The basic criteria for IMD to declare a heatwave currently does not take into account relative humidity, which is increasingly becoming a cause of humid heatwaves
Photo for representation. Courtesy: iStock The India Meteorological Department (IMD) declared 2024’s first heatwave conditions for isolated pockets of west Rajasthan on March 27, 2024. But there are many more places that likely suffered from humid heatwaves and will continue to do so in the next few days. These are not being accounted for in IMD’s heatwave data.
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. These temperature values are the thresholds set by IMD for the declaration of heatwaves in India.
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.
When the temperature crosses 45°C, the weather agency immediately declares a heatwave without considering the deviation from normal temperature for that particular place. But this criteria does not take into account relative humidity, which is increasingly becoming a cause of humid heatwaves.
During a humid heatwave, the temperature felt by the human body or by other animals and plants is much higher. 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.
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 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.
But 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.
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. 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.
Around mid-day on March 28, Sholapur in Maharashtra recorded a maximum temperature of 40°C and relative humidity of 53 per cent. This translates to a wet bulb temperature of 31.54°C.
Similarly, Jalgaon recorded a maximum temperature of 46.8°C and relative humidity of 29 per cent, which translates to a wet bulb temperature of 30.72°C.
Currently, this is not captured in the IMD definition of a heatwave as the weather agency does not calculate wet bulb temperature of a place, though there would be declaration of heatwave conditions for the Jalgaon region which falls under Madhya Maharashtra subdivision as its temperature exceeded 45°C.
The IMD has come up with two new terminologies for heat stress experienced by the place — ‘warm night conditions’ and ‘hot humid weather’.
In its statement dated March 28, it has indicated that ‘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’.
In its forecast for the next few days, the weather agency has also predicted that warm night conditions would prevail over Gujarat, Marathwada and Madhya Maharashtra on March 28 and March 29.
IMD has also predicted hot and humid weather for the Saurashtra and Kutch regions of Gujarat on March 28 and March 29 and for Rayalaseema, Tamil Nadu, Puducherry, Karaikal, Kerala and Mahe between March 28 and April 1. But what is meant by ‘hot and humid weather’ is not clearly defined by IMD.
Even the thresholds of 30°C and 35°C are not exactly accurate. There have to be separate thresholds for tropical countries and these need to be continuously monitored and humid heat alerts generated for the impacted population.
In a research paper published in the Journal of Applied Physiology of the American Physiological Society in December 2021, scientists had found that the wet bulb temperature threshold of 35°C cannot be applied to human adaptability across all climatic conditions. In high humidity conditions, that threshold could be well below 35°C. This was the first study with empirical evidence that evaluated the impact of wet bulb temperature on human health.
This means that there may be humid heatwaves close to the human survivability threshold of heat stress already occurring in India and they are not being monitored. More importantly, the people being impacted are not being informed about the occurrence of such conditions.
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