What is Heat Index?
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The heat index is a measure used to describe how hot it feels when relative humidity is factored in with the actual air temperature. Essentially, it combines the effects of heat and humidity to give a more comprehensive understanding of how hot it feels to the human body.
Here’s a breakdown of how it works:
* Air Temperature: The actual temperature of the air.
* Relative Humidity: The amount of moisture in the air relative to the maximum amount of moisture the air can hold at that temperature.
When humidity is high, sweat does not evaporate as efficiently from the skin, making it harder for the body to cool down. The heat index accounts for this by indicating a “feels-like” temperature that is often higher than the actual air temperature.
The heat index is especially useful for assessing potential heat-related health risks. For example, during a hot and humid day, the heat index can help people understand the potential strain on their body and take necessary precautions.
The heat index of a given combination of (dry-bulb) temperature and humidity is defined as the dry-bulb temperature which would feel the same if the water vapor pressure were 1.6 kPa. Quoting Steadman, “Thus, for instance, an apparent temperature of 24 °C (75 °F) refers to the same level of sultriness, and the same clothing requirements, as a dry-bulb temperature of 24 °C (75 °F) with a vapor pressure of 1.6 kPa.”
The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature. This has important considerations for the human body’s comfort. When the body gets too hot, it begins to perspire or sweat to cool itself off. If the perspiration is not able to evaporate, the body cannot regulate its temperature. Evaporation is a cooling process. When perspiration is evaporated off the body, it effectively reduces the body’s temperature. When the atmospheric moisture content (i.e. relative humidity) is high, the rate of evaporation from the body decreases. In other words, the human body feels warmer in humid conditions. The opposite is true when the relative humidity decreases because the rate of perspiration increases. The body actually feels cooler in arid conditions. There is direct relationship between the air temperature and relative humidity and the heat index, meaning as the air temperature and relative humidity increase (decrease), the heat index increases (decreases). Recently, the weather station near Dayrestan Airport in Iran’s southern registered the heat index of 82.2°C and a dew point of 36.1°C (97°F), which if confirmed will set a new record for the highest heat index.