It's Not The Heat, It's the Humidity

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Are we settling into summer?  Is our primary severe weather season over?  I'll quote Lee Corso and say, "Not so fast, my friends".  As I always say, severe weather can happen anytime of the year, but our primary severe weather season is spring and again later in fall.  It's very interesting to note that the two biggest tornado outbreaks in Arkansas weather history both took place OUTSIDE of our traditional spring severe weather season: January 21st, 1999 and June 5th, 1916.

IF our primary season is over (BIG IF), how will we look back at it?  Outside of 1 thunderstorm, it was very, very quiet.  However, 1 storm changed everything for our state and that's all it takes.  That 1 storm produced 5 tornadoes and killed 16 of our fellow Arkansans.

So as our heat and humidity starts to build, it's worth looking at the heat index, the combination of heat and humidity.  Did you know excessive heat is one of the leading weather related killers in the United States? Because the body can't cool itself efficiently with moisture in the air, you feel hotter than the actual temperature.  The equation which is used to come up with the heat index temperature is complex and assumes specific variables.  I thought it would be fun to take a look! It's important to remember that while we use the heat index, it's different for each individual person as the formula assumes everyone is 5'7'' tall,  weighs 147 pounds, etc.  Read all the variables below.

 Heat Index =    -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).


Here are just a few of the variables involved.  This is good water cooler chat!

* Vapor pressure . Ambient vapor pressure of the atmosphere. (1.6 kPa)
* Dimensions of a human. Determines the skin's surface area. (5' 7" tall, 147 pounds)
* Effective radiation area of skin. A ratio that depends upon skin surface area. (0.80)
* Significant diameter of a human. Based on the body's volume and density. (15.3 cm)
* Clothing cover. Long trousers and short-sleeved shirt is assumed. (84% coverage)
* Core temperature . Internal body temperature. (98.6°F)
* Core vapor pressure . Depends upon body's core temperature and salinity. (5.65 kPa)
* Activity. Determines metabolic output. (180 W m-2 of skin area for the model person walking outdoors at a speed of 3.1 mph)
* Effective wind speed. Vector sum of the body's movement and an average wind speed. Angle between vectors influences convection from skin surface (below). (5 kts)
* Clothing resistance to heat transfer. The magnitude of this value is based on the assumption that the clothing is 20% fiber and 80% air.
* Sweating rate. Assumes that sweat is uniform and not dripping from the body.
* Ventilation rate. The amount of heat lost via exhaling. (2-12%, depending upon humidity) * Skin resistance to heat transfer. A function of activity, skin temperature, among others.
* Skin resistance to moisture transfer. A function of the vapor-pressure difference across the skin (and, therefore, relative humidity). It decreases with increasing activity.
* Surface resistance to moisture transfer. Similar to heat transfer resistance but also depends upon conditions in the boundary layer just above skin's surface.