Dewpoint is probably the least understood weather term, and for good reason. Most people know it is related to humidity, but don’t know how. The technical description itself is somewhat difficult to understand. But making the jump from understanding WHAT dewpoint is, to understanding HOW it affects us is even more difficult. I will try to make it as easy to understand as possible.

Long, drawn-out explanation
If you want the simple answer, skip to the bottom.

Do this experiment. Or at least imagine yourself doing this experiment.

On a muggy, summer day, go to the store and by a can of soda off the shelf. Then go to a soda machine and buy another can of soda. One can is warm and the other is cold.

Take both cans outside. Wait a few moments and you will notice something happening. The cold can is starting to get wet while the warm can isn’t.

Dew is forming on the cold can. This is the same dew that you find on the grass on a cool morning. The temperature of the cold can was low enough that moisture in the air could condense on it. The temperature of the warm can was not low enough.

Now take the warm can and put it in a refrigerator for about 10 minutes so that it cools some. Then take it back outside. Did dew form on it? No? Then put it back in the refrigerator to cool it a little more and try again. Still no dew? Then try cooling it even more.

This may take awhile so be patient.

Eventually the can will get cold enough that dew will form. At that point, the temperature of the can will have reached the dewpoint.

Technical definition: The dewpoint is the temperature at which dew will start to form on an object.

If you did this experiment on a day that was not muggy, it would take longer for you to reach the dewpoint. That’s because there is less water in the air.

The more water there is in the air, the sooner that water will condense on the can. The less water there is in the air, the colder the can has to be for dew to form.

So the dewpoint is a way of measuring how much water is in the air.

Get to the point, Mark!

You already know that the more water there is in the air, the more “muggy” it feels. And now you know that dewpoint is a way of measuring how much water there is in the air.

So (here’s the point) if you know the dewpoint, you know how muggy it will feel when you go outside.

And here’s a dewpoint scale that will help you…

Less than 50....Not Muggy
50 - 59....….....slightly muggy
60 - 69....….....moderately muggy
70 - 79...…......very muggy

In our part of Pennsylvania the dewpoint almost never reaches 80 or above. But when it does, the air feels EXTREMELY muggy.

Dewpoint is a better measure of “mugginess” than relative humidity because the dewpoint isn’t affected by changes in the temperature. Look at these two examples…

On a day where the temperature is 40° and the dew point is 37°, the relative humidity is 90%. Even though the relative humidity is high, it would not feel “muggy”.

On a day where the temperature is 95° and the dew point is 75°, the relative humidity is only 50%. But this day would feel very muggy.

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