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
The difference is the direct exposure to open sky. At night objects will lose more energy to space through infrared radiation than they receive from space.
This allows the object like your motorcycle to cool down more than the air. On nights where the dew point is close to the air temperature the object can be below the dew point even though the air is not.
The infrared radiation for the motorcycle under the roof isn’t lost to space. It heats up the roof which then reradiates some of that energy back at the motorcycle.
In addition to the other answers that explain why the likelihood of dew may be less inside the shed, there is a good chance your buddy simply hasn’t noticed the condensation. Even in a much more air tight garage, under certain conditions I find water on my tools in the morning, and those are inside a toolbox as well.
The first difference is temperature. The temp inside of the shed doesn’t drop as quickly so there’s less condensation on the surfaces inside the shed. The second issue is moisture levels. A shed, or room doesn’t have to be completely airtight to keep moisture out. It just has to be mostly closed off the to outside elements. Obviously some moisture gets in but it will happen very slowly and by the time the sun rises and the temp comes back up, the space wont have accumulated a noticeable amount of moisture. This is why your home and garage remain dry even though neither of these are airtight. If your friends shed was in Alaska in the winter where it was perpetually dark, it would probably accumulate moisture over time.
Not surprising, since it’s not well know, that 53 mostly redundant comments so far and nobody has mentioned this key contributing factor, and the one that pretty much answers your question: ***the primary source of the moisture that produces morning (and night-time) dew is the topsoil itself.*** Inside the shed there’s no topsoil. (Under a forest canopy, the trees night-time aspiration is another big contributor, as are ground fogs when they occur.)
https://www.weather.gov/source/zhu/ZHU_Training_Page/fog_stuff/Dew_Frost/Dew_Frost.htm#:~:text=Second%2C%20the%20soil%20is%20often,condensation%20especially%20on%20clear%20nights.
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