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  • Writer's pictureTodd Yakoubian

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

What is the Heat Index?

It's a daily way of life in the summertime, but there's real science behind it. When moisture becomes excessive, the body has a difficult time cooling itself.

We have a natural A/C for our bodies. When we sweat, it evaporates giving us a cooling effect to our body temperature. However, if the air is loaded with moisture, the sweat we produce can't evaporate efficiently and it feels hotter than what the thermometer reads. We call the "apparent temperature", the HEAT INDEX! This is the mathematical formula... 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 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.

It's very possible the first heat advisories of the season will be issued next week. Now you know!

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