The "Heat Index" is the measure of the 'apparent temperature' on the body over and above the actual air temperature owing to the encumbering affects of humidity. With a high humidity, your ability to sweat away the heat is greatly diminished (and therefore so is your ability to keep cool). This diminishing can be calculated, and is actually calibrated to reflect an air temperature of similar difficulty if there wasn't any humidity issue. The highest levels of heat index can cause heat exhaustion and heat stroke.

To use the heat index chart, find the appropriate temperature along the side of the chart. Read across until you are opposite the correct humidity. The number which appears at the intersection of the temperature and humidity is the heat index.

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For those who are more ambitious who would like to calculate the heat index, here is the formula: