MadSci Network: Earth Sciences

Re: Does frost form at different speeds on different materials?

Date: Sun May 9 22:07:16 1999
Posted By: Samuel Conway, Senior Scientist, Message Pharmaceuticals, Aston, PA
Area of science: Earth Sciences
ID: 921913020.Es

Here's the annoyingly short answer:  "Yes and no."

Here's the full answer:

Frost will form at different speeds on different surfaces, but not because
of the composition of the surface.  It will form at different speeds due
to the temperature of the surface, which itself is related to its 


In other words, it will form faster on colder surfaces.  Darker surfaces,
such as blacktop and dark green grass, will absorb more sunlight than 
lighter surfaces like concrete or yellow grass, and hence will take longer
to cool down so frost can form.  A window in an unheated room will frost
over before a window in a heated room will.

The best illustration is seen in the road sign "Bridge freezes before
road surface."  The bridge has the same concrete surface as the road does
(except maybe in Pennsylvania, but that's because our highway department
has its head so far up its.....well, never mind), so why would it freeze

The answer is simple.  The road surface has warm, solid ground under it.
That ground takes a long time to cool down enough to allow ice to form on
the road.  The bridge, however, is just a thin strip of concrete suspended
in mid-air, with cold air blowing around under it.  It will cool off very

This is a good reason to ease off the gas pedal and keep the wheel very
steady when driving over a bridge in the wintertime!

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