Another very warm day here in Austin, for the 4th day in a row we cracked the 90 degree mark.  It was the 6th time we hit 90 this month and the 7th time this year, and they are starting to add up.  Once summer arrives, hitting 90-92 would be a welcome thing, however, its April.  Speaking of, does Mother Nature realize its April?  I mean, its in the 90s and its pretty summery muggy too.  So on air you might hear us starting to give not only the temperature, but the Heat Index or “feels like” temp.  So what exactly is the heat index you might ask, well its a “feels like” temperature calculation.  The Heat Index itself takes into account a lot of different variables, but the equation is actually rather simple.  The equation is long, but only requires you to know the relative humidity, and the air temperature.

I think there are 2 main reasons why we tell people what the Heat Index value is.  First, to make you aware that it might feel warmer outside than the temperature that we tell people it is outside.  Secondly, to warn people that it feels warmer than the air temperature outside.  Okay, so that sounds kinda the same, but it really is important.  When temperatures get warm, your body also heats up, the way we cool off is by sweating.  Sweating is only effective if the air around you is able to accept the sweat and evaporate it off your skin.  This gives your body a cooling effect that helps the heat transfer away from your body to keep your internal temperature where it is supposed to be.  On a very warm day, your body gets warmer faster, you sweat more, and you cool off properly.  Now on a very warm and very muggy day, your body gets warmer at the same rate as before, however when you sweat the cooling process is much different.  When the air around you is very moist, the rate of evaporation is much slower, and therefore does not cool/transfer heat at the same rate, and stays warmer longer.  If the humidity is high enough, your sweat just drips off your body, which has little to no cooling effect, and only continues to take fluids from your body.  If your body’s internal temp stays hot, you will sweat more to try to cool off more.    Since the only way to counter this process is to be prepared for it and stay well hydrated, we warn people about the heat index on sticky days.

Below is a Heat Index chart from the National Weather Service.  Temperatures are listed along the top, and along the side is the relative humidity value.  So on a day like today, with an air temperature of 90 this afternoon and humidity of 40%, that means the Heat Index value would have been 91 according to the chart.  According to the colors on the chart extreme caution should be exercised to avoid heat related disorders if you were outside for a prolonged period of time.