Christmas in Austin came wrapped in fog this year. Except for a trace of snow in 1939, weather records going back to the 1890s show Austin has never had a White Christmas. So the combination of humid air, warm daytime temperatures and cool nights that shrouded the city in a gauzy whiteness is the closest we will get to a White Christmas.
Fog is actually a cloud that touches the ground, formed when water vapor in the air condenses around microscopic dust, salt or other particles and changes into suspended water droplets or ice crystals. But did you know there are seven different kinds of fog?
Radiation fog occurs as the ground releases thermal energy absorbed during the day into the cooling night-air causing water vapor in the air to condense. It produces the fog that made Christmas spooky.
Advection fog is produced when warm moist air blows over a cool surface, usually water. This process produces San Francisco’s famously foggy weather.
Upslope or hill fog occurs when winds blow warm moist air up a slope. As the air expands adiabatically it cools and its moisture condenses. This process produces the spring and winter fogs on the Rocky and Appalachian Mountain ranges.
Freezing fog occurs when the water droplets in fog freezes and ices the surface of everything it touches. It typically occurs at night; people wake up to a “Winter Wonderland.” Beautiful views but dangerous driving conditions!
Frozen fog is different from freezing fog. Frozen fog occurs when the water droplets in fog are super-cooled below the liquid state and the ice crystals are suspended in the air. You will need to go to northern Alaska to see this type of fog in the USA.
Steam fog or sea smoke occurs when cool air moves over water that is still warm from the summer. Most often seen in fall and winter, another name for it is evaporation fog because water evaporates from the water body into the cooler air and condenses.
Valley fog occurs as air from higher elevations cools. The air becomes denser as it cools and flows down a slope where it is trapped in lower elevations under a layer of lighter, warmer air. Most common in fall and spring and densest in the early morning, this fog may last for days.
Information courtesy of the US Weather Service.