Context

The India Meteorological Department (IMD) declared 2024’s first heatwave conditions for isolated pockets of west Rajasthan.

About Heatwave

• About: Heatwaves are prolonged periods of extreme hot weather that can harm human health, the environment, and the economy.
• Heatwaves in India: India, as a tropical country, is especially subject to heatwaves, which have become increasingly common and severe in recent years.
• Criteria for Declaring Heatwave: The basic criteria for IMD to declare a heatwave is when the temperature of a place crosses 40 degrees Celsius (°C) in the plains, 37°C in the coastal areas and 30°C in the hills. 
  • Apart from this, the temperature of a particular day has to be above normal by at least 4.5°C for two consecutive days for a heatwave to be declared.
• Humid Heatwave: The basic criteria for IMD to declare a heatwave currently does not include taking into account relative humidity, which is increasingly becoming a cause of humid heat waves. 
• • During a humid heatwave, the temperature felt by the human body or by other animals and plants is much higher. 
• Relative Humidity: This happens even when the observed temperatures are lower than the thresholds because of relative humidity, which is a measure of the moisture levels in the atmosphere. 

• Relative Humidity Calculation: The combined impact of temperature and relative humidity can be captured by calculating the wet bulb temperature or heat index of a place. This takes into account both the variables and gives the actual felt temperature.
• Wet bulb temperature: It is the lowest temperature to which air can be cooled by the evaporation of water into the air at constant pressure. 
  • And this evaporation is constantly happening from the skin through sweating, which helps humans cool down when there is excessive heat.
• Cause of Heat Stroke: If there is higher moisture in the atmosphere, this cooling down takes place at a much slower pace or stops completely. At such a point, the temperature of the human body starts increasing, leading to heat stroke and death.
• Safe Limit of Wet bulb Temperature: Internationally, the agreed upon safe limit of wet bulb temperature is below 30°C and highest limit is 35°C, above which the possibility of human death becomes almost certain. 
• Hyperthermia: Between 30°C and 35°C, the human body undergoes hyperthermia, in which the body temperature increases leading to discomfort and multiple impacts on various organs including the brain and the heart.
• Non-Inclusion of Wet bulb temperature:  The IMD  does not calculate wet bulb temperature of a place
• Prevailing of Warm Night conditions: Warm night conditions prevailed in isolated pockets over north Gujarat, Marathwada and Madhya Maharashtra subdivisions, though it has not defined what it means by ‘warm night conditions’.