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Name: Benz
Status: student
Grade: 9-12
Country: United Kingdom
Date: N/A 


Question:
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?

regards Benz


Replies:
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.

Vince Calder


Benz,

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
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory



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