By Chris Spears

DENVER (CBS4) – If you’re from a humid part of the country then you know a lot about the heat index. This is a calculation made using air temperature and relative humidity to come up with what it actually feels like to those working or playing outside.

In many places east of the Rockies it’s common to have air temperatures in the 90s with a heat index well in excess of 100 degrees. This is because there is so much water vapor, or humidity present in the air, which is indicated by the dew point temperature.

Often those places will see the National Weather Service issue advisories and warnings about the heat. But you rarely, if ever, see this in Colorado, even when it gets really hot outside. The reason? Colorado’s dry climate.

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When you live in a humid location and the body sweats those beads of perspiration sit on the skin and insulate the body. There is little evaporation of the sweat because there is already so much water vapor in the air. When sweat doesn’t evaporate efficiently it causes the body’s internal temperature to rise to dangerous levels.

That’s where the heat index comes in, which is a “feels like” temperature trying to account for the lack of evaporation. To use this chart simply find your location’s approximate relative humidity on the left and air temperature at the top. Where that row and column meet you will find the current heat index.

In Colorado our heat index is negative because the air is so dry. When a person sweats the rate of evaporation is so great that it actually has a cooling effect on the current air temperature, by as much as 2 to 5 degrees. We are very fortunate to have dry air due to our high elevation and distant location from the primary source of humidity in the United States, which is the Gulf of Mexico.

While a negative heat index certainly makes the extreme heat more tolerable, it doesn’t lower the potential danger for those who work and play outside. You still need to drink plenty of water, wear sunscreen and avoid long exposure times to the heat.

Never leave pets or children inside of a parked vehicle, even with all four windows slightly open. When air temperatures hit 95°F or higher the interior temperature of a car or truck can top 115 to 130°F within a matter of minutes. Look before you lock!

Chris Spears


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