1
$\begingroup$

I noticed that dew only falls on horizontal surfaces, while vertical surfaces remain dry.

But dew is the condensation of water vapor contained in the air. And for the condensation process, the direction of the surface does not matter. If you bring a smooth cold object into a warm room, it will be covered with condensate evenly on all sides.

Why do vertical surfaces stay dry?

enter image description here

enter image description here

$\endgroup$
1
  • $\begingroup$ Dew doesn’t “fall’ from the sky like rain does. $\endgroup$
    – Ghoster
    Aug 25, 2023 at 17:20

4 Answers 4

7
$\begingroup$

And for the condensation process, the direction of the surface does not matter.

It can matter. The condensation process depends on the temperature. And in this case the temperature of the object depends at least in part on what it "sees".

On a clear evening, the objects around the vehicle, the ground, any trees, objects, buildings, still have quite a bit of thermal energy from the day. Overnight, these objects will radiate some of that energy away as heat.

The vertical surfaces of the car are oriented in a way to receive that heat. The roof of the car is not well-oriented to receive as much. On an evening with less moisture in the air, the radiation temperature of the sky can fall quite low. This allows the surfaces that "see" mostly sky to drop to a lower temperature, below that of the water vapor in the air.

If the car were parked under a carport or under a tree, this would block the ability to radiate to the sky and (on a night where condensation could otherwise occur) there would be much less condensation than the same car parked in the open.

$\endgroup$
2
  • 1
    $\begingroup$ This explanation makes sense. $\endgroup$ Aug 25, 2023 at 15:56
  • $\begingroup$ On my truck the side facing the house does not often get few, but the other side will. $\endgroup$
    – Jon Custer
    Aug 25, 2023 at 18:48
2
$\begingroup$

They only stay free of dew if they have higher temperature than the horizontal parts. My vertical windows of the car often have dew on it.

$\endgroup$
1
  • $\begingroup$ My experience, too. I Had to wipe copious condensation off the side windows (outside surfaces) of my car this morning. Autumn approaching in South West England ! $\endgroup$ Aug 25, 2023 at 15:15
-1
$\begingroup$

Dew is formed on all surfaces in accordance with the laws of thermodynamics, it is less observed on vertical surfaces because it was not observed at the time of its formation, and it fell down, or absorbed again through the surrounding air and re-entered in the form of air humidity

$\endgroup$
-2
$\begingroup$

Your own photos seem to provide the answer: one still sees dew even on the slanted, non-horizontal surfaces of your car's windshield, but gravity pulls it down.

The closer a surface is to vertical, the closer gravitational pull is to being parallel to the surface, and hence the more effective gravity will be at pulling the dew drops down. When the surface is perfectly vertical, the gravitational vector is parallel to the surface and hence has maximal effectiveness.

I've seen dew plenty of times on vertical surfaces as well, but gravity pulls the dew-drops down a vertical surface with maximal effectiveness. So you don't see them for long.

$\endgroup$
1
  • $\begingroup$ I believe that from the drops sliding on the glass there should have been traces. but the glass was perfectly clean. $\endgroup$ Aug 25, 2023 at 15:28

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.