Over the last few weeks, we have experienced several unique mornings during which fog...a phenomenon not usually associated with the middle of winter...has been reported throughout the region. This morning's fog formed as a result of low level moisture trapped beneath a layer of warm air aloft, and like the fog we typically see in the summertime it was made up of extremely tiny water droplets. This resulted in a very heavy frost on any object that was below freezing...including your car! Many of you probably had to spend a few extra minutes scraping your windshield before scooting off to work.
The last time we had fog was during the significant arctic outbreak during the middle of January. During that time, most of the fog was reported near the various rivers throughout our region...and it mainly formed as a result of the very large temperature difference between the air and the still-unfrozen water. The fog was different than what we experienced this morning, though, in that it was composed of ice crystals rather than water droplets....and was therefore classified as "ice fog" rather than normal fog.
Air temperatures during both episodes of fog were below freezing throughout the region....so why was this morning's fog composed of water droplets whereas the fog a few weeks ago was composed of ice crystals? It all comes down to just how cold it is! Water can still exist in liquid form down to about 0F....so fog that forms at temperatures above 0F will be composed of "supercooled" water droplets. Once temperatures dip below 0F, however, water can no long exist in liquid state....and "ice fog" forms as a result.