The heat index provides information about how hot it feels outside in the shade. It is a measure of the air temperature in relation to the relative humidity for a particular day.
The National Weather Service lists a heat index each day in the newspaper to alert people of the risk for a heat-related illness. Direct exposure to the sun can increase the risk for a heat-related illness on days when the heat index is high. Babies, older adults, or anyone with a health condition may have more risk of problems with the heat because of their age and general health.
A heat index of:
Prevention measures during days of high heat index will help reduce the risk of a heat-related illness. When the outdoor humidity is greater than 75%, losing body heat by sweating is not as effective, so other measures to keep cool are needed.
WebMD Medical Reference from Healthwise