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Check the Dew Point Before Exercising Outside
Heat Index and Dew Point Explained

How do you use the dew point and heat index?
How do you use the dew point and heat index?

The heat index and dew point are two ways to measure how good of a workout or run you’re going to get outside. But one measurement is a lot easier to use than the other. Here’s how they both work.

Think of the heat index as a measure of how hot it’s going to feel.

The heat index (also known as the apparent temperature) is what the temperature will feel like to the human body when relative humidity is combined with the air temperature.

When humidity and temperature are both high, your body reacts as if it’s hotter than the thermostat may indicate. If the heat index is above 90, you should keep your outside workouts shorter. If it’s above 105, hit the water or exercise inside. There’s a convenient chart made by NOAA (The National Oceanic and Atmospheric Administration) that you can use to figure out what the heat index is going to be.

Reading the heat index chart is simple. You find the temperature along the top, then move down that column until you hit the row that matches the relative humidity. The number you land on is the heat index.

Heat Index Chart

What the heat index numbers mean.

80 to 91 degrees Fahrenheit is a lower risk level, and you should take basic safety steps like wearing light reflective clothing, drinking plenty of water, and wearing head coverings.

91 to 103 degrees Fahrenheit moves the risk level to moderate, and you should be more careful monitoring your body temperature and how you feel.

103 and 115 degrees Fahrenheit moves you into the high-risk level, and it’s probably best to take your workouts indoors or reschedule to cooler times during the day.

115 degrees Fahrenheit or higher makes the risk very high to extreme, and you might not want to be outside at all.

The dew point is all about humidity.

Here’s the definition according to the National Weather Service.

“The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%. At this point the air cannot hold more water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation.”

The simple explanation is this: The higher the dew point, the greater amount of moisture that’s in the air and the muggier you’re going to feel.

The big advantage of the dew point is that the measurement will always tell you how it will feel outside, regardless of the actual temperature or humidity. If you have a dew point of 55 degrees Fahrenheit, that will always feel comfortable. But if the humidity is 55%, it can feel very different depending on the temperature.

How to read the dew point numbers.

Dew Point Fahrenheit
Dew Point Celcius
How it Feels
> 24
Extremely uncomfortable, humid and miserable.
Very uncomfortable, humid and oppressive.
Moderately uncomfortable and humid.
Slightly uncomfortable, humid and feeling “sticky.”
Very comfortable and pleasant.
Comfortable, but the air is drier.
< 32
< 0
Comfort varies, for some, the air may be too dry.

Pay attention to the dew point when you go out, and pretty soon, you’ll know just how comfortable or uncomfortable you’ll feel when you’re outside. Remember this: As the dew point increases, it gets harder and harder for the body to cool itself. Sweat helps keep you cool by evaporating and creating a cooling effect. If the dew point is high, there’s more moisture in the air, so sweat doesn’t evaporate as quickly.

Here are some websites that post the dew point.

National Weather Service: https://www.weather.gov/arx/why_dewpoint_vs_humidity

Weather.com: https://weather.com/maps/currentdewpoint

Weather Street: https://weatherstreet.com/states/u-s-dewpoint-temperature-forecast.htm

Weather Underground: https://www.wunderground.com/

For those of you on vacation, watch the dew point closely. If it’s higher or lower than you’re used to, you need to adjust your run or outdoor workout accordingly.

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