Too Hot to Handle: Why mercury is rising and How does India measure heatwave?

New Delhi, Apr 06: You thought March was the hottest month? Well you are wrong. It's going to be a sizzling April too.

India Meteorological Department Director General Mrutyunjay Mohapatra said that Northwest India and adjoining parts of central India are predicted to see "more intense and frequent heatwave" conditions in April.

"We are expecting that the temperature will be higher than normal in the entire northwest India and the adjoining central India, starting with Gujarat, Rajasthan and up to east Madhya Pradesh and Chhattisgarh," he said at a virtual event on "Building Climate Resilience for the Most Heat Vulnerable".

The IMD had earlier predicted that April will be "more severe" than March and the temperatures will be higher than normal, Mohapatra said.

"The frequency of intense heatwave conditions will be higher in April as compared to March. And, we expect the heatwave conditions to continue till April 15 in some parts," he added.

March warmest month in 122 years

India recorded its warmest March in 122 years with a severe heatwave scorching large swathes of the country during the month.

Why mercury is rising?

The weather department attributed the heat to the lack of rainfall due to the absence of active western disturbances over north India and any major system over south India.

The country as a whole recorded a rainfall of 8.9 mm, which was 71 per cent less than its long period average rainfall of 30.4 mm. It was also the third lowest precipitation in March since 1901 after 7.2 mm in 1909 and 8.7 mm in 1908.

India is witnessing a repeat of 2021 conditions, when temperatures touched 40 degrees Celsius as early as in February in some parts of the country.

What is 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. In certain countries it is defined in term of the heat index based on temperature and humidity or based on extreme percentile of the temperatures.

What is criterion for declaring heat wave?

Heat wave is considered if maximum temperature of a station reaches at least 400C or more for Plains and at least 300C or more for Hilly regions.

a) Based on Departure from Normal

Heat Wave: Departure from normal is 4.50C to 6.40C
Severe Heat Wave: Departure from normal is >6.40C

b) Based on Actual Maximum Temperature

Heat Wave: When actual maximum temperature ≥ 450C
Severe Heat Wave: When actual maximum temperature ≥470C

If above criteria met at least in 2 stations in a Meteorological sub-division for at least two
consecutive days and it declared on the second day.

What are favorable conditions for Heat wave?

a. 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).

b. Absence of moisture in the upper atmosphere (As the presence of moisture restricts the temperature rise).

c. The sky should be practically cloudless (To allow maximum insulation over the region).
d. 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.

How heat discomfort is determined?

It is determined by a combination of meteorological (temperature, Relative Humidity, wind, direct sunshine), social/cultural (clothing, occupation, accommodation) and physiological (health, fitness, age, level of acclimatization) factors.