Forecast Secret: The Dewpoint - | WBTV Charlotte

Forecast Secret: The Dewpoint

CHARLOTTE, NC (WBTV) - Have you ever heard us say the overnight low is dependent upon the dewpoint?  The lower the dewpoint, the more the temperature can drop. The higher the dewpoint, the more moderate (and possibly muggy) the overnight low will be.

First of all, what is the dewpoint? It's a measure of how much water vapor is in the atmosphere. It's a better indicator of how much moisture is present since relative humidity is just that – relative to the temperature. The relative humidity changes all day as the temperature changes. The dewpoint is more absolute.

If you want to have an idea of how low the temp will drop, look at the dewpoint. The temperature can't drop below that number because when the temp and dewpoint are equal, the air is holding all the moisture it can. When the temp drops to the dewpoint, latent heat will then be released as the air condenses instead of the temp falling farther.

When the dewpoint is very low and the air is quite dry, the temperature can fall quite a bit. There's nothing to keep the heat in. With a lot of moisture, some of the heat will be radiated back to the earth's surface, keeping the temp from falling. It acts sort of like insulation!

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