Weather Bee | Temperature alone cannot predict distress as monsoon approaches
The combination of humidity and temperature can pose health risks, emphasising the need for caution beyond just thermometer readings
Experts have pointed to an abundance of moisture in the atmosphere as one of the reasons behind the thunderstorm in Mumbai on May 13 that left at least 14 people dead and 74 injured. As the monsoon season approaches, moisture or humidity is also likely to become the cause of another concern. People can suffer adverse health effects even if the thermometer shows temperature to be relatively benign. Here is how.
The Global Forecast System (GFS) of the US National Weather Service produces hourly forecasts of temperature over a period of five days. This can be used to calculate the average maximum temperature in the next five days. HT calculated this average for the May 14 to May 18 period, which is shown in the map below.
As the above map shows, almost the entire northern half of India is likely to average a maximum of 40°C or higher in the five days ending May 18. On the other hand, almost all of peninsular India is likely to average below this threshold. To be sure, the minimum threshold for declaring a heat wave in coastal areas is a maximum temperature of 37°C (it is 40°C for plains), according to the India Meteorological Department (IMD). However, even this threshold is not being met by the peninsular states. No area in Kerala, Tamil Nadu, or Karnataka is averaging beyond the 37°C threshold; and only around 15% of Andhra Pradesh is likely to average a maximum of more than 37°C.
The above statistics illustrate that heat wave reports are more likely to arrive from northern states in the coming days than from peninsular states. However, this does not mean that there is no danger in the peninsular states in the ongoing week. This can be seen from another map below, which shows the average maximum of relative humidity in the five days ending May 18.
The map for relative humidity is the exact opposite of the map for maximum temperature. Most of peninsular India is likely to be very humid. On the Kerala and Karnataka coast, this moisture is likely to average a peak of over 90%. This means that the air there will hold 90% or more of the water it can hold at the temperature prevailing at the time (this is what is meant by the "relative" in "relative humidity").
While it is not necessary that the hour of maximum relative humidity will coincide with the hour of maximum temperature, the map shows that peninsular India is likely to experience much higher levels of humidity than the northern half of India. If high humidity coincides with even relatively low temperatures, it can produce a devastating effect on the human body. This is because if the air is already holding over 90% of the water it can hold at a particular temperature, human beings can’t maintain their body temperature by sweating. This is similar to the uncomfortable feeling after a shower on a hot day without an exhaust fan.
What is the degree of danger that the combination of humidity and temperature will produce? This is quantified using an adjusted temperature value called the heat index. The relation between heat index and humidity is not necessarily linear. This means that even though 99% humidity at 20°C temperature is not dangerous, but is very dangerous at 30°C. In effect, the relative humidity map alone is not sufficient to see where humidity is likely to be dangerous. HT used the formula for heat index from the National Oceanic and Atmospheric Administration (NOAA) of the US government on each hour’s temperature and humidity forecast to calculate the average maximum heat index in the five days ending May 18. As expected, coastal weather does not look as pleasant as it did on the map for maximum temperature.
To be sure, NOAA advises “extreme caution” at even 32.8°C heat index, a level most areas in the country are likely to experience in the ongoing week. Moreover, large parts of northern India are likely to experience a heat index of over 40°C, a threshold NOAA considers “dangerous”. However, what this analysis seeks to point out is that thermometer temperature – while useful on days of low humidity – should not be the only guide for safety as the Indian summer approaches the monsoon season. This is when humidity is likely to increase even beyond coastal regions.
Abhishek Jha, HT’s senior data journalist, analyses one big weather trend in the context of the ongoing climate crisis every week, using weather data from ground and satellite observations spanning decades.
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