Melting Frost on a Windshield

May 02, 2022

Water drops and frost
Photographer: Glenn McCreery

Summary Author: Glenn McCreery

The photo above shows frost that formed on the outside of my car's windshield following a very cold night and warming morning air temperature. Frost builds on surfaces when its temperature is lower than freezing and moisture (humidity) is in the air at or above freezing temperature. The vapor sublimates directly to form the solid ice crystals of frost without going through the liquid phase, but these crystals do not necessarily have the six-sided symmetry of three-dimensional snow crystalsAdhesive forces between the sublimating ice crystals and microscopic surface irregularities help dictate the more irregular two-dimensional frost geometry.

When the frost crystals melt, the resultant liquid drops tend to form shapes that minimize the energy of the drops. The resultant surface tension cohesive forces between water molecules, and adhesive forces between water and the surface and ice crystals, pull the drops into more hemispherical shapes. This melting process may be seen on the left side of the photograph. Melting first occurred on the left side of the windshield because the glass is curved, and the Sun was shining from this side. Photo taken on November 21, 2020.

Photo details: Olympus TG 4, 25mm (35mm equivalent), f/2.8, 1/80 second exposure, ISO-100.


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