Physics Photo of the Week

December 14, 2012

Two forms of "Frost"

When cars are left out in the open in freezing temperatures frost often forms on the vehicle's roof and windshields.  These two pictures of the icy rooftop of my pickup truck are quite different.  The photo on the left shows a "frosty" coating of tiny ice crystals on the roof of the truck.  The frost appears white on account of the many thousands of needle-like ice crystals (PPOW for March 2, 2007).  The frost was formed from water vapor in the air freezing directly into ice - without first forming liquid water.  Click on the images for closer views.

The image on the right was photographed on a different day.  The temperature was just as cold, and the car top showed about the same amount of ice.  However, the form of the ice is frozen dew, not the frosty appearance of thousands of needle-like ice crystals.  The air contained much more water vapor.  The dew point (the temperature at which the water vapor condenses out of the air) was above the freezing point for water that liquid dew drops formed above the freezing point.  Later in the night the temperature dropped below freezing and the dew drops froze into frozen water drops.  The dew drops in the right hand image are frozen ice as indicated by the small hand scraping.

When true frost was formed (on the left image), the air was so dry that the dew point was below freezing.  When the "dew point" is finally reached (below freezing) the water vapor condenses directly into ice - it bypasses the liquid state.  This process is called sublimation, where water vapor condenses directly into ice - skipping the liquid state.  Snow flakes are sublimated ice crystals formed in the sky also directly from the water vapor.

Due to Christmas break at Warren Wilson College - this is the final Physics Photo of the Week for 2012.  PPOW returns to this site on January 25, 2013.  Be sure to browse the PPOW archives in the link below.  Have a very cheerful Christmas, and enjoy the frosty mornings!

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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