and humid weather condition commonly known as the 'October heat'.
- While north India experiences dry weather during the retreating monsoon, the eastern part of the Peninsula receives rain in October and November, which are the rainiest months of the year.
- The rain in this season is often caused by cyclonic depressions originating over the Andaman Sea and crossing the eastern coast of the southern Peninsula.
- Cyclonic storms are less common in the Arabian Sea.
- The densely populated deltas of Godavari, Krishna, and Kaveri are the preferred targets of these cyclonic storms.
- The Coromandel coast receives a significant portion of its rainfall from these depressions and cyclones.