WEEBLES WOBBLE, IT'S NOT THE HEAT BUT THE HUMIDITY, BUT WHAT IS THE HEAT INDEX?

Happy Father's Day Weekend and happy Summer!  We just had a record wet Spring and now the pattern is changing again.  With the remnants of Tropical Storm Bill long gone the old summer ridge has a chance to take hold and more than likely will dominate our weather for the next few months. The summer ridge refers to an upper level high pressure system that usually forms in the summertime and wobbles around back and fourth across Oklahoma during the summer months. 

Here's a look at what the upper level Summer ridge looks like on a weather map.  This is the forecast for next week from one of our medium range forecast models.  The circle over Oklahoma is the center of the high pressure system Monday afternoon.

Sometimes the summer heat dome is right over Oklahoma and other times it shifts west or even east but bottom line our sensible weather is more or less controlled by this summer heat dome as it wobbles around like a weeble all summer long.  And just like a weeble it wobbles but never really falls down until late September when the jet stream dives south again and we call that Fall.  

So expect hot weather to take hold and persist for a while.  Rain chances won't be completely zero with the best rain chances occurring when the center of the heat dome wobbles or shifts west (northwest flow) or shifts east (tropical flow).  In those cases our rain chances will go up but otherwise as long as the high pressure ridge is nearby our weather won't change all that much.

The one thing different about this Summer is what happened this past Spring.  All that rain has broken the drought and placed significant moisture back into the soil.  What does this mean for our Summer?  Two things, #1 the moisture in the ground will be evaporating into the air and since the evaporating process is a cooling process actual air temps might not be that hot this summer.  #2 with all that moisture being evaporated back into the atmosphere the humidity and heat index temps could be out of control!  It's a trade off.

But what is the heat index?  The heat index is NOT a real temperature.  It's an apparent temperature or a feels like temp.  Example:  if the air temp is 100 and the heat index is 212 water will not boil!  The heat index is an estimation of what it feels like with the actual air temp and the humidity combined but not a real temp.

Here's the heat index chart. If the temp remains constant and the humidity goes up the feels like temp will go up accordingly.

Notice, if you look at this chart above, if the actual air temp is 98 degrees and the humidity is around 50% the feels like temp or heat index is a whopping 113 degrees!  So the humidity does matter.  We deal with this every Summer in Oklahoma but this Summer with so much moisture evaporating back into the air the heat index might be a bigger factor compared to the actual temp. Hence the saying, "it's not the heat, it's the humidity" absolutely applies this year!

Did you ever wonder how we calculate the heat index.  Check this out, this is the heat index equation.  So you say you want to be a meteorologist do ya?

1. HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 6.83783x10-3T2 - 5.481717x10-2R2 + 1.22874x10-3T2R + 8.5282x10-4TR2 - 1.99x10-6T2R2
   
   where T = ambient dry bulb temperature (°F)
   R = relative humidity (integer percentage) 

I made this calculation every night during the Summer right before showtime for 27 years!  Just kidding.  Thankfully, we have computers!

But what the heck does all of this really mean to you and me?

As I stated earlier evaporation is a cooling process.  There's an equation for that too but I'll skip that this go around.  

If you're outside playing in Summer heat your body is going to perspire, sweat.  You body sweats as a natural cooling mechanism.  As the moisture or sweat on your skin evaporates into the surrounding air it cools your skin off.  So sweating is how your body cools off.  Makes sense, right.

Here's the important part...If the air surrounding your body is humid then it takes longer for your sweat to evaporate into that air and your body can't cool down.  If the air is dry then your sweat evaporates fast and you cool off.  This is why you feel hotter when it's humid outside!  Your body has a harder time cooling off.

So the heat index is an attempt to measure what it might feel like when it's hot on humid days when your body can't cool off.  On dry days there is no heat index but as the humidity goes up so does the inability of your body to cool down so the heat index rises.

There ya go.  You now know more than the average tv weathercaster.

Have a great weekend and try not to make dad sweat too much on Sunday.