Surface Hoar Formation
January 05, 2021
Photographer: Glenn McCreery
Summary Author: Glenn McCreery
Frost forms when water vapor condenses directly into ice. Large frost crystals (larger than about one-quarter inch or 0.6 cm across) that form are often called hoarfrost. The most common form of hoarfrost is surface hoar, which can occur, for example, on the top of a snowbank that’s warmed during the day and cooled at night under a clear, calm sky. Meltwater that evaporates from within the snow is later recrystallized onto the colder surface. On sunny days, surface hoar is particularly noticeable when it glitters in reflected sunlight.
This photo was captured one morning several weeks ago while cross country skiing above Pine Creek Pass, Teton County, Idaho, at an elevation of approximately 7,000 ft (2,133 m). The previous day’s high temperature was approximately 34 degrees F (1 degree C), but overnight temperatures dropped to about 4 degrees F (-15.5 degrees C), nearly ideal for hoarfrost formation.
Note that in mountainous areas, if surface hoar is covered by new snow, it can form a very weak layer that’s prone to sliding, resulting in avalanche warnings being posted. Photo taken on November 29, 2020.
Photo Details: Olympus TG4 camera, 30mm (35mm equivalent); f:9.0; ISO 100; 1/320 second exposure for top photo and 1/200 second exposure for bottom photo (Macro mode for bottom photo).