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Do the Dew

Do the Dew

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On our farm we raise some alfalfa hay along with some other crops. We like to bale the hay when it is dry but catch it when it has some dew on it but it can have too much dew at times. So my question is what things do I need to watch for to help determine if there is going to be any dew at night or if there will be a lot.

I do know that clear, calm nights seem to bring on more dew and that when there are clouds and wind there is little dew. Why would this be and does temperature have anything to do with it. Oh, and what does dew point mean?

Thank you for your response.



Dew forms when the temperature has reached its dewpoint and moisture begins to condense. The definition of dewpoint is "The temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur." It's the temperature you need to get saturation or condensation to happen. If you think of a glass of ice water, go ahead and get one, put it out next to you. Afer a few minutes you see condensation on the outside of the glass. Did the water go through the glass? Nope! The on the glass surface has been cooled to its dewpoint and dew forms.

You are right in saying that clear nights with light wind cause more dew to form than cloudy and windy nights. Clear skies allow for maximum radiational cooling, so a clear night will lower the temperature more and we can get more dew to form. Light wind or calm wind will prevent the mixing of air from the suface to the drier air above. The moisture concentration, especially if the soil is moist will be greatest at the surface. As that air cools near the ground, it will form dew.

Dry surfaces are less likely to have dew or heavy amounts of dew. So if you haven't gotten rain in a week there won't be much dew. But if it rained, say yesterday, you can have dew for the next few nights. Soil moisture has a lot to "do" with dew.

You can also look at the dewpoint depression or the DD as we like to say. If this number is very far away from the temperature, then you'll need to cool the air a lot to form dew.

If you're looking for dew, you'll have to figure out the forecast. Will the night be clear, calm, has it rained lately? Those questions will help you forecast dew.

Answered by KSL Meteorologist Dina Freedman.

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