Apple #120: Fog vs. Dew

I was driving home one night and moisture kept forming on my windshield so that I had to use my wipers. I wondered if this was technically fog or dew. And what's the difference between the two, anyway?

• Basically, the difference is that fog is moisture that has condensed but stays in the air, floating just above the surface of the earth in a cloud. Dew is water vapor that has condensed on the surface of things.
  

That's the simplest way to think of it. But of course there's more to it than that.

• This is dew:

• It appears on the surfaces of things in the early morning or in the evening.

• After a warm day that begins to cool, the surface temperature of things cools faster than the air. When the air cools too, it can't hold as much moisture. So the moisture that was in the air forms on the surface of cooler objects.

• Most often, the things that collect dew are thin or small objects like blades of grass or strands in a spider's web or the shallow cup of a fallen leaf.

• If the temperature is cold enough, the dew freezes and becomes ice, which is better known as frost.

Dew
(photo from University of Aberdeen School of Physics)

• Now here's fog:

• Fog is a cloud that touches the ground, but just barely.

• There are actually lots of different kinds of fog, which can form under many circumstances:
  
• Fog at the end of the day
• Fog when wind comes off water and blows over land
• Fog when rain falls out of a damp cloud into drier air below
• Fog when a clump of cold air passes over much warmer water and makes steam
• Valley fog in mountain valleys
• Upslope fog as air rises up the slope of a mountain and cools and condenses as it does so.

Fog
(photo from the University of Houston's Fog in the Woods)

• For my money, the most exciting types of fog are ice fog and freezing fog.
  

• Ice fog happens when the moisture in the fog freezes into ice crystals. Temperatures have to be below freezing, and this usually happens in places like Alaska or Canada or in the Arctic.

• Freezing fog happens when the droplets in fog freeze onto surfaces. This forms a particular ice called "rime ice." It happens most often on mountainsides exposed to low clouds. It's the same thing as what happens in a freezer that's not frost-free. It looks like the wind blew and froze the moisture instantly into place.

Rime ice
(photo from Grandfather Mountain)


The moisture on my windshield certainly wasn't ice fog or freezing fog. It wasn't so late at night that you could say it might as well have been morning, which would lead me to say it couldn't be dew. However, there were no cloudy patches in the air around me that I was driving through (I do like to drive through fog. I like to make a verbal "foom" sound effect when the car first punches into it.)

The temperature probably had been quite a bit warmer earlier in the day and had cooled fairly significantly by the time I got in my car to drive home. So I'm going to say that the moisture on my windshield that I kept having to wipe away was dew.

And that's probably why using the defroster also helped, because it raised the air temperature so that the water on the window could evaporate away from my window into the air.

This also means that that old song "The Foggy Foggy Dew" is more than just inexplicable, it's enormously confusing.

Sources
Wikipedia, Fog
Wikipedia, Dew
Wikipedia, Dewpoint
Thanks, Wikipedia!
Onelook, Fog
Onelook, Dew