This article is about how to calculate heat index. Heat index is a measure of how hot it feels to the human body when the air temperature and relative humidity are combined. It is also known as the "apparent temperature" or the "feels-like temperature" because it reflects the effect of humidity on the perception of heat.
How to Calculate Heat Index?
The heat index can be calculated using a complex formula that involves many coefficients and variables. The formula was developed by George Winterling in 1978 and adopted by the National Weather Service (NWS) in 1979. The formula is:
Heat Index (HI) = c1 + c2T + c3R + c4TR + c5T² + c6R² + c7T²R + c8TR² + c9T²R²
Where,
HI is the heat index in degrees Fahrenheit
T is the air temperature in degrees Fahrenheit
R is the relative humidity as a percentage
c1 to c9 are constants with the following values:
c1 = -42.379
c2 = -2.04901523
c3 = -10.14333127
c4 = -0.22475541
c5 = -6.83783 x 10^-3
c6 = -5.481717 x 10^-2
c7 = -1.22874 x 10^-3
c8 = 8.5282 x 10^-4
c9 = -1.99 x 10^-6
To use this formula, you need to have the air temperature and relative humidity values in Fahrenheit and percentage, respectively. You can obtain these values from your local weather station, app, or website. Then, you plug in these values into the formula and perform the calculations.
However, this formula is only valid for temperatures above 80°F and relative humidity above 40%. For lower values, the heat index is approximately equal to the air temperature.
Another way to calculate the heat index is to use a chart or a calculator that has been prepared by the NWS or other sources. You can find these online or in print, and they are very easy to use. You just need to find the intersection of the temperature and humidity values on the chart or enter them into the calculator, and you will get the heat index.
For example, using the NWS online calculator, if the temperature is 90°F and the humidity is 60%, then the heat index is:
HI = 100°F
This is much lower than the previous estimates, but still very hot!
Why Is Heat Index Important?
Knowing the heat index is important because it can help you prevent heat-related illnesses and injuries, such as heat stroke, heat exhaustion, heat cramps, heat rash, and sunburn. These can occur when your body overheats and cannot cool down properly.
The NWS has issued a heat index chart that shows the risk level of heat-related illnesses based on the heat index values and exposure time. The chart also provides some recommendations on how to protect yourself from the heat, such as drinking plenty of fluids, wearing light-colored and loose-fitting clothing, staying in shaded or air-conditioned areas, avoiding strenuous activities, and checking on vulnerable people.
The chart is divided into four categories: caution, extreme caution, danger, and extreme danger.
Caution: Heat index values of 80°F to 90°F. Fatigue is possible with prolonged exposure and activity. Drink plenty of fluids and stay in cool places.
Extreme caution: Heat index values of 91°F to 103°F. Sunstroke, heat cramps, and heat exhaustion are possible with prolonged exposure and activity. Drink plenty of fluids, avoid alcohol and caffeine, wear light-colored and loose-fitting clothing, and limit outdoor activities.
Danger: Heat index values of 104°F to 124°F. Sunstroke, heat cramps, and heat exhaustion are likely, and heat stroke is possible with prolonged exposure and activity. Drink plenty of fluids, avoid alcohol and caffeine, wear light-colored and loose-fitting clothing, limit outdoor activities, and seek medical attention if symptoms occur.
Extreme danger: Heat index values of 125°F or higher. Heat stroke is highly likely with continued exposure. Seek immediate medical attention if symptoms occur.
Bottom Line
In this article, we have discussed how to calculate heat index. The heat index also does not account for other factors that affect thermal comfort, such as wind speed, clothing, activity level, and individual differences.
