Weather Whys March 12, 2015

Courtesy Texas A&M Atmospheric Sciences

Q:  Some areas near here were recently under a “freezing fog” warning. What is freezing fog?
A:  “Freezing fog is similar to regular fog, with the exception that it occurs at temperatures below 32 degrees,” explains Brent McRoberts of Texas A&M University. “Freezing fog is composed of liquid droplets, but unlike regular fog, it will freeze when it contacts a surface, similar to a freezing rain event. The droplets sometimes form crystals similar to snowflakes, and this is called frozen fog. The ingredients for all of this to happen are the same as regular fog: clear nighttime skies, low-level moisture and light winds.”

Q: Is it rare and where does it happen most often?

A: “Freezing fog is most common at the tops of mountains, but it can occur anywhere when cold air passes over warm, moist land,” McRoberts adds. “Freezing fog is relatively rare since anytime you have fog, the air temperature must cool to the exact level as the dewpoint temperature. It is uncommon in Texas to have the sufficient moisture and sub-freezing temperature necessary for freezing fog formation.”