Loo

Topic: Climate of India and the Monsoon · NCERT: Class 7, Ch 2 "Understanding the Weather"; Class 7, Ch 3 "Climates of India"; Class 11, Ch 4 "Climate"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"; Class 11, Ch 8 "Solar Radiation, Heat Balance and Temperature"

Meaning

Loo is a strong, hot, dry and dusty wind that blows from the west and north-west over the northern plains of India. It blows on May and June afternoons, from Punjab to Bihar, and is strongest between Delhi and Patna.

• It is the clearest sign of north India's hot weather season.
• It is the most direct link between that season and heat-wave deaths from heatstroke and dehydration.

Explanation

How the Loo forms (cause → process → result)

• The sun's apparent northward march heats the north:
• After the spring equinox (21 March), the sun seems to move north towards the Tropic of Cancer (23.5°N). The Earth's tilt causes this. The sun itself does not move.

• The heat belt (the zone of highest temperature) moves from the Deccan (March, about 38°C) to Gujarat and Madhya Pradesh (April, 38–43°C). It then reaches north-west India (May, about 48°C).

• Very hot air builds up in the north-west:

• The Thar Desert and north-west India become extremely hot.

• The air over the land expands and rises. A long thermal low forms, stretching from the Thar to the Chota Nagpur Plateau. A thermal low is an area of low pressure caused by heating.

• The wind blows east down the plains:

• The hot air moves east over dry land, so it picks up no moisture.

• Air temperatures reach 45°C or more. This matches the IMD's level for an actual-temperature heat wave in the plains [2].

• Result: a scorching, dusty wind. It causes heatstroke, where the body overheats and can no longer cool itself. It also causes dehydration, where the body loses too much water.

Its main features

• Nature: hot, dry and dusty. It is not a rain-bearing wind.
• Direction: from the west and north-west.
• Time of day: it starts in the afternoon and often goes on until midnight.
• Season: May and June, the hottest part of the hot weather season in the north. May is the hottest month in north India.
• Type: a local wind, meaning a wind that blows over a limited area for a short season because of local heating.

Conditions it needs

• The same weather that brings 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 full sunlight reaches the ground;

• an anticyclone sits over the area. This is a high-pressure system where air sinks and warms, and it stops clouds from forming.

• Distance from the sea: the northern plains are far from the sea. No sea breeze or moist air cools them, unlike the south (26–32°C).

The Loo and the heat-wave yardstick

• The Loo is a wind. A heat wave is an officially declared condition that the Loo often helps create.
• IMD base level: a heat wave can be declared only when a station's maximum temperature reaches at least 40°C in the plains, 37°C on the coast or 30°C in the hills [2].
• Plains: a heat wave means an actual maximum of 45°C or more. A severe heat wave means 47°C or more [2].
• Departure from normal: a heat wave is +4.5°C to +6.4°C above normal. A severe heat wave is more than +6.4°C above normal [2].
• The criteria must be met at at least 2 stations in one meteorological sub-division [2].
• Warm nights: a warm night means a minimum temperature 4.5°C or more above normal [2]. When the Loo blows until midnight, the body gets no rest from the heat.

Where it occurs

• India, northern plains: from Punjab to Bihar, across Haryana, Delhi and Uttar Pradesh.
• Strongest belt: between Delhi and Patna in the Ganga plain.
• Source region: the Thar Desert (Rajasthan) and north-west India, where hot air builds up before it moves east.
• Wider heat zone: heat waves hit mainly north-western India in summer [4]. They usually come between March and June, and in rare years last into July [4].
• Not found: on the Peninsula or the coasts. There the sea keeps temperatures steadier, and isotherms (lines joining places with the same temperature) run parallel to the coast.

Don't confuse with

• Nor'westers (Kalbaisakhi in Bengal, Bardoisila in Assam): these are moist, violent evening thunderstorms in the east and north-east with hail and squalls. They help tea, jute and rice. The Loo is a dry afternoon wind of the north and brings no rain.
• Evening dust storms: these blow in May evenings over Punjab, Haryana, eastern Rajasthan and UP. A light shower or a cool breeze often follows, bringing brief relief. The Loo brings no relief, only heat.
• Mango showers: these are pre-monsoon thundershowers in Kerala and coastal Karnataka that help mangoes ripen. They are wet and come from moist sea air. The Loo is dry and comes from land air.
• Heat wave: this is an IMD-defined period of unusually high temperature, measured by thresholds [2]. The Loo is a wind that often brings heat-wave conditions.

Prelims Hooks

• Loo region: northern plains, Punjab to Bihar. It is strongest between Delhi and Patna.
• Loo season and timing: May–June afternoons, often lasting until midnight.
• Loo nature: hot, dry and dusty, blowing from the west and north-west. A common trap is to call it rain-bearing or moist.
• Matching local storms: Loo = north; Kalbaisakhi = West Bengal; Bardoisila = Assam; mango showers = Kerala and coastal Karnataka.
• IMD heat wave (plains): actual maximum of 45°C or more. A severe heat wave is 47°C or more [2]. The Loo pushes temperatures to these levels.
• Hottest month: May in the north, the Loo belt. It is April in the west and south (NCERT Class 9).

Mains Points

• Loo as a disaster risk (GS-III):
• The Loo and heat waves (March–June, sometimes into July) cause heatstroke deaths [4].
• Outdoor workers and the urban poor face the highest risk. Warm nights (minimum temperature 4.5°C or more above normal) make the danger worse [2].

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

• The Loo and the monsoon link (GS-I):

• The same strong heating that drives the Loo deepens the thermal low over the Thar and the north-west. This low draws in south-west winds, and their strong inflow by mid-June starts the monsoon rains.

• So the scorching Loo season is part of the process that "pulls in" the monsoon.

• Regional contrast within one season (GS-I):

• In the same months, the dry north-west has the Loo and dust storms, the east has nor'westers, and the south-west coast has mango and blossom showers.
• Distance from the sea, latitude and the meeting of moist and dry air explain these differences.

Related concepts

• Hot weather season
• Mango showers
• Blossom showers
• Nor'westers

Read more

• The hot weather season and its local storms