# Air pressure and humidity **URL:** \ **Category:** Factual Questions\ **Created:** [September 22, 2004, 8:10pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548 "2004-09-22T20:10:54Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![MaxTheVool](https://avatars.discourse-cdn.com/v4/letter/m/9fc29f/32.png) [@MaxTheVool](https://boards.straightdope.com/u/MaxTheVool)\ **Post date:** [September 22, 2004, 8:10pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/1 "2004-09-22T20:10:54Z")
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? ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [September 23, 2004, 12:47am UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/2 "2004-09-23T00:47:18Z")
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. ---
**Author:** ![Doctor\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/doctor_jackson/32/32_2.png) [@Doctor\_Jackson](https://boards.straightdope.com/u/Doctor_Jackson)\ **Post date:** [September 23, 2004, 1:51pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/3 "2004-09-23T13:51:02Z")
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](http://www.edwardwillett.com/Columns/homeruns.htm) > [@](#): > > And humid air is less dense than dry air, because the water molecules in it weigh less than the oxygen and nitrogen molecules they displace. Air at 80 percent humidity is one percent less dense than dry air. Even one percent is significant, because just a five percent difference in drag can change a fly ball into a home run–or vice versa. Google “homeruns +humidity” for more. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [September 23, 2004, 1:54pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/4 "2004-09-23T13:54:44Z")
> [@Doctor Jackson](#): > > Actually, the opposite is true… 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”? ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [September 23, 2004, 2:30pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/5 "2004-09-23T14:30:44Z")
**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. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [September 23, 2004, 2:36pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/6 "2004-09-23T14:36:02Z")
> [@Crafter\_Man](#): > > 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. 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. ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [September 23, 2004, 3:16pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/7 "2004-09-23T15:16:33Z")
> [@Crafter\_Man](#): > > 1. **Humid air is more thermally conductive than dry air**. Um, no it’s not. Dry air is more thermally conductive. 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? ---
**Author:** ![Inigo\_Montoya](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/inigo_montoya/32/124_2.png) [@Inigo\_Montoya](https://boards.straightdope.com/u/Inigo_Montoya)\ **Post date:** [September 23, 2004, 3:37pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/8 "2004-09-23T15:37:54Z")
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! ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [September 23, 2004, 3:43pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/9 "2004-09-23T15:43:14Z")
> [@Dag Otto](#): > > 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? I was referring to thermal conductivity, not thermal convection. And for a given mol fraction of water, the thermal conductivity increases with increasing temperature. ---
**Author:** ![Dag\_Otto](https://avatars.discourse-cdn.com/v4/letter/d/439d5e/32.png) [@Dag\_Otto](https://boards.straightdope.com/u/Dag_Otto)\ **Post date:** [September 23, 2004, 4:51pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/10 "2004-09-23T16:51:58Z")
> [@Crafter\_Man](#): > > I was referring to thermal conductivity, not thermal convection. Right, I got that. I was just wondering if ‘most people’ didn’t confuse the two. > [@Crafter\_Man](#): > > It’s strange… what **most people** believe to be properties of water vapor are 180 degrees out of phase from the truth. ---
**Author:** ![MaxTheVool](https://avatars.discourse-cdn.com/v4/letter/m/9fc29f/32.png) [@MaxTheVool](https://boards.straightdope.com/u/MaxTheVool)\ **Post date:** [September 23, 2004, 9:47pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/11 "2004-09-23T21:47:24Z")
> [@Q.E.D.](#): > > 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”? And you thought you wouldn’t get a response to that… > [@a poor guy who has to email me in order to get his response posted](#): > > The answer is that I play tennis throughout the New Jersey summer (for the past 30 years, and 6 years in Baltimore before that), and the humidity varies a lot. > > On less humid days, the ball tends to go too far, and your topspin is less effective in making it drop into the court. On more humid days, you really have to drive the ball just to get it deep into the other player’s court. > > Don’t take my word for it – ask anyone who plays a lot of tennis outdoors in the summer. So if someone says the ball travels farther on humid days, that statement is not consistent with my experience. Reminds me of the old Groucho Marx line: “Who are you going to believe – me or your own eyes?” ---
**Author:** ![W.Fikere\_Tomba](https://avatars.discourse-cdn.com/v4/letter/w/f9ae1b/32.png) [@W.Fikere\_Tomba](https://boards.straightdope.com/u/W.Fikere_Tomba)\ **Post date:** [September 23, 2004, 10:59pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/12 "2004-09-23T22:59:36Z")
> [@](#): > > On less humid days, the ball tends to go too far, and your topspin is less effective in making it drop into the court. On more humid days, you really have to drive the ball just to get it deep into the other player’s court. Maybe the ball becomes waterlogged on humid days. Could a tennis ball gain enough mass that way to create these effects? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [September 24, 2004, 4:08am UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/13 "2004-09-24T04:08:16Z")
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. ---
**Author:** ![MaxTheVool](https://avatars.discourse-cdn.com/v4/letter/m/9fc29f/32.png) [@MaxTheVool](https://boards.straightdope.com/u/MaxTheVool)\ **Post date:** [September 24, 2004, 7:08pm UTC](https://boards.straightdope.com/t/air-pressure-and-humidity/265548/14 "2004-09-24T19:08:05Z")
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