Dew Drops Only in Morning
Name: Shawn R. W.
Why do we only see dew drops in the early morning?MO
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
After sunrise, the ground begins warm up and usually the wind increases.
Both these events warm the air and evaporate the dew.
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
David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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Update: June 2012