How can there be fog when the temperatures are so cold?

Dear Tom,

I was driving near Champaign, IL, in early December, in dense fog with a temperature of 7 degrees (on my car thermometer). How can there be fog with it being so cold?

Maggie Robinson,

Dear Maggie,

Fog consists of tiny droplets of water in the air at or near the Earth’s surface. Fog droplets persist in liquid form even at air temperatures well below freezing because, generally, the liquid droplets need a surface to freeze upon. Liquid fog can occur even at temperatures well below zero degrees, but below -40 degrees the fog particles do indeed freeze (and this is called ice fog).

Your experience with fog at a temperature of 7 degrees is not unusual. As air is cooled, it can contain progressively less and less water vapor and its relative humidity increases until, when its humidity reaches 100 percent, the water vapor begins to condense into fog.