Heatwave warnings in India: what the IMD colours mean and what to do

Updated September 13, 2026 · Claude AC · 3 sources cited

Contents (8)

What IMD actually means by «heatwave»
The colour codes, translated into household decisions
The night is what does the damage
Who is actually at risk
Recognising heat illness before it becomes an emergency
The household routine for an orange or red day
Heat action plans: what your city may already owe you
Frequently asked questions

Every summer the same sentence runs along the bottom of the news: orange alert for heatwave conditions. Most households read it as «it will be hot», which it already was yesterday, and carry on. The alert is saying something more specific than that, and the specificity is the point — it is a statement about how far the coming days depart from what this particular place normally does, which predicts who ends up in a hospital far better than the raw number of degrees.

This is a cooling site, so the honest framing is this: an air conditioner is one tool in a heatwave, not a plan. The households that come through a severe week badly are usually the ones whose plan quietly assumed uninterrupted electricity.

What IMD actually means by «heatwave»

The India Meteorological Department does not declare a heatwave on an absolute number alone. A day is judged against the station’s own normal for that date, because 42 °C is an ordinary May afternoon in parts of Rajasthan and a public-health event on the Konkan coast. The published criteria run on two tracks.

Element	IMD criterion
Entry threshold	The question does not arise until the maximum reaches about 40 °C in the plains, about 37 °C in coastal areas and about 30 °C in hill stations
Heatwave (departure track)	Maximum temperature 4.5 °C to 6.4 °C above the station’s normal
Severe heatwave (departure track)	Departure of 6.5 °C or more above normal
Heatwave (absolute track)	Maximum at or above 45 °C, whatever the departure
Severe heatwave (absolute track)	Maximum at or above 47 °C
Declaration	Conditions met at two or more stations in a subdivision on two consecutive days; declared on the second day

IMD also flags warm nights — nights whose minimum stays far above normal. That category deserves more public attention than it gets, for the reason set out below.

The colour codes, translated into household decisions

The four-colour scheme IMD uses for rain and cyclones applies to heat as well. It is an impact-based warning issued for the days ahead, and each level carries a plain-language action.

Colour	Official sense	What it means at home
Green	No action	Normal summer routine
Yellow	Watch — be updated	Heat is building. Check on elderly neighbours, store water, move outdoor work to early morning
Orange	Alert — be prepared	Shift errands out of the middle of the day, set up the cool room, prepare for a power cut, watch infants and anyone on regular medication
Red	Warning — take action	Treat outdoor exposure between noon and 3 p.m. as avoidable risk; vulnerable people should not be left alone all day

The night is what does the damage

A hot afternoon is survivable if the body gets to unload heat afterwards. What makes a heat spell dangerous is a run of nights that never drop — the pattern behind IMD’s warm-night category, and the reason mortality in Indian cities tracks multi-day spells rather than single record afternoons.

Indian building stock makes this worse in a very physical way. A bare concrete terrace absorbs sun all day and radiates it downward for hours after sunset, which is why a top-floor room is often at its worst at ten at night; our guide to the top-floor flat explains why the roof, not the machine, decides that fight. If a household can do only one thing before next summer, the roof beats the tonnage.

Who is actually at risk

Group	Why, specifically
Outdoor workers — construction, delivery, farm, street vending	Exertional heat load on top of ambient heat, often with no shade and no scheduled break
Older adults	Reduced thirst sensation and sweating response; several common medications affect fluid balance or heat regulation
Infants and small children	High surface-to-mass ratio, limited ability to say what is wrong, entirely dependent on an adult noticing
Pregnant women	Higher baseline heat load and greater fluid requirements
People with heart, kidney or respiratory conditions	Heat strain adds directly to an already loaded system
Anyone living alone	Every bad outcome above gets worse when nobody checks

Recognising heat illness before it becomes an emergency

Stage	Signs	What to do
Heat cramps	Painful muscle spasms, heavy sweating, usually after exertion	Stop, move into shade, sip water or ORS, rest before resuming
Heat exhaustion	Weakness, dizziness, nausea, headache, clammy skin, fast weak pulse	Lie down somewhere cool, loosen clothing, cool the skin with a damp cloth, fluids in small sips; seek care if it does not improve
Heat stroke	Very high body temperature with confusion, slurred speech, unsteadiness or unconsciousness — sweating may have stopped	A medical emergency. Call 108 or 112 and begin cooling in the same minute — shade, remove excess clothing, water on the skin, air movement — while help is on the way

The distinction that matters is mental status. Someone tired and queasy needs shade and fluids; someone confused, aggressive or not making sense needs an ambulance and active cooling, not a debate about which comes first.

