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Why Your Local Weather Authority uses dew points to describe humidity

You may have heard of “relative humidity,” but it’s not a good indicator of comfort level

Explaining "humidity" - Dew point vs. Relative humidity
Explaining "humidity" - Dew point vs. Relative humidity (Copyright 2021 by WSLS 10 - All rights reserved.)

ROANOKE, Va. – Our much-anticipated break from the June humidity has arrived!

Your Local Weather Authority has been forecasting drier air to move into our corner of the Commonwealth and that forecast has been verified.

You’ve probably heard Chris or Jeff talk about dew points to describe the change in how it’s feeling outside, rather than relative humidity.

Let me explain our reasoning!

Dew points truly give us a measure of moisture in a parcel of air. If you see the dew point is elevated (think 65 or 70 degrees), you know it’s feeling muggier.

On the other hand, relative humidity is simply a ratio of the temperature and dew point. So there will be times when the relative humidity is 100%, but it’s not feeling all that humid outside.

Below, you’ll see the past 24-hour weather conditions at Roanoke-Blacksburg Regional Airport. I’ve specifically highlighted the dew point and relative humidity.

List of 24-hour weather conditions at Roanoke-Blacksburg Regional Airport (Copyright 2021 by WSLS 10 - All rights reserved.)

Look at the red box. That’s the readings from roughly 3 to 7 a.m. The dew point stays relatively unchanged, but the relative humidity values fluctuate!

If you stepped outside during that timeframe, you probably would not feel much difference in the “comfort” level. The relative humidity only rose to 83% at 4 a.m. because the temperature and dew point got closer together.

The relative humidity started going down again once the sun rose and temperatures started to warm up.

So now that you have a better understanding of dew points and what they mean for our comfort level, you might be wondering when the muggies will return!

Tracking moisture levels - Sunday 8 a.m. (Copyright 2021 by WSLS 10 - All rights reserved.)

We believe a disturbance in the Bay of Campeche will gain tropical characteristics and move north into the Gulf of Mexico. It may eventually come onshore in the United States.

Once that happens, tropical moisture will be sent our way and it will feel more humid outside this weekend in Southwest and Central Virginia.


About the Author:

Justin McKee presents the weather forecast on 10 News Saturday and Sunday mornings from 6 to 8 a.m. He also fills in for other meteorologists during the week.