The story so far: Earlier in August, Iran recorded a scorching heat index of 70 degrees Celsius (°C) in the coastal part of the country, a metric at which survival of life is unfathomable, if not impossible. The country had also declared public holidays on August 2 and 3 on account of “unprecedented heat,” Reuters reported.
This is not the first time this year that Iran is dealing with extreme heat. In July as well, U.S.-based weather observer Colin Mccarthy reported that the Persian Gulf Airport reported a heat index of 66.7 °C.
What is heat index?
Heat index, also known as apparent temperature, is a measure of how the temperature feels to humans. Relative humidity is an important factor that determines heat index, along with air temperature.
Heat index calculation
A complex formula to calculate heat index was published by Dr. Robert Steadman, a professor in the textiles and clothing department of Colorado State University, in 1979. Dr. Steadman published two papers titled The Assessment of Sultriness – Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science, and Part II: Effects of Wind, Extra Radiation and Barometric Pressure on Apparent Temperature, both describing his calculations of heat index.
Dr. Steadman’s study considers a “typical adult human of either sex, with a height of 1.7 metres and a weight of 67 kg.” Dew point, which is the temperature at which gas is transformed into liquid state, is an important factor in the calculation of heat index. In terms of atmospheric moisture, it’s the temperature at which air cannot hold any more water vapour, and droplets of water begin to form. Dr. Steadman used 14 °C as the dew point in his calculations.
Some countries have developed their own corresponding indices to measure heat index instead of using the one developed by Dr. Steadman. Canada uses Humidex, its own measure of heat index, based on the following table.
The U.S. National Weather Service (NWS) uses the following chart to determine heat index:
NWS Heat Index table | Photo Credit: U.S. National Weather Service
In 2024, India is scheduled to launch its own heat index to quantify the impact of heat on its population and generate impact-based heat wave alerts for specific locations.
Why is it important to measure heat index?
Hot air can hold more moisture than cold air. Therefore, when temperature rises, the air’s capacity to hold moisture also increases, thus affecting the apparent temperature or heat index. Humidity is typically higher during heat waves — which is why the heat index at the time is usually higher than just the temperature, because humid air can feel hotter to humans.
High humidity can lead to heat stress, meaning the body is unable to get rid of excess heat. Humans usually maintain a core temperature in the range of 36.1 to 37.2 °C. When the body is unable to get rid of excess heat, the heart rate increases due to a rise in core temperature, leading to heat-related exhaustion and rashes, among other symptoms. It can also be fatal if not addressed promptly.
At high temperatures, the human body can lose excess heat through perspiration and cool itself. But when humidity is high as well, it is difficult to sweat and then for that sweat to evaporate because the air around is already saturated with moisture. This makes it difficult for the body to lose heat. On the other hand, if the humidity is low, evaporation of sweat is easier, thus making the apparent temperature feel close to the actual air temperature.
This is why a measure of heat index is more useful than just the temperature to gauge the impact of heat on humans.
For example, the NWS chart above shows that if the temperature is 31 °C (88 degrees Fahrenheit), a relative humidity of 40% only means that people need to be cautious of fatigue with prolonged exposure and/or physical activity. But, if the relative humidity at the same temperature is 95%, heat cramps or heat exhaustion is likely, whereas prolonged exposure and/or physical activity can also lead to a heat stroke.
A heat index value of 67°C or above can be extremely dangerous for people and animals who have direct and prolonged exposure, says Tashina Madappa Cheranda, senior associate at the Centre for Study of Science, Technology and Policy.
“With climate change, it is likely that we will continue to witness record-breaking heat index values across the world. We will need to prepare and adapt to such extreme conditions by investing in early warning, making changes to work timings, and finding sustainable cooling solutions,” she added.
- Heat index, also known as apparent temperature, is a measure of how the temperature feels to humans. Relative humidity is an important factor that determines heat index, along with air temperature.
- A complex formula to calculate heat index was published by Dr. Robert Steadman, a professor in the textiles and clothing department of Colorado State University, in 1979. In 2024, India is scheduled to launch its own heat index to quantify the impact of heat on its population and generate impact-based heat wave alerts for specific locations.
- A heat index value of 67°C or above can be extremely dangerous for people and animals who have direct and prolonged exposure, says Tashina Madappa Cheranda, senior associate at the Centre for Study of Science, Technology and Policy.
Published - September 06, 2023 04:43 pm IST
