MadSci Network: Earth Sciences

Re: Why does the wind blow into the canyon during morning and blow out at night

Date: Sat May 10 17:23:03 2003
Posted By: In Koo Kim, Grad student, Physical Chemistry, Harvard
Area of science: Earth Sciences
ID: 1052437286.Es

This question is difficult to answer without knowing more about the
particular geographical layout of this canyon and it's position with
respect to surrounding bodies of water or mountains. However, that said, we
do know that breezes often occur due to temperature and pressure
differences between two areas.  If you're by an ocean, in the mornings and
evenings, the ocean and land heat and cool at different rates.  Water and
land have different heat capacities.  This causes differences in the
temperature of the air over the respective areas, and a resulting
difference in local pressures.  These temperature and pressure differences
are equilibriated when the cool and warm air masses mix.  Near the coast,
this mixing of two air massess is witnessed as the commonly termed "sea

It doesn't just have to be an ocean either.  The only thing that matters is
that two nearby areas have significantly differing effective heat
capacities.  It could also be something like a canyon.  Deep inside the
canyon, the sun doesn't penetrate deeply and transfer it's radiative
forcing as quickly as in the open.  Therefore, a hypothesis for your
particular canyon may be that during the morning, the area outside the
canyon heats up at a faster rate than inside the canyon. This may result in
a temperature differential and felt as the rush of air.  This also happens
in the evenings, although in reverse direction, because the area in the
open may cool faster (release it's heat to outer space) than the area deep
inside the canyon.

One question for you is: is the breeze noticeably weaker on humid days? 
Can you think of why this might be?

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