For the past five days, Rajasthan, Delhi, Uttar Pradesh, Madhya Pradesh, and Maharashtra have been experiencing severe to very severe heatwave conditions. Here is why this summer is slightly unusual.

Heatwaves being more frequent phenomena, the UPSC may end up asking a prelim as well as mains question about it.  It may ask Q. What are heat waves and how are they classified? What are the external factors on which it is depended?

A MCQ may be a statement based question mentioning the criteria for declaring a heatwave.

What is a heatwave and when is it declared?

Heatwaves occur over India between March and June.

• IMD declares a heatwave event when the maximum (day) temperature for a location in the plains crosses 40 degrees Celsius.
• Over the hills, the threshold temperature is 30 degrees Celsius.

Following criteria are used to declare heatwave:

To declare heatwave, the below criteria should be met at least in 2 stations in a Meteorological subdivision for at least two consecutive days and it will be declared on the second day.

a) Based on Departure from Normal

• Heat Wave: Departure from normal is 4.5°C to 6.4°C
• Severe Heat Wave: Departure from normal is >6.4°C

b) Based on Actual Maximum Temperature (for plains only)

• Heat Wave: When actual maximum temperature ≥ 45°C
• Severe Heat Wave: When actual maximum temperature ≥47°C

How long can a heatwave spell last?

• A heatwave spell generally lasts for a minimum of four days. On some occasions, it can extend up to seven or ten days.
• The longest recorded heatwave spell, in recent years, was between 18 – 31 May 2015.
• This spell had severely affected parts of West Bengal along with Odisha, Andhra Pradesh, and Telangana.
• Heatwave conditions occurring in May have been observed to last longer, as the season reaches its peak this month.
• Whereas those reported in June often die down sooner, often due to the onset of Southwest monsoon over the location or in its neighbourhood.

Does all of India experience heatwave conditions?

• Heatwaves are common over the Core Heatwave Zone (CHZ) — Rajasthan, Punjab, Haryana, Chandigarh, Delhi, West Madhya Pradesh, Uttar Pradesh, Chhattisgarh, Orissa, Vidarbha in Maharashtra.
• The CHZ also includes parts of Gangetic West Bengal, Coastal Andhra Pradesh and Telangana, as categorised by IMD.
• Several recent studies indicate that CHZ experience more than six heatwave days per year during these four months.
• Many places in the northwest and cities along southeastern coast report eight heatwave days per season.
• However, the regions in the extreme north, northeast and southwestern India are lesser prone to heatwaves.

Whats’ so unusual this year?

• Summer season reaches its peak by May 15 in India when the day temperatures across north, west, and central India cross 40 degrees and hover close to 45 degrees then on.
• This year, north India did not experience such temperatures till May 21.
• It was mainly because of the continuous inflow of Western Disturbances that influenced the weather in the north till as late as April.
• Since last winter, there was frequent passing of Western Disturbances over the north, appearing after every five to seven days.

What are these Western Disturbances?

• Originating in the Mediterranean Sea, Western Disturbances are eastward-moving winds that blow in lower atmospheric levels.
• They affect the local weather of a region during its onward journey.
• Between January and March this year, there were about 20 Western Disturbances, a record of sorts.
• When Western Disturbances interact with weather systems heading from the two southern seas, that is, warm winds blowing in from the Bay of Bengal or the Arabian Sea, they cause snowfall or rainfall over the north.
• A significant influence of Western Disturbances is experienced from December to February. However, this year, its influence persisted until early May.
• The recent Western Disturbances got support from easterly winds blowing over from the Bay of Bengal.

Has cyclone Amphan influenced the current heatwave?

• Since the event of severe heat has emerged immediately after the passing of Cyclone Amphan, experts confirm its role in leading to the present heatwave spell.
• Cyclone Amphan, which was a massive Super Storm covering 700 km, managed to drag maximum moisture from over the Bay of Bengal to entire Peninsula.
• All the moisture that was otherwise built during the thunderstorm and rainfall got gradually depleted from over vast areas as the storm advanced towards West Bengal and Bangladesh between May 16 and 20.
• It has now triggered dry north-westerly winds to blow over Rajasthan, Madhya Pradesh, Uttar Pradesh and Maharashtra causing severe heatwave.

Get an IAS/IPS ranker as your 1: 1 personal mentor for UPSC 2024

Attend Now