What exactly is the heat index?
Summers in Houston are brutal! We all know that. We also know the adage, "It's not the heat. It's the HUMIDITY!" Oh, how true it is!
Actually, both the heat AND the humidity matter when it comes to feeling miserable during the summer. The combination of temperature and relative humidity is expressed as the heat index, a simple number that tells us just how dangerous the heat/humidity combination is at any given time.
When the temperature rises into the 90s during the summer, our bodies have to work to stay cool. The mechanism our bodies employ to cool off is sweating. Sweat builds on our skin. It evaporates into the air. The evaporation process actually pulls heat away from our skin and cools us off.
For Houstonians, though, there is one big problem with the sweating process -- it's not very efficient! In the summertime in Houston, the air is already loaded with moisture, which is reflected in high relative humidity values and that sticky, thick-feeling air that hits us like a ton of bricks when we step outside. When the air is already almost full-to-capacity with water vapor, the sweat on our skin can't evaporate easily. When our sweat can't evaporate easily, we lose our natural cooling mechanism and we retain too much body heat. The result: Heat stress, heat exhaustion, and heat stroke.
We sometimes call the heat index the "feels like" temperature. It's a loosely descriptive term that indicates how high humidity levels give the illusion that the temperature is higher than it actually is. For example, if the temperature is 92 degrees and the relative humidity is 60 percent, the heat index, or "feels like" temperature, is 105! The body's natural cooling system has to work much harder -- as if it was 105 degrees outside instead of 92 degrees -- to keep us cool.
The table above (provided from the National Weather Service's great website section on the dangers of heat, which you can link to here) shows the heat index resulting from any given temperature and relative humidity. Notice how we reach the danger level when the heat index creeps toward 105 degrees. That's when, if we aren't very careful, we can suffer serious damage if we overheat.
The map below is a snapshot of heat index values around the Greater Houston area at 4pm on Friday, June 14, 2013. Temperatures were in the low to mid-90s but the heat index was well above 100 in most places!
These numbers are approaching "dangerous" levels. And we're only in the middle of June! We've got several months to go during which heat index readings will be at or above 100 degrees on a daily basis.
If you've lived in Houston for any length of time, you know how brutal the heat can get. Make sure you're aware that excessive heat can be dangerous and take the necessary precautions when you're outside for long periods of time or if you're doing strenuous activity outside. Check out the National Weather Service's heat page so that you're fully prepared!