Heatwaves: Here is what you should know

Children cool down in a fountain outside the Columbus shopping centre in Vienna on August 11, 2015 due to a heatwave. 

Photo credit: File

The South Africa heatwave brings me bad memories of my 2017 experience in Rajasthan State, India. The region is home to Thar Desert, commonly known as the Great Indian Desert, and from March to June every year, heatwaves are the order of the day.

Here are some facts you should know about this heated-weather phenomenon often linked to climate change.

What is a heatwave? 
According to climate experts, a heat wave is a period of unusually hot weather that typically lasts two or more days and to be considered a heat wave, the temperatures have to be outside the historical averages for a given area.

How does a heatwave form? 
Heat waves are generally the result of trapped air. Scientists explain that, for example, in the US during the 2012 heatwave, air was trapped above much of North America for a long period of time.

As opposed to cycling around the globe, it simply stayed put and warmed like the air inside an oven. 


Extreme heatwaves, drought and devastating flooding have impacted millions of people and cost billions in damage in 2022, according to the United Nations. Graphic shows selected extreme weather events of 2022.

Photo credit: Pool I Nation Media Group

This means that high-pressure systems force air downward and as a result, the force prevents air near the ground from rising while the sinking air acts like a cap which then traps warm ground air in place.

It is very important to note that without rising air, there is no rain, and nothing to prevent the hot air from getting hotter. In the case of America,  a weather pattern that normally pulls air toward the east was also weaker at the time and that  meant that there was little that could be done to push this high-pressure cap out of the way.

Are all heatwaves the same?
Heatwaves are classified into three types, based on intensity.

Low-intensity heatwaves: They are more frequent during summer. Most people can cope during these heatwaves.

Severe heatwaves: They are less frequent and are likely to be more challenging for vulnerable people such as the elderly, particularly those with medical conditions.

Extreme heatwaves: They are rare. They are a problem for people who don't take precautions to keep cool—even for people who are healthy. People who work or exercise outdoors are also at greater risk of being affected.

What do experts consider to declare it a heatwave?
Scientists declare a heatwave if the maximum temperature of a station reaches at least 40 degrees Celsius or more for plains and at least 30 degrees Celsius or more for hilly regions.

Here is a breakdown of the numbers they look

1. Based on departure from normal:

A.  Heat Wave: Departure from normal is 4.5 degrees Celsius to 6.4 degrees Celsius.

B.  Severe Heat Wave: Departure from normal is >6.4 degrees Celsius.

2. Based on Actual Maximum Temperature:

A.  Heat Wave: When actual maximum temperature ≥ 45 degrees Celsius 

B.  Severe Heat Wave: When actual maximum temperature ≥47 degrees Celsius 

This means that if the above criteria met at least in two  stations in a meteorological sub-division for at least two consecutive days, the scientists  declare it on the second day.

And for Coastal regions? 
For coastal regions, when the maximum temperature departure is 4.5 degrees Celsius or more from normal, a heatwave may be called  provided  that the actual maximum temperature is 37 degrees Celsius  and above. 

How exactly are heatwaves monitored? 
Weather experts use a big network of surface observatories covering an entire country to measure various metrological parameters like temperature, relative humidity, pressure, wind speed as well as direction.

Now from the daily maximum temperature station data realized, the climatology of maximum temperature is then prepared to find out normal maximum temperature of the day for  that particular station and as a result a heatwave is called as per their findings from the data. 

What conditions make it conducive for heatwaves to manouvre? 
There are four conditions namely prevalence of hot dry air over a region or transportation, the sky should be particularly cloudless, absence of moisture in the upper atmosphere as well as large altitude anti-cyclonic flow over the area. 

What is a heat index? 
This refers to the combination of air temperature and relative humidity, it is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature according to experts. 

What is heat discomfort and how is it determined? 
This is the state of your body overheating and as a result you may have a high temperature, bumps on your skin, muscle spasms, headache, dizziness, nausea or a number of other symptoms.

It is determined by a combination of meteorological factors such as temperature, relative humidity, wind and direct sunshine, social-cultural factors  such as clothing, occupation, accommodation and   physiological factors  such as health, fitness, age and level of acclimatisation.

How do experts issue a heatwave warning? 
To achieve an impact in their messaging , weather experts use a colour code system  where red denotes an  extreme heat alert and a heatwave that will be present  for more than six days, yellow is a heat alert that tells the public that there will be heatwaves at isolated pockets for two days, orange informs it is a severe heat alert for the day and though not really severe but the heatwave will be persistent for more than four days  while green notifies that all is normal and that maximum temperatures are near normal. 

Does a heatwave affect the health of a person?
Yes. They come along with dehydration, heat cramps, heat exhaustion and/or heat stroke. Some people may faint and develop swellings and this is usually  accompanied by fever below 39 degrees Celsius while heat exhaustion may lead to fatigue, weakness, dizziness, headache, nausea, vomiting, muscle cramps and sweating.

Experts observe that heat strokes are fatal and show up when the body is at a temperature of 40 degrees Celcius  accompanied   seizures or a coma. 

How can one protect themselves during a heatwave?
Experts advise you to avoid  going out in the sun, especially between  noon and 3pm, to drink sufficient water and as often as possible, even if not thirsty, to wear lightweight, light-coloured, loose and porous cotton clothes, to use  protective goggles, umbrella/hat, shoes or chappals while going out in sun, to avoid strenuous activities when the outside temperature is high and to avoid  working outside between 12 noon and 3pm .

While travelling, they encourage that you carry water with you, avoid alcohol, tea, coffee and carbonated soft drinks which dehydrate  the body but instead re-hydrate with things like lemon-water and that you should also avoid  high-protein foods.

Remember not to eat stale food. For those working outside, make sure that you put on a hat or use an umbrella if not a damp cloth on your head, neck, face and limbs .

Make sure you take a cold bath and use fans. They highlight that you should not leave children or pets in parked vehicles and If you feel like fainting or ill, go see a doctor immediately. 

Who is at greater risk of a heatwave?
The most at risk are those who have come from a cooler climate and are not used to a hot climate. They should not move about in open field for a period of one week till the body is acclimatized to heat and should drink plenty of water. Experts explain that acclimatisation is achieved by gradual exposure to the hot environment during heat wave.

What should you do in case someone close to you is affected by a heat stroke during a heatwave?
The first thing to do is to lay the person in a cool place, under a shade and then wipe them with a wet cloth/wash the body frequently. Pour normal temperature water on the head. The main thing is to bring down the body temperature.

After that, give them oral rehydration salts to drink or  lemon-water, whatever is useful, to rehydrate the body and then  take the patient to the nearest health care facility immediately because they need immediate hospitalisation.