Why Do Heat Index and Wind Chill Temperatures Exist?

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Unlike air temperature which tells how warm or cool the actual air around you is, apparent temperature tells you how warm or cool your body thinks the air is. The apparent, or "feels-like" temperature, takes into account the real air temperature plus how other weather conditions, like humidity and wind, can modify what the air feels like.

Not familiar with this term? More than likely, the two types of apparent temperature -- wind chill and heat index -- are more recognizable.

 

The Heat Index: How Humidity Makes Air Feel Hotter

During summer, most people are concerned with what the daily high temperature will be. But if you really want an idea of how hot it will get, you'd do better to pay attention to the heat index temperature. The heat index is a measure of how hot it feels outdoors as a result of air temperature and relative humidity combined.

If you've ever stepped outside on a fair 70 degree day and found that it feels more like 80 degrees, then you've experienced the heat index firsthand. Here's what happens. When the human body overheats, it cools itself by perspiring, or sweating; heat is then removed from the body by evaporation of that sweat. Humidity, however, slows the rate of this evaporation. The more moisture the surrounding air contains, the less moisture it's able to absorb from the skin's surface through evaporation. With less evaporation occurring, less heat is removed from the body, and thus, you feel hotter.

For example, an air temperature of 86°F and a relative humidity of 90% can make it feel like a steamy 105°F outside your door!

The Wind Chill: Winds Blow Heat Away From the Body

The opposite of the heat index is the wind chill temperature. It measures how cold it feels outdoors when wind speed is factored in with the actual air temperature.

Why does the wind make it feel cooler? Well, during the wintertime, our bodies heat (through convection) a thin layer of air just next to our skin. This layer of warm air helps insulate us from the surrounding cold. But when the cold winter wind blows across our exposed skin or clothes, it carries this warmth away from our bodies. The faster the wind blows, the faster the heat is carried away. If the skin or clothes are wet, the wind will lower the temperature even more quickly, since moving air evaporates the moisture at a quicker rate than still air would.

Apparent Temperatures Can Have REAL Health Affects

Although the heat index isn't a "real" temperature, our bodies react to it like it is. When the heat index is expected to exceed 105-110°F for 2 or more consecutive days, the NOAA National Weather Service will issue excessive heat alerts for an area. At these apparent temperatures, the skin essentially can't breathe. If the body overheats to 105.1°F or more, it is at risk for heat illnesses, such as heat stroke.

Similarly, the body's response to a loss of heat by the wind chill is to move heat away from the internal areas to the surface in able to maintain an appropriate body temperature there.

The drawback to this is, if the body is unable to replenish the heat being lost, a drop in core body temperature occurs. And if core temperature drops below 95°F (the required temperature for keeping up normal body functions) frostbite and hypothermia could occur.

When does apparent temperature "kick in?"

Heat index and wind chill temperatures only exist on random days and at certain times of the year. What determines when this is?

The heat index is activated when...

  • the air temperature is 80°F (27°C) or higher,
  • the dew point temperature is 54°F (12°C) or higher, and
  • the relative humidity is 40% or more.

Wind chill is activated when...

  • the air temperature is 40°F (4°C) or less, and
  • wind speed is 3 mph or greater.

Heat Index and Wind Chill Charts

If the wind chill or heat index is activated, these temperatures will be shown in your current weather, alongside the real air temperature.

 

To see how different weather conditions mix to create heat indices and wind chills, check out this heat index chart and wind chill chart, courtesy of the National Oceanic and Atmospheric Administration (NOAA).