Here’s the latest edition of Weather whys, a series of posts by Braniff Davis explaining the science behind weather phenomenon affecting Texas.
A reader asked us a great question on Eric’s post this morning about the meaning of a heat index. In a place as hot and humid as Houston, we see meteorologists refer to the heat index all the time, saying things like, “The air temperature is 97 degrees, but the humidity makes it feel like 107 degrees outside.” But what does this actually mean?
With heat indices, there’s a little bit of meteorology—and a little bit of perception. The heat index is an ‘apparent temperature,’ or a measure of how hot air actually feels against your skin. When our bodies sweat, that sweat evaporates off of our skin into the atmosphere. This evaporation of moisture releases latent heat away from the body and, in turn, cools you down. However, when there is already a lot of moisture in the air, the sweat doesn’t evaporate as easily. This makes it harder for our bodies to cool, and therefore, the temperature you feel on your skin is much hotter than the actual air temperature.
HOW DO WE KNOW HOW HOT IT FEELS?
Heat index is computed using an equation, but we generally just use the values in the table above to quickly determine the heat index, based on the observed temperature and relative humidity. The higher the temperature, and the higher the humidity, the higher the apparent temperature. Using this table, we can quickly see that, on a hypothetical day with an air temperature of 90 degrees F and humidity of 80 percent (not a completely unreasonable scenario in Houston), it would feel like it is 113 degrees F outside. Of course, as the temperature and humidity changes throughout the day, this apparent temperature changes, as well.
An important thing to note is that these values are calculated in the shade. According to the National Weather Service, exposure to full sunshine can increase the heat index by as much as 15 degrees F!
The higher the heat index, the greater you are at risk for heat exhaustion, heat cramps, or heat stroke. Although the heat index is only a perceived temperature, the risks are very real. And since summer is going to be around a while, everyone should familiarize themselves with the signs of heat-related illness. Stay cool!