Weatherz School: Heat index

It’s a complaint as old as time; “The heat I can handle, it’s the humidity that’s the problem.” Not only is there science to back this old adage, but one could say that our disdain for humid heat is simply human nature.

It all begins with sweat. Our bodies like to maintain a temperature around 98.6 degrees. When our body temperature rises, sweat is released from our skin. The water from the sweat evaporates, which draws heat out of the body through a process known as evaporative cooling. But, we need the sweat to evaporate in order for the body to cool. In muggy weather, sweat fails to evaporate from your skin. This keeps the body warmer and leaves us feeling ‘sticky.’

This is why the apparent temperature, or heat index, is directly related to humidity. If we have a temperature of 95 and a dew point of 55, sweat evaporates efficiently and the heat index is actually lower than the air temperature at 93. This is hot, but it’s below Heat Advisory criteria. However, a 75 degree dew point leads to a heat index of 107. The air temperature hasn’t changed, but because of the increase in humidity, it’s now hot enough for an excessive heat warning.

The criteria for a Heat Advisory depends on what climate you live in because our bodies become acclimated to our environment. A Heat Advisory is issued for Maine or the mountains when the heat index is expected to reach 95 degrees. Meanwhile, the same advisory doesn’t go into effect for parts of the Southern U.S. until the heat index reaches 110.

Sadly, extreme heat is the number one weather related killer in the U.S. The important thing is, heat related deaths are preventable. That’s why we need to practice heat safety when the heat index is high.