Heat wave

Also called: Heatwave, Heatwave criteria of IMD · Topic: Natural Hazards and Disaster Management in India · NCERT: Class 7, Ch 2 "Understanding the Weather"; Class 7, Ch 3 "Climates of India"; Class 11, Ch 6 "Natural Hazards and Disasters: Causes, Consequences and Management"

Meaning

A heat wave is a spell of air temperature much hotter than normal for a place. In India, the IMD (India Meteorological Department) judges it only by the maximum (day) temperature [5]. Heat waves make people ill and can kill them, especially outdoor workers, the old and slum dwellers. Heat is also not a notified disaster on the national list used for relief funds, so it is often called a "silent disaster".

Explanation

How a heat wave forms in northwest India

• Cause: strong sun on dry land
• In May-June the sun is almost overhead above north India.

• The dry, bare land heats up quickly.

• Process: no clouds and hot winds

• Skies are clear and the air holds little moisture, so no clouds block the sun.

• Hot winds from the Thar Desert carry the heat east over the plains.

• Result: the loo

• Loo means the hot, dry and dusty west winds that blow in the afternoon over Punjab, Haryana, Rajasthan and Madhya Pradesh in April-June.
• NCERT (Class 11, Table 6.1) groups the loo with cold waves, thunderstorms, dust storms and forest fires as an atmospheric hazard (a danger that comes from the air and the weather).

IMD criteria for declaring a heat wave [5]

• Step 1: a starting condition must be met
• Maximum temperature ≥ 40°C in the plains
• ≥ 37°C at coastal stations

• ≥ 30°C in the hills

• Step 2: then either test applies

Basis	Heat wave	Severe heat wave
Departure from normal	4.5°C to 6.4°C above normal	More than 6.4°C above normal (6.5°C or more)
Actual maximum (plains)	≥ 45°C	≥ 47°C

• Key point: the night (minimum) temperature plays no part in the official definition [5].

Urban heat island: why cities suffer more

• Urban heat island (UHI) means a city that is warmer than the countryside around it.
• Cause: concrete, asphalt and roofs soak up heat during the day. Vehicles, ACs and factories add more heat, and there are few trees.
• Process: the stored heat escapes slowly at night, so the city does not cool down.
• Result: hot nights add to the heat of the day. City dwellers, outdoor workers and slum dwellers suffer most.

Heat Action Plans (HAPs): the main response

• Heat Action Plan means an early warning and preparedness plan for extreme heat. It lists short-term and long-term actions to prepare, share information and coordinate the response, so that fewer vulnerable people fall ill or die [2].
• A typical HAP has four parts:
• Early warning: colour-coded IMD alerts.
• Public awareness: drink water, stay out of the noon sun.
• Health-system readiness: cool hospital wards, stocks of ORS.

• Long-term steps: cool roofs, shade and green cover, changed work hours.

• NDMA heat-wave guidelines (2016, revised 2019) guide the plans that states and cities make.

Where it occurs

• Core region in India: Punjab, Haryana, Rajasthan and Madhya Pradesh, which form the belt of the loo.
• Wider spread: heat waves also hit central and east India.
• Season: April-June, in the hot, dry pre-monsoon months.
• Source of the heat: the Thar Desert (Rajasthan), whose hot winds spread east over the northern plains.
• Cities: urban heat islands make heat waves worse in big cities.
• Ahmedabad (Gujarat): after a deadly heat wave in 2010, the city made South Asia's first Heat Action Plan in 2013.
• National coverage: NDMA and the state governments jointly carry out HAPs in 23 heat-prone states [2][3]. NDMA and IMD work with these states to build the plans [3].
• Yearly review: before each summer, NDMA reviews how ready the states are for heat wave and lightning together [4].

Don't confuse with

• Cold wave: judged by the minimum (night) temperature (≤ 10°C in the plains, ≤ 0°C in the hills), and it strikes in December-February. A heat wave is judged only by the maximum temperature [5][6].
• Loo: the loo is the wind (hot, dry west winds in April-June). A heat wave is the spell of high temperature. The loo is one cause of heat waves in northwest India. It is not a monsoon or winter wind.
• Urban heat island: a lasting difference between a city and the countryside around it, felt most at night. A heat wave is a temporary spell over a whole region, measured against the normal temperature for that place.
• Dust storm: a strong wind that lifts loose, dry soil. It hits the same belt (Rajasthan, Punjab, Haryana, western Uttar Pradesh) in the same months (April-June), but it is defined by wind and dust, not by temperature.

Prelims Hooks

• IMD starting thresholds: plains ≥ 40°C, coastal stations ≥ 37°C, hills ≥ 30°C [5].
• Actual maximum in the plains: ≥ 45°C is a heat wave and ≥ 47°C is a severe heat wave [5].
• Departure from normal: 4.5°C to 6.4°C above normal is a heat wave. More than 6.4°C (6.5°C or more) is a severe heat wave [5].
• Trap: a heat wave is defined only by the maximum temperature. A cold wave is defined by the minimum temperature [5][6].
• First Heat Action Plan in South Asia: Ahmedabad, 2013, made after the 2010 heat wave.
• Legal status: heat is not a notified disaster on the national list used for relief funds. NDMA heat-wave guidelines came in 2016 and were revised in 2019.

Mains Points

• Heat is a growing "silent disaster" (GS-III, disaster management):
• Urban heat islands trap heat at night, and outdoor informal workers have no shelter from it.

• Because heat is not a notified disaster, it gets no relief funds, and this holds back the response.

• Heat Action Plans are the proven policy answer:

• Ahmedabad's plan (2013) showed that early warning, cooler shelters and changed work hours save lives.

• Extending the plans through NDMA's 23-state HAP network, with IMD colour-coded alerts, is the main way forward [2][3].

• Long-term fixes for cities and regions:

• Cool roofs, more shade and green cover, and readier hospitals (cool wards, ORS stocks) cut the harm from heat.
• NDMA's yearly review of heat-wave and lightning preparedness ahead of summer supports the Sendai Framework priority on early warning [4].

Related concepts

• Heat Action Plan
• Cold wave
• Thunderstorm and lightning
• Forest fire

Read more

• Weather extremes and fire: heat and cold waves, lightning, dust storms and forest fires