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
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 firstname.lastname@example.org.
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.