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About The Heat Index


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.


GS I: Geography

Dimensions of the Article:

  1. What is heat index?
  2. Heat Index Calculation: Understanding the Science Behind It
  3. The Significance of Measuring Heat Index

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.

Color Codes for Experimental Heat Index:

The Experimental Heat Index uses color codes to represent different ranges of “feel like” temperatures:

  • Green: Experimental Heat Index below 35°C
  • Yellow: Experimental Heat Index between 36°C and 45°C
  • Orange: Experimental Heat Index between

Heat Index Calculation: Understanding the Science Behind It

Origins of the Formula
  • Dr. Robert Steadman, a professor at Colorado State University, unveiled a complex formula for calculating the heat index in 1979.
  • His work included two pivotal papers: “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.” These papers detailed his intricate calculations for the heat index.
Standard Assumptions
  • Dr. Steadman’s study employs a hypothetical “typical adult human of either sex,” characterized by a height of 1.7 meters and a weight of 67 kg.
Dew Point’s Significance
  • The heat index calculation places significant importance on the dew point, which marks the temperature at which gas transitions into a liquid state.
  • In terms of atmospheric moisture, it signifies the temperature at which air becomes saturated with water vapor, leading to the formation of water droplets.
  • Dr. Steadman’s calculations use a dew point of 14°C.
National Variations
  • Various countries have devised their unique heat index systems instead of relying on Dr. Steadman’s formula.
  • Canada, for instance, uses the “Humidex,” its distinct measure of heat index, based on specific criteria.
  • The U.S. National Weather Service (NWS) utilizes its chart to determine the heat index.
  • India’s Forthcoming Heat Index
  • India is poised to launch its heat index in 2024, tailored to gauge the impact of heat on its population.
  • This initiative will also generate location-specific, impact-based heat wave alerts, enhancing public safety.

The Significance of Measuring Heat Index

Effect of Temperature and Humidity
  • As temperature increases, the air’s capacity to hold moisture rises, impacting the apparent temperature or heat index.
  • Heat waves often coincide with elevated humidity levels, resulting in a heat index higher than the actual temperature.
  • Humid air can make heat feel more intense to humans.
Heat-Related Stress
  • High humidity can lead to heat stress, where the body struggles to dissipate excess heat.
  • The human body typically maintains a core temperature between 36.1 to 37.2°C.
  • Inability to shed excess heat can elevate heart rate, causing heat-related issues like exhaustion and rashes.
  • If not addressed promptly, heat stress can be fatal.
The Role of Humidity
  • At high temperatures, the body can cool itself through perspiration.
  • However, in high humidity, it’s challenging for sweat to evaporate as the air is already moisture-saturated.
  • Low humidity, on the other hand, supports efficient sweat evaporation, making the apparent temperature closely align with the actual air temperature.
Heat Index’s Value
  • The heat index serves as a more comprehensive metric than mere temperature in assessing heat’s impact on humans.
  • For instance, the U.S. National Weather Service’s chart illustrates that at 31°C, a 40% relative humidity calls for caution with prolonged exposure and physical activity.
  • However, at the same temperature, a 95% relative humidity implies a risk of heat cramps, exhaustion, and heat stroke during prolonged exposure or physical exertion.
Addressing Extreme Heat
  • Heat index values exceeding 67°C pose severe danger to individuals and animals with direct, prolonged exposure.
  • As climate change drives record-breaking heat indices worldwide, preparations and adaptations are crucial.
  • Strategies include investing in early warnings, adjusting work schedules, and implementing sustainable cooling solutions to combat extreme conditions.

-Source: The Hindu

May 2024