The household routine for an orange or red day

Drink before you are thirsty, and prefer water, buttermilk, nimbu-paani or ORS. NDMA’s standing advice is to avoid alcohol, tea, coffee and carbonated drinks during heat spells, and to skip heavy, stale or very high-protein meals.
Move the day around the sun. Outdoor work and errands before 11 a.m. or after 5 p.m.; noon to three is the window NDMA singles out.
Close the house by day, open it by night. Curtains and shutters on the sun-facing side from mid-morning; cross-ventilation once the outdoor air is genuinely cooler than the indoor air. The full priority order is in our guide to cooling an Indian room without an AC.
Designate one cool room and let the household sleep in it. One properly sized machine cooling one room well beats three rooms cooled badly, and it halves the bill — the arithmetic is in our guide to what an AC adds to an Indian electricity bill.
Set the AC sensibly rather than aggressively. A moderate setpoint holds through the afternoon and lowers the chance of a protection trip on the hottest day of the year; see what temperature to set your AC to.
Plan for the cut. A wet cloth, stored water, a fan or cooler on the inverter, and a realistic view of what that inverter can carry — our guide to running an AC on an inverter battery is blunt about the limits.
Check on someone. A phone call to an elderly relative at four in the afternoon is the cheapest intervention in this entire article.

One caution about fans in extreme heat

Air movement helps as long as sweat can evaporate and the air is cooler than skin. When air temperature climbs well above body temperature, particularly in dry conditions, a fan blowing over an already dehydrated person adds heat rather than removing it — which is why health guidance stops treating fans as sufficient on the most extreme days, and why evaporative coolers behave so differently by climate (the humidity logic is set out in our comparison of air coolers and ACs, city by city). Fans plus fluids and wetted skin: yes. A fan alone for a frail person at 46 °C: not enough.

Heat action plans: what your city may already owe you

Since Ahmedabad published South Asia’s first city heat action plan in 2013, a large number of Indian cities and states have followed under NDMA’s national heat guidelines. A heat action plan is not a pamphlet: it defines the temperature triggers at which school timings shift, worksite hours change, water points open, hospitals prepare for heat cases and alerts go out through local channels. Whether it works depends entirely on whether the local body executes it. Knowing whether your city has one, and what its triggers are, changes what you can reasonably ask of a school, an employer or a ward office during a red-alert week.

Frequently asked questions

Does an orange alert mean schools will close?

Not automatically. Closures and timing changes are decided by state governments and local authorities, usually guided by their heat action plan, not by IMD. The alert is a weather warning; the school decision is an administrative one that often follows a day later. Watch the district or state announcement rather than inferring it from the colour.

What does a «severe heatwave» add to an ordinary one?

Severity in IMD’s scheme is about the size of the departure from normal, or crossing the absolute threshold of roughly 47 °C in the plains. The practical difference for a household is duration and night-time relief: severe spells tend to arrive with nights that do not cool, and it is accumulation across days that fills hospital beds.

What is the safest AC setting during a heatwave?

A moderate setpoint held steadily. Driving a machine to its lowest setting on a 45 °C afternoon does not cool the room faster — capacity falls as outdoor temperature rises, as our guide on whether an AC really cools at 52 °C explains — and it raises the risk of a high-pressure trip exactly when you need the machine running. Clean the filters before the season, keep the outdoor unit’s airflow path clear, and accept a comfortable room rather than a cold one.

Can an air cooler keep a room safe in a heatwave?

In genuinely dry heat, an evaporative cooler with good ventilation gives a real and useful temperature drop. In humid coastal heat it gives very little cooling and adds moisture, which is the worse of the two variables for the body’s ability to shed heat. The verdict depends on your city’s humidity at peak, which is exactly what our city-by-city pages for India are built to answer.