# Air pressure and humidity

A non-SDMB-enabled acquaintance has posed the following question, which I figure the teeming millions should easily be able to answer:

I have read in physics books that the extent to which air affects the path of a moving object (a baseball, a tennis ball, etc.) is positively related to the density of the air. And I know that balls travel farther at high altitudes, where the air is less dense.

I have also obsrved day-to-day variation in the extent to which the air slows down the ball. The slowing effect of the air is much greater on humid days than on dry days of the same temperature.

However, on humid days, the barometer registers a lower air pressure than on dry days. The lower barometer reading seems to imply that the air is less dense on humid days, when I observe it to have a greater effect on the ball.

That seems to be a contradiction. What is the explanation?

Totally unqualified WAG (to keep the OP on the front page until the real help arrives)

Water molecules have greater mass than nitrogen & oxygen which makes up the majority of air.

High pressure forces moisture to precipitate and lower the humidity (amount of water molecules floating around and getting in the way)

Despite higher density of nitrogen & oxygen, the cumulative inertia of the air against the ball is less in the absence of the heavier water molecules.

Think about walking through a cloud of styrofoam beads blowing at you at 80 mph. This is high pressure, low humidity. Now add a few 1 inch ball bearings into that 80 mph mix, but only half as many styrofoam beads. This represents low pressure, high humidity.

The balbearings are going to slow you down.

Actually, the opposite is true - a baseball will fly farther in humid air than in dry air. This discussion comes up fairly often in Atlanta, GA since we have both high humidity and a lot of home runs hit at our major league ball park.

Here’s one cite:
http://www.edwardwillett.com/Columns/homeruns.htm

Which begs the question (yeah, I know, but I like that usage, dammit!), how did the OP’s friend “observe” the “day-to-day variation in the extent to which the air slows down the ball”?

Doctor Jackson is correct.

It’s strange… what most people believe to be properties of water vapor are 180 degrees out of phase from the truth. Two examples come to mind:

1. Water vapor is dense. No it’s not. If you allowed a room to come to mechanical and thermal equilibrium, you’ll find the highest concentration of water molecules near the ceiling. Water vapor is light stuff.

2. Humid air is more thermally conductive than dry air. Um, no it’s not. Dry air is more thermally conductive.

Exactly. What people tend to forget is that although a water molecule is heavier than an oxygen or nitrogen atom, those gasses are found as diatomic molecules in the air, and as such, are heavier than water.

Tht might be explained by a misunderstanding between conductive heat transfer and convective heat transfer. Doesn’t humid air have a higher convective coefficient than dry air?

Whew…glad I announced it was a WAG!

But DO come to me for the REAL science when you need to tell whether someone is a witch!

I was referring to thermal conductivity, not thermal convection. And for a given mol fraction of water, the thermal conductivity increases with increasing temperature.

Right, I got that. I was just wondering if ‘most people’ didn’t confuse the two.

And you thought you wouldn’t get a response to that…

Maybe the ball becomes waterlogged on humid days. Could a tennis ball gain enough mass that way to create these effects?

Do they still make high-quality tennis raquet strings out of catgut (don’t worry, it was never made from kitties as far as I can tell)? If so, there’s your answer. If you’ve got gut strings, then humidity is going to affect the tension. On dry days, they’ll contract, thus becoming tighter; the reverse will be true on humid days. A similar caveat applies for wood-frame raquents, though not quite to the same extent.

So, have we got any atmospheric physicists in the house?

So far, we seem to have reached the following conclusions:
(1) Water vapor is less dense than air
(2) “spin” on balls is more effective when the air pressure is higher (taken to an extreme, it won’t do anything at all in a vacuum)
(3) “spin” on balls is also more effective when there is more humidity
(4) this seems to be a contradiction
(5) it may have something to do with the ball and/or racquet getting damp