The hot weather season and its local storms

Detail

1. What the hot weather season is

• Hot weather season (summer or pre-monsoon season): the time between the cold winter and the coming of the south-west monsoon. It runs from March to May, and in the north it runs into early June.
• Hottest month (NCERT Class 9):
• April in the west and south of India.
• May in the north of India.

• The sun's rays fall straight down on the south first, and on the north later.

• The IMD (India Meteorological Department) uses March–May as its pre-monsoon season for thunderstorms [5]. Heat waves usually come between March and June, and in rare years they last into July [4].

2. Temperature: the heat belt moves north

Why the heat moves north:

• After the spring equinox (21 March), the sun seems to move north towards the Tropic of Cancer (23.5°N). This is its apparent northward march. The Earth's tilt causes it; the sun itself does not move.
• The noon sun stands higher in the sky further north, so the land there takes in more heat.
• So the heat belt (the zone of highest temperature) moves from south to north between March and May.

Month	Hottest area	Maximum temperature (NCERT)
March	Deccan Plateau	about 38°C
April	Gujarat and Madhya Pradesh	38–43°C
May	North-west India (Rajasthan, Punjab, Haryana)	about 48°C

The south stays milder:

• South India: 26–32°C. Three things keep it cooler:
• the sea around it on three sides (a maritime influence: the sea warms and cools slowly, so it keeps the land's temperature steady);
• its nearness to the equator, so summer days and winter days are not very different;

• sea breezes (cool winds that blow from the sea to the land in the afternoon).

• Western Ghats hills: below 25°C, because the air gets cooler with height.

Coasts: heat rises inland, not northward:

• Isotherms (lines on a map that join places with the same temperature) run parallel to the coast.
• So on the coasts, temperature rises from the coast towards the interior. It does not rise from north to south.
• On the coasts, the minimum temperature (the lowest temperature of the day, usually at night) rarely falls below 26°C. The nights stay warm and humid.

3. Heat waves: the official yardstick

• Heat wave (NDMA): a period of unusually high temperature, above the normal maximum. It is seen mainly in north-western India in summer [4].
• When a heat wave can be declared (IMD): a station's maximum temperature must first reach at least 40°C in the plains, 37°C on the coast or 30°C in the hills [2].

Basis	Heat wave	Severe heat wave
Departure from normal (how far above the usual maximum)	+4.5°C to +6.4°C	more than +6.4°C
Actual maximum temperature (plains only)	45°C or more	47°C or more

• The criteria must be met at at least 2 stations in one meteorological sub-division (an IMD weather area, roughly a state or part of a state) [2].
• Warm night: the minimum temperature is 4.5°C or more above normal [2]. Warm nights are dangerous because the body gets no rest from the heat.
• Weather conditions behind a heat wave [3]:
• hot, dry air comes in from the north-west;
• there is little moisture in the air;
• the sky is clear, so the full sunlight reaches the ground;

• an anticyclone (a high-pressure system in which air sinks and warms) sits over the area and stops clouds from forming.

• Trend: heat waves are becoming more frequent and more intense each year [4]. Check current IMD data for the latest counts of heat-wave days. Heat-wave policy and Heat Action Plans are covered in natural-hazards-disasters.

• Wet-bulb risk: wet-bulb temperature is the lowest temperature that air can be cooled to by evaporating water into it. When it is high, sweat cannot evaporate and the body cannot cool itself. This is a growing danger on India's humid coasts. (Beyond NCERT.)

4. Pressure and winds

Low pressure builds over the north:

• The land heats up strongly → the air over it expands and rises → pressure falls over north and north-west India.
• A long thermal low (an area of low pressure caused by heating) forms. It stretches from the Thar Desert in the north-west to Chota Nagpur Plateau (Jharkhand) in the east and south-east.
• The ITCZ moves north with it. ITCZ (Inter Tropical Convergence Zone): the low-pressure belt near the equator where the trade winds of the two hemispheres meet. By July its centre lies at about 25°N, over the Ganga plain. Here it is also called the monsoon trough.

