A weather advisory is issued to bring to the public’s attention on a situation that may cause some inconvenience or difficulty for travelers or people who have to be outdoors. One type of weather advisory is the heat advisory. Before discussing when a heat advisory is issued by the National Weather Service, we need to define the ‘heat index’.

The heat index indicates how hot it feels. It is expressed as a function of air temperature and the relative humidity. At the same pressure and temperature, water evaporates more slowly in air that has a high relative humidity and more quickly in air that has a low relative humidity. This fact is of prime importance to the public because high humidity makes perspiration an inefficient way of removing heat by evaporation. This can lead to uncomfortable and even life-threatening conditions

A high temperature in combination with a high relative humidity prevents evaporation and makes it seem hotter than it really is. A combination of high temperature and high humidity leads to extreme heat indices as much as 40° F above the actual air temperature. In these situations, exercising outside can be fatal.
For our area, a heat advisory is issued when the heat index is predicted to be 100°F or higher during the day, or when we have four days of a heat index of 95-99. Under these conditions, the National Weather Service advises people to limit their time outdoors, drink plenty of fluids and check on neighbors, family and friends.