I keep a motorcycle outdoors under a waterproof cover, but noticed that with morning dew the bike is still noticeablely wet on the inside of the cover.
Meanwhile a buddy has his bike in a plywood shed that is by no means air tight but has 4 walls and a roof, but no insulation or air handling fans/AC and he says dew is never an issue..what’s the difference?
In: Planetary Science
Dew forms because the ground (or other objects) radiate heat into space on clear nights, and thus get cooler than the air near the ground.
If the air has a lot of water vapor, then the vapor pressure of liquid water at the temperature of the object is lower than the partial pressure of the vapor in the air. It’s exactly the reverse of evaporation, which happens when the vapor pressure of water at the temperature of objects on the ground (or, say, the surface of a body of water) is higher than the partial pressure of water vapor in the air.
At night, on clear nights (when clouds aren’t reflecting infrared radiation back to the ground), objects on the ground cool down via radiation, and that sets up conditions for dew.
Something being covered — not insulated, just covered — can prevent dew formation. For example, when frost forms (which is just solid dew), typically you will see less of it under evergreen trees, or deciduous trees in the autumn before the leaves fall. That’s because the ground there is warmer, insulated from radiative cooling by the boughs of the tree above.
The plywood is doing a sufficient job of preventing radiative cooling; it’s working as a cloud. The waterproof covering is not.
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