Re: Baseball, air pressure, and humidity

[David Bowman <dbowman@tiger.gtc.georgetown.ky.us>]

Ed Schweber writes:

> During today's Yankee/Baltimore game the announcers said that the
> ball was carrying further than it did yesterday. 
>
>    Can this be true? Can air pressure or humidity have a noticeable
> effect 
> on air friction. Or is it perhaps that humidity effects the resiliancy
> of the ball?
>
>    Do any of you have any knowledge; either based on theory or from
> playing the game?  
>
>   I'm looking forward to your responses.

I did not get a chance to see either game, but I suspect that the most likely
reason for the difference between them was a change in the wind speed and/or
direction.

Besides wind effects, there is a smaller effect on the resistance of the air
on the moving ball due to changes in the *density* of the air.  The density is
directly proportional to the drag.  Air density is typically proportionally
increased by a reduction in the absolute temperature and/or by an increase in
the air pressure.  There is also an effect on the air density due to a change
in the composition of the air caused by a change in the absolute humidity,
with humid air being less dense than dry air.  There is an even smaller effect
on the air resistance due to a change in the air viscosity (caused
predominantly by a change in the temperature), but this effect is not as great
as the effect of a change in the density unless the ball is moving *so* slowly
through the air that the drag is dominated by v^1 viscous Stokes drag rather
than the usual v^2 type caused by the ball pushing/plowing its way through a
massive cylinder of air which is blocking its path.  (Needless to say, the
viscous effect is completely unimportant in a baseball game.)

David Bowman
dbowman@gtc.georgetown.ky.us