• Mercury hovered around 40 degrees Celsius in large parts of the country on April 17 as the IMD predicted heat wave conditions in parts of east India over the next four days and the northwest region over the next two days. 

• In the national capital, heat wave conditions persisted for the second consecutive day with some weather stations recording the maximum temperature at least five degrees Celsius above normal. 

• According to the India Meteorological Department (IMD), the Gangetic West Bengal and Bihar may see heat wave conditions for four days on the trot. Sikkim, Odisha and Jharkhand may also experience such conditions over the next two to three days.

• West Bengal and Tripura have ordered closure of schools, colleges and universities for a week due to the prevailing heatwave conditions.

• Andhra Pradesh and its southern coastal regions are expected to experience a heat wave for three days. 

What is a heat wave?

• Higher daily peak temperatures and longer, more intense heat waves are becoming increasingly frequent globally due to climate change. India too is feeling the impact of climate change in terms of increased instances of heat waves which are more intense in nature with each passing year, and have a devastating impact on human health thereby increasing the number of heat wave casualties.

• Heat waves do not fetch as much public attention as more dramatic disasters such as earthquakes and floods, but until more recently they were taking a substantial toll of lives in India.

• A heat wave is a period of abnormally high temperatures, more than the normal maximum temperature that occurs during the summer season. 

• Heat waves typically occur between March and June, and in some rare cases even extend till July. 

• The extreme temperatures and resultant atmospheric conditions adversely affect people as they cause physiological stress, sometimes resulting in death.

The IMD has given the following criteria for heat waves:

i) Heat wave need not be considered till maximum temperature of a station reaches at least 40°C for plains and at least 30°C for hilly regions.

ii) When normal maximum temperature of a station is less than or equal to 40°C heat wave departure from normal is 5°C to 6°C severe heat wave departure from normal is 7°C or more.

iii) When normal maximum temperature of a station is more than 40°C heat wave departure from normal is 4°C to 5°C severe heat wave departure from normal is 6°C or more.

iv) When actual maximum temperature remains 45°C or more irrespective of normal maximum temperature, heat waves should be declared. 

• The health impact of heat waves typically involve dehydration, heat cramps, heat exhaustion and/or heat stroke.

What is heat index?

The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature.

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