Dear Tom,
I am having a discussion with a friend and the issue is baseballs sailing through the air. Do they fly farther through moist air or through dry air? 

Ron Callignart, Boston, Massachusetts

Dear Ron,

When a baseball flies through the air, it must push aside the air through which it is flying. That uses inertial energy that would otherwise carry the ball forward. The denser the air, the more resistance it will encounter and the quicker it will decelerate and fall.  Conversely, anything that causes air to be less dense will result in the ball traveling farther.

Air becomes less dense as one increases in elevation, when air warms, and when the air contains a greater percentage of water vapor. The latter may sound counterintuitive, but read on:

Recall that nitrogen and oxygen make up about 99 percent of dry air yielding a molecular weight of about 28.9. The molecular weight of a water molecule, however, is only 18. Thus, when water vapor is added to dry air, the lighter water molecules displace heavier nitrogen and oxygen molecules and the air becomes less dense. The result is that moist air will cause baseballs to fly slightly farther than they would in dry air.