• Marshall Armstrong

Humidity And Dew Point

Humidity and Dew point can sometimes be confusing. They are measurements of different things. When we see a weather forecast on television the meteorologist uses these terms but often, they don't explain what they mean. So let's see if we can make sense of these two weather terms.

Humidity is the measurement of water vapor in the air. When the humidity is 100% it means that the atmosphere is completely saturated with moisture, and can hold no more. When the weather is hot and sticky we often state that it feels really humid outside. But humidity is not always associated with heat. This morning, my home weather station showed that the air temperature was 52° F., and the humidity was 99%. At 52° F, it did not feel "sticky" outside. So humidity is simply the amount of moisture (water vapor) in the air.

What then, is Dew Point? Simply put, Dew Point is the temperature at which the air would have to cool down to in order for condensation to occur. Condensation is the changing of water vapor (gas) to liquid water (dew). Did you notice that I said Dew Point is a temperature? That's right, Dew Point is measured as a temperature. But what does this mean?

It means that if the Dew Point if 52° F and the air temperature cools overnight (or any time, really) to 52° F, condensation will occur and you'll have dew covering everything. Condensation actually starts happening as the Dew Point and temperature get close together. Dew Point is a much more accurate indicator of how "sticky" it will feel outside. If the Dew Point is in the 50's or lower it will feel pretty comfortable outside, unless you live in arid areas where it's dry. When the Dew Point climbs up into the 60's you start to notice the "humidity" feeling and when it's up in the 70's, it feels tropical outside.

So now we know the difference between, humidity and dew point. If you want to know how it's going to "feel" outside, check the Dew Point, not the Humidity!

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