Winds circulate around this trough:

• South-westerly winds blow along the west coast and the West Bengal–Bangladesh coast.
• Easterly or south-easterly winds blow over north Bengal and Bihar.
• These are "displaced" equatorial easterlies:
• the south-east trade winds of the southern hemisphere cross the equator;
• the Coriolis force (a turning force from the Earth's spin that bends winds to the right in the northern hemisphere) turns them;

• they become south-westerly winds blowing into the low.

• Their strong inflow by mid-June brings the south-west monsoon rains. The hot season "pulls in" the monsoon.

Evening dust storms:

• In May, evening dust storms blow over Punjab, Haryana, eastern Rajasthan and Uttar Pradesh.
• They are often followed by a light shower or a cool breeze, which brings brief relief from the heat.

Violent local storms where two air masses meet:

• At the edges of the trough, moist air from the sea meets hot, dry air from the land.
• The hot ground makes the air rise fast. This is convection: rising warm air cools, its water vapour condenses, and tall thunderclouds form.
• Result: short but violent local storms with strong winds, torrential rain and hail.

5. Famous local storms of the hot weather season

Local storm	Region	Time	Nature	Effect
Loo	Northern plains, Punjab to Bihar; strongest between Delhi and Patna	May–June, afternoons	Hot, dry, dusty wind	Heatstroke, even death
Mango showers	Kerala and coastal Karnataka	End of summer (April–May)	Pre-monsoon thundershowers	Help mangoes ripen early
Blossom (cherry/coffee) showers	Kerala and nearby Karnataka	Pre-monsoon	Local showers	Coffee plants flower
Nor'westers	West Bengal, Assam, and the east and north-east	Evenings, March–May	Violent thunderstorms with squalls	Help tea, jute and rice

Loo

• Loo: a strong, hot and dry wind that blows from the west and north-west over the northern plains in summer.
• Where: from Punjab to Bihar. It is strongest between Delhi and Patna.
• When: in May and June, in the afternoon. It often goes on until midnight.
• Why it forms:
• very hot air builds up over the Thar Desert and north-west India;
• the wind blows east down the plains, over dry land, so it picks up no moisture;

• air temperatures reach 45°C or more. These match the IMD's heat-wave levels [2].

• Result: heatstroke (the body overheats and can no longer cool itself) and dehydration (the body loses too much water). It is the most direct link between the hot season and heat-wave deaths.

Mango showers

• Mango showers: pre-monsoon showers near the end of summer.
• Where: Kerala and coastal Karnataka.
• Why they form: moist sea air plus strong heating during the day makes thunderclouds.
• Effect: they help mangoes ripen early, which is where the name comes from.

Blossom showers

• Blossom showers, also called cherry blossom or coffee showers, fall in Kerala and nearby areas of Karnataka.
• Effect: they make coffee plants flower. Coffee is grown in the Western Ghats in Kodagu, Chikkamagaluru and Hassan (Karnataka) and in Wayanad (Kerala). A good blossom shower followed by dry days helps decide the size of the coffee crop.

Nor'westers (Kalbaisakhi and Bardoisila)

• Nor'westers: dreaded evening thunderstorms of the pre-monsoon season in Bengal and Assam.
• Local names:
• Kalbaisakhi in Bengal, meaning "calamity of the month of Baisakh" (Baisakh is the Bengali month from mid-April to mid-May);

• Bardoisila in Assam.

• Season and path: they are severe thunderstorms that occur from March to May. They mostly move from north-west to south-east over east and north-east India, which is where the name comes from. Some move from south-west to north-east over Nepal, Bhutan and Bangladesh [5].

• How they form:
• moist air comes in from the Bay of Bengal;
• the land below is very hot, so the air is unstable (it rises easily);
• the moist air rises fast and builds tall cumulonimbus clouds (dark, towering thunderclouds);

• in the evening the storm breaks.

