Thursday, July 21, 2011

7/21 - 2pm - Enough Excessive Heat to Go Around

93° in Louisville is hot on its own, but when you couple that with a very high 77° dew point (which puts relative humidity at 60%), you're talking about a heat index of 107°. We're well on our way to a high near 97° this afternoon. It's just hot. An Excessive Heat Warning, on left shaded in purple, remains in effect for the entire Louisville area until Saturday night as heat index values soar past 110° during the maximum heating of the day. Regardless of where you're from and the weather you're used to, your body cannot sustain itself in the hot, humid air we have in Louisville and much of the Midwest right now. Be sure to drink plenty of water and limit your activity outside this week to avoid heat-related illness.

Heat Index?
NC State Climate Office

I've been mentioning the term heat index for weeks now, so what exactly is the heat index you ask? Most people know it as the "feels like" temperature or what it feels like because of the humidity. That's basically it in a nutshell, but there's more to it. Mathematically, the heat index is a product of an equation that uses temperature and relative humidity. Factors like perspiration and body size averages are held constant, so the heat index is an educated guess at the "temperature" that most people are feeling. Don't let the "educated guess" usage fool you though, because higher humidity and in turn heat index values lessen the amount of evaporational cooling on your skin. If your body can't cool properly through this evaporation, you can overheat quite easily. It's interesting and useful to note that heat index values are based on what it feels like in the shade. Being in the sun can increase the heat index by up to 15°. Below is a table from NWS Pueblo, CO with heat index values and associated heat disorders:

With the heat we'll have a chance for afternoon scattered storms, which will increase significantly as we get closer to the passage of a cold front on Monday. Today's storms have been and should be staying south of Louisville, so we'll see no relief from the heat today!

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