QUESTION:
Is the outer edge of the earth's atmosphere a perfectly smooth sphere?

ANSWER from Dave Kock on May 12:
The earth's atmosphere really does not have an edge but rather the
amount of air gradually gets less and less as you go further and further
up from the surface of the Earth.

The amount of air decreases by about 1/2 every 3 1/2 miles up.  That is,
if you go up 3.5 miles there is only half as much air.  If you go up
another 3.5 miles to 7 miles altitude (about where many jet airplanes fly)
there is only about 1/2 of 1/2 or 1/4 as much air.

The amount of air eventually becomes very small and at some point it is
no more than the vacuum of space.  But even in the vacuum of space there 
is still some very small amount of matter.  

Try this as an illustration. 
Take a sheet of paper and cut off two one inch wide strips that are 11
inches long.  Let the first strip represent the amount of air at the
surface of the Earth.  Cut the other strip in half and lay it next to
the first.  This represents the amount of air at 3.5 miles up (a
little higher than the telescopes on top of Mauna Kea in Hawaii).
Take the other half of the strip that you cut and cut it in half and
lay one piece of it next to the first two.  This represents the amount
of air at 7 miles up.  All of the weather is confined to this portion
of the atmosphere.  Continue this as often as you can cut the
remaining strip in half.  If you want to get more strips next to each
other (go higher in the atmosphere and further into space) you need to
start with a longer strip.  Try to get a roll of adding machine paper
or a roll of toilet paper.  You might be able to represent the
atmosphere at the height the ozone layer at 25 miles but probably not
as far out as the Space shuttle at 150 miles altitude.

Please don't image though that the atmosphere has separate steps of
change in it.  It is a very smooth change with altitude and there
are no edges.