• Where they start: in Jharkhand, Odisha and sub-Himalayan West Bengal. They grow as they move over Bangladesh and north-east India [6].

• Mesoscale Convective Complexes (MCCs): sometimes many storm cells join into one large, long-lasting storm system. These are called MCCs [6].
• Hazards: heavy showers, lightning, hail, squalls (sudden strong winds), downbursts (strong downward blasts of air) and sometimes tornadoes [5].
• Benefits: the rain helps tea (Assam), jute and rice (Bengal).
• Monitoring: the IMD watches these storms every 30–60 minutes using INSAT satellite images and overlapping radar coverage [6].

6. North versus east and south: one season, three faces

Region	Main weather in the hot season	Why
North-west and north (Rajasthan, Punjab, Haryana, Delhi, UP, Bihar)	Loo, dust storms, heat waves up to about 48°C	Far from the sea; dry air from the north-west; clear skies
East and north-east (West Bengal, Odisha, Jharkhand, Assam)	Nor'westers (Kalbaisakhi, Bardoisila) with hail	Moist Bay of Bengal air meets hot, dry land air
South-west coast (Kerala, coastal Karnataka)	Mango showers and blossom showers; 26–32°C	Sea nearby, moist air, closeness to the equator

Prelims Hooks

• Hottest month: April in the west and south; May in the north (NCERT Class 9).
• Order of the heat belt's peak: Deccan (March, about 38°C) → Gujarat and MP (April, 38–43°C) → north-west India (May, about 48°C).
• Loo is strongest between Delhi and Patna. It is hot and dry, not a rain-bearing wind.
• Kalbaisakhi = West Bengal; Bardoisila = Assam. Both are names for nor'westers, which help tea, jute and rice.
• Mango showers = Kerala and coastal Karnataka (mangoes ripen). Blossom/cherry showers = Kerala and nearby Karnataka (coffee flowers). A common trap is to swap the crop.
• On the coasts, isotherms run parallel to the coastline, so temperature rises from coast to interior, not from north to south.
• IMD heat-wave threshold: at least 40°C plains / 37°C coast / 30°C hills. A severe heat wave in the plains means an actual maximum of 47°C or more, or more than 6.4°C above normal [2].
• Nor'westers mostly move from north-west to south-east. They can bring hail, squalls, downbursts and tornadoes [5].
• Dust storms in May hit Punjab, Haryana, eastern Rajasthan and UP, not the Peninsula.

Mains Points

• How the hot season sets up the monsoon (GS-I):
• strong heating of north-west India and the Thar → a deep thermal low and a northward-moving ITCZ (about 25°N by July);
• this low draws in the displaced equatorial easterlies as south-west winds;

• their strong inflow by mid-June starts the monsoon rains. A hotter, longer pre-monsoon can therefore shape when the monsoon arrives and how strong it is.

• Local storms as both risk and resource (GS-I/III):

• nor'westers water the tea, jute and rice crops of the east;
• but their hail, lightning, squalls and tornadoes damage crops, homes and power lines [5];

• INSAT and radar nowcasting (short-range forecasts for the next few hours) reduce the loss [6].

• Heat-wave hazard (GS-III, disasters):

• the Loo and heat waves (March–June, sometimes into July) cause heatstroke deaths [4];
• the risk is highest for outdoor workers and the urban poor, and warm nights (minimum 4.5°C or more above normal) make it worse [2];

• heat waves are becoming more frequent and more intense [4]. This needs Heat Action Plans, early IMD warnings and changes to work hours.

• Regional contrast in one season (GS-I): the dry, scorching north-west (Loo, dust storms), the stormy east (Kalbaisakhi, Bardoisila) and the mild, showery south-west coast (mango and blossom showers) show how distance from the sea, latitude and where moist and dry air meet control India's climate.