The Intergovernmental Panel on Climate Change (IPCC)’s Sixth Assessment Report (AR6) states that humidity is a crucial factor when estimating the physiological stress extreme heat inflicts on the human body. The report used an alternative metric, known as the “wet bulb temperature” to measure exposure to extreme heat, instead of the traditional “dry bulb” temperature usually measured with a regular thermometer.
Since March 2022, South Asia has been experiencing consecutive heatwaves and breaking historical temperature records.
Understanding Heatwaves
A heatwave is defined as a period of unusually high temperatures, exceeding the normal maximum temperature recorded during the summer season, especially in North-Western and South Central regions of India. Typically, heatwaves occur between March and June, and sometimes may extend till July. According to the India Meteorological Department (IMD), heatwaves are classified based on regions and their temperature ranges.
Criteria to Classify Heatwaves
Heatwaves are categorized when the maximum temperature of a station reaches at least 40°C for Plains and 30°C for Hilly regions. For stations with a normal temperature of less than or equal to 40°C, a heatwave condition is declared when the temperature increases by 5°C to 6°C from the normal temperature. If the temperature rises by 7°C or more, it is considered a severe heat wave condition. For a station with a normal maximum temperature of over 40°C, if the increase in temperature is 4°C to 5°C, it is deemed a heat wave condition. An increase of 6°C or more is considered a severe heatwave. Furthermore, any temperature over 45°C is declared a heatwave, irrespective of the normal maximum temperature.
The Role of Humidity in Heat Exposure
Humidity is critical in measuring heat exposure because humans lose heat produced within their bodies through sweat which evaporates on the skin. The evaporation’s cooling effect is essential for maintaining a stable body temperature. However, as humidity rises, sweat does not evaporate, making it difficult to regulate body temperature and causing discomfort in humid places.
According to the report, sustained exposures to wet bulb temperatures above 35°C can be fatal, while above 32°C can be dangerous for intense physical activity. Achieving wet bulb temperatures over 35°C is exceedingly hard and such conditions are rarely observed now.
Underlying Health Conditions and Heatwaves
The report also highlights that inability to stabilize core body temperature might be due to various reasons, apart from the evident physiological and political issues. Increased strain on the heart due to elevated temperature could be fatal for those with existing cardiac conditions, which are leading causes of death during heatwaves. Respiratory problems and diabetes might also lead to fatalities as these conditions hinder the body’s ability to efficiently transfer heat to the environment.
The Concept of Wet Bulb Temperature
Wet bulb temperature (WBT) is the lowest temperature attainable through the evaporation of water into the air at a constant pressure. It is a limit that considers both heat and humidity beyond which humans cannot tolerate high temperatures. WBT can be measured using a thermometer with the bulb wrapped in wet muslin, with the evaporation of water from the thermometer indicating a “wet bulb temperature” lower than the “dry bulb temperature”.
Understanding Dry Bulb Temperature
Dry Bulb temperature is the most frequently used air property. Commonly referred to as “air temperature”, when people talk about the temperature of the air, they are usually referring to the dry bulb temperature. It refers to the ambient air temperature and can be measured using a normal thermometer freely exposed to the air but shielded from radiation and moisture. It is an indicator of heat content.