What is a heat wave?

Qualitatively, heat wave is a condition of air temperature which becomes fatal to human body when exposed. Quantitatively, it is defined based on the temperature thresholds over a region in terms of actual temperature or its departure from normal.
According to the IMD, a region has a heat wave if its ambient temperature deviates by at least 4.5-6.4°C from the long-term average. There is also a heat wave if the maximum temperature crosses 45°C (or 37°C at a hill-station).

What is the period of heat wave over India?

It is occurring mainly during March to June and in some rare cases even in July. The peak month of the heat wave over India is May.

Transportation / Prevalence of hot dry air over a region (There should be a region of warm dry air and appropriate flow pattern for transporting hot air over the region).
Absence of moisture in the upper atmosphere (As the presence of moisture restricts the temperature rise).
The sky should be practically cloudless (To allow maximum insulation over the region). Large amplitude anti-cyclonic flow over the area.
Heat waves generally develop over Northwest India and spread gradually eastwards & southwards but not westwards (since the prevailing winds during the season are westerly to northwesterly). But on some occasions, heat wave may also develop over any region in situ under the favorable conditions.

Heat waves are formed for one of two reasons — warmer air is flowing in from elsewhere or it is being produced locally. It is a local phenomenon when the air is warmed by higher land surface temperature or because the air sinking down from above is compressed along the way, producing hot air near the surface.

In spring, India typically has air flowing in from the west-northwest. In the context of climate change, West Asia is warming faster than other regions in latitudes similarly close to the equator and serves as a source of the warm air that blows into India. Likewise, air flowing in from the northwest rolls in over the mountains of Afghanistan and Pakistan. Some of the compression also happens on the leeward side of these mountains, entering India with a bristling warmth.
While air flowing in over the oceans is expected to bring cooler air, the Arabian Sea is warming faster than most other ocean regions.
The strong upper atmospheric westerly winds, that come in from the Atlantic Ocean over to India during spring, control the near-surface winds. Any time winds flow from the west to the east. The energy to run past the earth near the surface, against surface friction, can only come from above. This descending air compresses and warms up to generate some heat waves.
Lapse rate — the rate at which temperatures cool from the surface to the upper atmosphere — is declining under global warming. In other words, global warming tends to warm the upper atmosphere faster than the air near the surface. This in turn means that the sinking air is warmer due to global warming, and thus produces heat waves as it sinks and compresses.

Heat waves are caused by a strong high pressure settling in at 10,000-25,000 ft. and refusing to move. This causes warm air to sink. The result is a dome of hot air that traps the heat near the ground and prevents cooling convection currents from forming clouds.

The health impacts of Heat Waves typically involve dehydration, heat cramps, heat exhaustion and/or heat stroke.

HAPs are India’s primary policy response to economically damaging and life -threatening heatwaves. They prescribe a variety of preparatory activities, disaster responses, and post-heatwave response measures across state, district, and city government departments to decrease the impact of heatwaves.

Heat index, also known as apparent temperature, is a measure of how the temperature feels to humans.
Relative humidity is an important factor that determines heat index, along with air temperature. Experimental Heat Index has been launched by IMD to provide general guidance for the regions within India where, the apparent temperature/feel like temperature (considering the impact of humidity along with the temperature) are on the higher side causing discomfort for the people. At present, heat index is derived using the heat index equation similar to what is used by National Weather Service, National Oceanic and Atmospheric Administration (NOAA), USA.
Heat index provides information about the impact of humidity on the high temperatures and thus provides a feel like temperature for human beings which can be used as an indication for human discomfort. It provides guidance towards additional care to be taken by people to reduce discomfort. Colour codes used for Experimental Heat Index are as follows:
- Green: – Experimental heat Index less than 35 deg C
- Yellow: – Experimental heat Index in the range 36-45 deg C
- Orange: – Experimental heat Index in the range 46-55 deg C
- Red: – Experimental heat Index greater than 55 deg C

Avoid going out in the sun, especially between 12.00 noon and 3.00 p.m. o Drink sufficient water and as often as possible, even if not thirsty
Wear lightweight, light-coloured, loose, and porous cotton clothes. Use protective goggles, umbrella/hat, shoes or chappals while going out in sun.
Avoid strenuous activities when the outside temperature is high. Avoid working outside between 12 noon and 3 p.m.
Carry Water while travelling
Avoid alcohol, tea, coffee and carbonated soft drinks, which dehydrates the body.
Avoid high-protein food and do not eat stale food.
If you work outside, use a hat or an umbrella and also use a damp cloth on your head, neck, face and limbs
Do not leave children or pets in parked vehicles o If you feel faint or ill, see a doctor immediately.
Use ORS, homemade drinks like lassi, torani (rice walemon water, buttermilk, etc. which helps to re-hydrate the body.
Keep animals in shade and give them plenty of water to drink.
Keep your home cool, use curtains, shutters or sunshade and open windows at night
Use fans, damp clothing and take bath in cold water frequently.