• Large swathes of India reeled from searing heat and stifling humidity earlier this week with maximum temperatures soaring to 45°C, straining power grids and prompting health warnings from government agencies. 

• Intensely hot conditions are expected in east India until May 1 and the south peninsular region over the next five days, the India Meteorological Department (IMD) said.

• The weather office issued a red alert, warning that extreme heat could scorch parts of Andhra Pradesh, Bihar, West Bengal and Odisha over the next two to three days. 

• An orange warning is in place for parts of Telangana, Karnataka and Sikkim.

• Heatwave to severe heatwave conditions prevailed in parts of West Bengal, Gujarat, Bihar, Sikkim, Odisha, Jharkhand, Kerala and Andhra Pradesh.

The meteorological department uses four colour codes for weather warnings. They are: 

i) Green (no action needed)

ii) Yellow (watch and stay updated)

iii) Orange (be prepared)

iv) Red (take action).

• The IMD also said warm night conditions are likely in east Uttar Pradesh and Karnataka over the next five days. High night temperatures are considered dangerous because the body doesn’t get a chance to cool down.

• Increasing nighttime heat is more common in cities because of the “urban heat island” effect, in which metro areas are significantly hotter than their surroundings.

• “Urban heat islands” occur when cities replace natural land cover with dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat. 

• The intense heat could strain power grids and result in water shortages in parts of India.

What is a heatwave?

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

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

• Heatwaves 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 heatwaves:

i) Heatwave 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 heatwave departure from normal is 5°C to 6°C severe heatwave departure from normal is 7°C or more.

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

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

• The health impact of heatwaves 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.

What is warm night?

• It is considered only when maximum temperature remains 40°C or more. 

It is defined based on departures or actual minimum temperatures as follows:

Warm night: minimum temperature departure is 4.5°C to 6.4°C

Very warm night: minimum temperature departure is >6.4°C.

Manorama Yearbook app is now available on Google Play Store and iOS App Store