What is freezing fog? Here's what's behind a first-of-its-kind advisory from NWS Pittsburgh.

The fog on Thursday morning prompted the National Weather Service in Pittsburgh to issue a freezing fog advisory for some counties where the temperature was at or below freezing. 

This was the first time this office has issued a freezing fog advisory, but other National Weather Service offices across the country have been doing this for years and continue to issue this when warranted.

Freezing fog itself is not a regular occurrence, but it is also not a rare phenomenon and has occurred many times before in western Pennsylvania and other parts of the world. 

Fog is essentially a cloud based near ground level. Most clouds are made up of supercooled liquid water droplets that can exist in temperatures as cold as -10 degrees to -40 degrees (aka well below freezing). When these droplets interact with a surface that is at or below freezing, they can freeze on contact, creating a coating of ice. These droplets can also create a coating of rime ice on trees and other exposed surfaces as shown in the figure below. 

Believe it or not, anytime we fly through clouds that are made of liquid droplets, we are essentially dealing with the same phenomenon just thousands of feet above ground.