Sunday, January 17, 2010
FROG... or Hoarfrost
Pewaukee temps dipped into the teens last night and a fog rolled in. This morning, we were treated to a Jack Frost show on all the trees and shrubs.
Our local weather-dudes call this "frog" - meaning frozen fog. The technical term is actually "hoarfrost." Frost is to dew as snowflakes are to raindrops. When water vapor condenses into liquid water, you get raindrops and dew. When temperatures drop, water vapor condenses directly into ice, and you get snowflakes and frost. When frost forms as tiny ice crystals covering the ground, it's just called frost.
But sometimes the frost grains grow larger and are called hoarfrost crystals. This often happens when the skies are clear, or covered with high, thin clouds, and there is little or no wind to prevent the settling of foggy moisture.
These ice crystals are so large, that they almost look like the trees have a salty coating on them like pretzels!