• Todd Yakoubian

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

It will feel like 110°, so why isn't the temperature 110°? A meme reflecting that thought has been circulating around the web the past few weeks. It all boils down to science! See what I did there?


You know what realtors say about real estate? Location, location, location. Meteorologists say, "humidity, humidity, humidity". That's the key to the next few days. There will be plenty of it in the air, especially after 4'' of rain Saturday in spots.

Sweating is a natural way the human body cools itself. If you can't sweat efficiently, that's a problem! When you step outside on a humid day, the sweat you produce has a difficult time evaporating because the air is already loaded with moisture. When you step outside on a dry day, there's plenty of room in the air for that moisture to evaporate into. Make sense? Think of it this way. On a hot and very humid day working in the yard, you walk inside an air conditioned house. You know what cooling sensation? That's the sweat evaporating off your body into that cooler and drier air inside.


There's an actual formula to determine the effects of high humidity on the human body.


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.


Don't worry, we have computers working the math for you!


The extreme heat and humidity will last through Tuesday. As a front works into the state, showers and strong storms will develop late Tuesday into Wednesday. This spell of hot and humid weather should not last long, but PLEASE take care of yourself and others! Heat is the #1 weather related killer in the United States. Football practice, band practice, etc are in full swing right now as school gets ready to start. PLEASE take breaks and fuel your body with the right liquids.






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Copyright by Meteorologist Todd Yakoubian 2020

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