Ticker for November 15, 2000

                
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November 15, 2000  November 15, 2000  November 15, 2000  November 15, 2000


Rare Phenomenon (and Physics Lesson) Occurs at Pawnee

A quick look at the pressure trace on the November 13th Pawnee meteogram
reveals a strange pressure trace.



Is this representative of the atmosphere?  No.  Well, sort of.  The
atmosphere had something to do with the trace.

What happened at Pawnee Monday morning also happened on two other dates
in the history of the network.  Ice formed in the air port of the box
which houses the  barometer.  When the ice closed off the box to the
outside world, the box became a constant-volume system.  In such a
system, according to Charles' Law, pressure is directly proportional
to temperature.

This is evident in the pressure trace.  When the cutoff occurred around
6:00 am, the temperature was falling.  So, too, was the closed-system
pressure trace.  As the temperature rose shortly following sunrise, so
did the pressure inside the box.  When the air temperature rose above
the freezing mark (32F, at about 9:30 am), the ice melted, the seal was
broken, the box equilibrated with the outside atmosphere, and the
pressure trace shot back to the appropriate value.

The barometer may have lost track of the atmosphere for a brief time
in this rare event, but it provided a fun physics lesson in the process.