Dew Collecting and Color
Country: United Kingdom
I am doing some research on maximising dew collection and I was
reading one of your previous questions and answers regarding colour and
radiation. I am wondering what colour would be most efficient in
facilitating dew? Would it be black as black should cool quicker and
become colder than the local air temperature and dew point can be reached
in the right conditions. or white which may not have stored as much heat
and thus cool quicker and collect dew when the conditions are correct?
I think that your experiment is overwhelmed by other factors than color
differences, and variables that you cannot control.
Several of many issues are:
1. Air speed (wind).
2. Sunlight, i.e. (cloud cover).
3. Surface (dew collection surface).
4. Quantitative measurement of
the amount of the amount of dew formed.
All of these, and others, require
careful control that is often difficult to control.
In principle "black" should absorb the most radiation (not necessarily
visible) -- it could be infrared.
These various factors make your experiment, that seems so simple, very
difficult when executed.
At night, infrared radiation (IR) dominates. Most objects,
no matter what colour, absorb IR about equally and release
energy in the infrared about equally. Thus, most objects,
if placed in the same conditions, will cool at the same rate
at night. Therefore, there is probably no advantage of one
colour over another for a dew collecting surface.
There are a few conditions, the first of which you touched on,
that will affect dew collection. First and probably most
important is the temperature of the object at sunset. If
one object is much warmer than another, the warmer object will
release energy more quickly, at least initially, but overall will
take longer to cool to the dewpoint. So, the lower the temperature
of the object at sunset, the more likely that dew may form on it.
However, dew collection is also dependent on whether the object can
cool to the dewpoint. If the air temperature cools to the dewpoint
temperature at night, objects in the air will probably cool to or
below the dewpoint and have dew form on them. Objects can actually
cool to a slightly lower temperature than the surrounding air
(because they are better radiators of energy than air that contains
water vapor, which tends to absorb IR and thus not cool as fast)
and therefore slight dew can form on objects even though the air
temperature does not quite get down to the dewpoint temperature.
If an object is located near a warmer object, it will absorb IR
from the warmer object, keeping it warmer than it would be if
it were not near the warmer object. Thus, dew is less likely to
form on objects that are close to houses (which usually stay warmer
than the air), as opposed to objects that are out in the middle
of your lawn.
There are other factors that affect dew formation, including
wind speed. Higher wind speeds can keep the air near the Earth's
surface mixed up, distributing energy from the surface into the
air above, thereby reducing cooling of the air during the night
and reducing the likelihood of dew formation.
David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory
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Update: June 2012