Why no dew on car in the morning?
October 11, 2015 10:07 AM   Subscribe

Why is it that in the morning that a car parked overnight in the garage has no condensation on it but a car parked outside in the driveway is covered with dew? The garage in question is completely open on one end since it's an old horse barn with no doors. I would think the atmosphere in the garage (relative humidity and temp) would be virtually identical to the outside.
posted by Kevin S to Science & Nature (8 answers total) 1 user marked this as a favorite
 
In the garage there is a lower volume of air, from which moisture condenses. The floor is always dry to begin with. I do not mean the atmosphere is less dense, but the cube of air in the garage has a more finite and stable climate.
posted by Oyéah at 10:18 AM on October 11, 2015


Best answer: Dew is water vapour in the air that condenses on surfaces that are colder than the dew point of the air. The air inside and outside your garage have pretty much the same temperature and humidity, but the temperature of the surfaces relative to the humidity level of the air is what causes dew. When those surfaces get cold enough, the moisture in the air condenses onto them.

So, the car outside is colder than the car inside the garage. Why? The car outside, especially on a clear night, radiates heat out. For those surfaces pointing at the sky, essentially zero heat is radiated back to the car, so those surfaces get cold enough for dew to form. For the car inside the garage, heat radiates out, but much of that heat loss is balanced by heat radiating in from the surfaces above it, so it doesn't get as cold and dew doesn't form as the surface temperature stays above the dew point.
posted by ssg at 10:23 AM on October 11, 2015 [8 favorites]


Best answer: And, to take this a little further: Why is the delicate balance where the exposed surface is colder than the dew point but the covered surface is above the dew point maintained for hours? Because of the energy released when water is converted from vapour to liquid. The huge amount of energy required to make water vapour (e.g. to boil away a pot of water) is released when that process runs in reverse, when the water changes from vapour to liquid form. So when dew starts to form, the dew itself increases the temperature of the surface that it forms on. Where does that energy go? For your outside car, it can radiate away into the clear night sky. For the covered car, much of the extra energy is reflected or re-radiated from the surfaces above.

Because the amount of energy released when water changes from vapour to liquid is large, as soon as a little bit of dew starts to form on the covered car, it heats it up so that it stays above the dew point. Only the uncovered car can radiate heat out fast enough to keep the surface temperature below the dew point and thus to keep collecting dew.

Essentially, in order to collect dew you need to be able to get rid of the huge amount of energy that condensing water releases quite quickly. You need a big reservoir that can take a lot of energy without reflecting it back to you. The sky is such a reservoir, the ceiling of your garage is not.
posted by ssg at 10:43 AM on October 11, 2015 [4 favorites]


Best answer: If you park your car up next to a house you'll notice less dew on the side facing the house. I assume that's a demonstration of just how much heat is radiating from the house (not from inside the house, but just stored heat accumulated in the bricks during the warmth of daytime).
posted by emilyw at 10:47 AM on October 11, 2015


Best answer: If the surfaces of the car are radiating out to a clear sky, they will approach the lower temperature of the sky, and hence be a cooler surface for water to condense on.
posted by nickggully at 11:17 AM on October 11, 2015 [1 favorite]


Response by poster: Thanks, the radiation explanation makes a lot of sense. Given all the time I've devoted to understanding the physics of global warning and radiant energy, I should have thought of it.
posted by Kevin S at 12:12 PM on October 11, 2015


I think also that, as the night grew colder, the heavy, cold air settled closer to the ground. Some water vapor that could have reached your car actually settled on the roof of the garage.
posted by Cool Papa Bell at 12:14 PM on October 11, 2015 [2 favorites]


Response by poster: Cool papa bell, interesting point. From a little googling I see that you might be referring to "nocturnal inversion". But that would seem to be an exception condition in which there would be ground fog?
posted by Kevin S at 1:28 PM on October 11, 2015


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