Thursday, June 25, 2009

Today Was Hot...

...but it wasn't the hottest day of the year. However, it still made the list of the five hottest so far:

1) 95° on June 20
2) 94° on June 2
3) 92° Today
4) 92° on May 25
5) 91° on June 3

Heat Safety


Central Virginia is entering a prolonged stretch of hot weather, where afternoon highs are expected to climb into the mid-90's each afternoon Thursday through the beginning of next week. Humidity levels are also rising, and that means heat index values will rise as well into the 98-103 range. Heat index is what the air temperature feels like to a human in the shade. In full sunshine, it will feel even hotter as you add the radiational heating effect to your skin.

Heat index takes into account the actual air temperature as measured by a thermometer, the level of moisture content in the atmosphere at that temperature, and also how human bodies' ability to perspire is compromised in higher humidity levels. When you get hot, your body perspires to carry heat away from the surface of your skin as the sweat evaporates. Evaporation is a cooling process, taking heat away from your body and transferring it into the atmosphere. However, when moisture content is relatively high in the atmosphere around your body, your sweat does not evaporate as effectively. Thus, the heat remains on your body. You can calculate heat index values using this tool. Or, you can view it in a chart form.

I cannot emphasize enough that the heat index value is a measurement for what it feels like to you in the shade. In the sun, it will feel even hotter. Add another 10 to 15 degrees on top of the heat index value to get what it feels like to you outside in the sun.

Here are some thresholds to guide you (remember, these are for shade):
Heat Index 90-105: Use Extreme Caution outdoors. Heat cramps and heat exhaustion can occur, with heat stroke possible.
Heat Index 105-130: Danger. Heat cramps and heat exhaustion are likely, with heat stroke probable with extended outdoor exertion.
Heat Index 130+: Extreme Danger. Heat stroke is imminent.

Play safe this weekend!
--Carrie