What is the Heat Index?

The heat index is the felt temperature outside, given the current shade temperature and humidity level. When there is high relative humidity along with high temperatures, it can feel even hotter than it really is. The heat index is used by meteorologists and weather experts in the US. In Canada, the Humidex is used instead.

To calculate the current heat index, you need to know the current outside temperature and the percent relative humidity. You can get this information from the news or online weather reports. The formula below will tell you what the outside weather feels like for a given temperature and humidity level. You can also use the convenient heat index calculator on the left.

Heat Index Equation

If the outside temperature is T degrees Fahrenheit and the humidity is H percent, then the felt temperature is given by the expression

(2.049)T + (10.143)H - 42.379 - (0.225)TH - (0.00684)T² - (0.0548)H² +
(0.00123)T²H + (0.000853)TH² - (0.00000199)T²H²

Example: Suppose the current temperature is 90°F and the humidity is 55%. Then the felt temperature is

(2.049)90 + (10.143)55 - 42.379 - (0.225)(90)55 - (0.00684)90² - (0.0548)55² +
(0.00123)(90²)55 + (0.000853)(90)55² - (0.00000199)(90²)55²
= 96.4°F

Even though the outside temperature is 90°F, the humidity makes it feel more like 96.4°F

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