Getting hotter by the year

The rising heat and humidity levels are putting an additional stress on people across India with a greater chance of increase in heat-related illnesses. Thus warns a first-of-its-kind scientific study that has analysed monthly mean maximum temperature and relative humidity records for 60 years from 283 surface meteorological stations across the country.

Based on the data analysis, the authors of the study, who are scientists with India Meteorological Department (IMD) and Indian Institute of Tropical Meteorology (IITM), have recorded an increasing trend of heat index in almost all regions of India. “Averaged over the country, HI (heat index) is increasing during summer and monsoon seasons at the rate of +0.56oC per decade and +0.32oC per decade respectively. The increasing HI (heat index) indicates high level of discomfort in both the seasons which is primarily due to increase in humidity in summer season and maximum temperature in monsoon season,” reads the study published recently in the Journal of Climate Change.
Simply put, heat index is the human-perceived apparent temperature.

Recording the changes

Heat index combines air temperature and relative humidity to determine how hot it actually feels to the people. For example, an air temperature of 30oC with a relative humidity of 60% translates to an apparent temperature, or heat index of 33oC. The difference between heat index and air temperature is the measure of heat stress placed on the human body by elevated atmospheric water vapour content.

The calculation of heat index is based on two broad parameters - ambient temperature and relative humidity. It is a known fact that a rise in ambient temperature makes people feet hot. However, a rise in humidity levels is also associated with hot weather. The human body normally cools itself by perspiration in which the water in the sweat evaporates and carries heat away from the body.

However, when the relative humidity is high, the evaporation rate of sweat is reduced. This means heat is removed from the body at a lower rate, causing it to retain more heat than it would in dry air. The higher the heat index, the hotter the weather feels.

Based on discomfort and health effects, researchers have classified heat index into four broad categories – warm (27-32oC), hot (32-41oC), very hot (41-54oC), and extremely hot (more than 54oC).

When heat index is ‘very hot’, there is a possibility of heatstroke, whose likelihood increases during extremely hot heat index of more than 54oC. The recent study has recorded the highest mean summer (March to May) heat index of 52oC in southeast coastal areas of the country. The highest mean monsoon (June to September) heat index has been registered in the Indo-Gangetic belt ranging between 47oC and 52oC. People living in these regions of the country are most prone to heat-related illnesses and mortality.

To calculate the heat index, the researchscientists collated data on mean maximum temperature (TMAX) and mean monthly relative humidity (RH) between 1951 and 2010 from 283 surface meteorological stations under the network on IMD. The data was analysed for calculating bio-meteorological heat index for various locations in India. The results show that summer season (March to May) heat index is steadily increasing at a rate of 0.56oC per decade, which is significant at 95 per cent level of confidence. For monsoon season (June to September), the heat index rise across India is 0.32oC per decade.

The study has also recorded an increasing trend of average relative humidity (RH) in summer season at 1.16% per decade, and an increasing trend of mean maximum temperature (TMAX) at 0.08oC per decade during monsoon season. This means that humidity levels are increasing during summer months, whereas the monsoon months are becoming hotter across India. As part of the analysis, the research scientists have also calculated heat index for 25 metropolises (million plus population) in India. The results show highest increase in summer heat index in Amritsar (Punjab) at 0.95oC per decade. Chennai and Bengaluru’s summer heat index trends show a rise of 0.73oC per decade and 0.51oC per decade respectively.

For other southern cities, the summer heat index rise is 0.69oC per decade for Hyderabad, 0.63oC per decade for Coimbatore, 0.33oC per decade for Madurai, and 0.39oC per decade for Tiruchchirappalli. The cities of Kolkata, Pune and Vishakhapatnam show a declining trend in summer heat index. Heat index during monsoon months has also registered an upward trend. For instance, Hyderabad and Coimbatore are witnessing a rise of 0.67oC per decade and 0.66oC per decade in their monsoon heat index respectively.

“The combination of heat and high humidity may cause discomfort, heat related health problems, heat stroke or even death to humans and animals,” warn the research scientists. Due to various factors, including the climate change, ambient temperatures are on the rise in India.

Stilling of winds

“As temperature rises, the capacity of atmosphere to hold moisture or water vapour increases almost exponentially. This leads to high humidity. Thus, we see an increase in relative humidity during summer months in India,” says Dr A K Jaswal, one of the authors of the study.

In monsoon, humidity is already high. However, ambient temperatures are also rising.

“This is because cloud cover is decreasing across India. Thin cloud cover means more solar radiation can enter our atmosphere, thereby increasing temperature,” explains Jaswal. According to him, decrease in cloud cover has also led to a decrease in number of rainy days, which, in turn, contributes to the rising heat. “Heavy precipitation is increasing,” he adds.

There is another meteorological phenomenon - stilling of winds - which is contributing towards rising heat index in India.

According to Jaswal, wind speeds are decreasing in India, which is a cause of worry. Wind helps sweat to evaporate and the body to cool. It also picks up heat through convection from the skin’s surface. Decrease in wind speed means more heat stress on vulnerable sections of the society.

The recent study concludes that due to “increasing temperatures and humidity and decrease in wind speed in India, the frequency, duration and intensity of higher heat stress days are likely to increase substantially in the future.”

Keeping in mind the rising heat index, which gets as high as 52oC in some regions of the country, it is imperative that the Indian government prepares an exhaustive plan to deal with its human health consequences and a possible increase in mortality.