METEOROLOGIST JEFF HABY
Morning condensation (dew) is very common in some regions and can easily be forecasted. The favorable weather
elements for dew include clear skies, light wind, decent soil moisture, and low night-time
Dew forms when the temperature becomes equal to the
dewpoint. This often happens first at ground level for two
reasons. First, longwave emission causes the earth's surface to cool at night. Condensation requires the
temperature to decrease to the dewpoint. Second, the soil is often the moisture source for the dew. Warm and
moist soils will help with the formation of dew as the soil cools overnight.
of warm and moist soil during the night will cause condensation especially on clear nights. Clear skies allow
for the maximum release of
longwave radiation to space. Cloudy skies will reflect and absorb while re-emitting longwave radiation back
to the surface and that prevents as much cooling from occurring. Light wind prevents the mixing of air right
at the surface with drier air aloft. Heavier dew will tend to occur when the wind is light as opposed to when
the wind is strong. Especially when
soils are moist, the moisture concentration will be higher near the earth's surface than higher above the earth's
surface. As the air with higher moisture concentration cools, this air will produce condensation first.
moisture is EXTREMELY critical to producing dew (especially heavy dew). Dry regions that have not received
rain in over a week or two are much less likely to have morning dew (especially a heavy dew). Once the soil
gets a good soaking from a rain, it takes several days for the soil to lose the moisture through evaporation.
If nights are clear after a good rain, dew can be expected every morning for the next few days (especially in
regions with abundant vegetation, clear skies and light wind). The dewpoint depression is important because it
determines how much the air will need to cool to reach saturation. With a large dewpoint depression (greater
than 25 units of F), quite a bit of night-time cooling will need to take place in order to produce dew. A low dewpoint
depression with the other factors favorable for dew is more likely to produce heavy dew.
Dew is important to
forecast since it impacts people. Dew can produce a thick film of water all over the car in the morning
(can be especially annoying for people that don't have a garage). Time has to be spent wiping the water
off the windows in order to see
on-coming traffic. Dew is also important to agriculture. Dew recharges the soil moisture and limits evaporation
from the soil during the time the dew is forming. Dew can make the mowing of the lawn more difficult. It is
much easier to mow the lawn in the late afternoon when the dew has evaporated than it is in the morning. Wet
grass clumps together and sticks to everything. Also, you are more prone to getting a dirty shoe when walking
on dew covered grass as compared to dry grass.