Everything You Wanted To Know About The Heat Index, But Were Afraid To Ask

It's not the heat, it's the humidity.  At least that's what we tell ourselves during the long, hot summer months. 

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.