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Name: Shawn R. W.
Status: student
Age: 30s
Location: N/A
Country: N/A
Date: 5/26/2004


Question:
Why do we only see dew drops in the early morning?MO


Replies:
Dear Shawn-

We only see dew in the morning because of the way dew forms. For dew to form, air must be cooled to its condensation point. There must be little movement to the air or it will remain mixed and not form dew, but will form fog instead. (Fog is formed in other ways as well). These conditions usually occur overnight, when the winds die down, and the air becomes still. Radiational cooling from the ground or structures near the surface, such as automobiles, mailboxes, etc. cools the air in contact with the ground or structures. The humidity in the air condenses into droplets of moisture we call dew.

After sunrise, the ground begins warm up and usually the wind increases. Both these events warm the air and evaporate the dew.

Wendell Bechtold


Shawn,

Dew (or frost) forms on surfaces at night when the surface cools to the dew (or frost) point (the temperature at which saturation occurs). This is helped by evaporation of water from the surface (soil, grass), adding to the moisture in the air. Normally you get dew/frost on a night without clouds, as radiational cooling of the Earth's surface increases the rate of cooling. Since there usually isn't much wind at night, this allows moisture from the surface to build up near the surface, enhancing the effect. If there is wind, the moisture is removed from the vicinity of the surface and dew will not form.

After the Sun rises, the surface and the air are warmed, raising the temperature and causing the dew to evaporate. Thus we normally only see dew in the morning. However, I can remember some days when dew formed on a clear night, then clouds moved in in the early morning and stayed around for most of the day, leaving the dew in place for several hours.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory


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