Published Tuesday, May 9, 2000, in the San Jose Mercury News


Special to the Mercury News

With spring in the air and the sounds of hickory (or aluminum) hitting horsehide, let's talk about weather and baseball this week.

Q.  How does the humidity on a foggy evening affect a batted or thrown baseball?

A.  The air is approximately 21 percent oxygen, 78 percent nitrogen and some very small amounts of other gases. Now if we look at the weight of the molecules (i.e., molecular weight) of dry air we see it has a value of 29. But gaseous water vapor has a molecular weight of only 18. Because the air around us is a mixture of dry air and water vapor, its actual molecular weight falls somewhere between 18 and 29, with it being less on humid days. When the air becomes more humid, its density lessens, so it creates less drag or friction, which would slow a baseball or allow it to curve.

However, the dynamics aren't quite that simple because high humidity can cause a baseball to gain a small amount of water, which makes it slightly heavier. The bottom line is that the effect of humidity, all other things being equal, is negligible.

Q.  Well, if humidity has little effect, how about temperature and barometric pressure?

A.  Temperature and pressure have a more noticeable effect. Air becomes less dense as it warms, so there will less wind resistance to a moving ball. And a decrease in barometric pressure also makes the air less dense. A home run ball would
go as much as 20 feet farther on a 95-degree day with a barometric pressure of 29.50 inches of mercury than it would
on a 45-degree day with a pressure of 30.50 inches.

Q.   Would a batted ball travel farther in the mountains, where the barometric pressure is lower?

A.  Absolutely. Pressure is probably the single biggest atmospheric effect on a baseball. Atmospheric pressure, and the density of the air, decreases with altitude and, consequently, so does the drag on the ball.  The atmospheric pressure at Turner Field in Atlanta -- at 1,000 feet elevation, it's the second-highest ballpark in the major leagues -- is about 4 percent less than sea level
pressure. Coors Field in Denver's thin, mile-high air has only 83 percent of sea level air pressure. Consequently, if the altitude
and humidity are taken into account, a 400-foot home run at sea level would go about 408 feet in Atlanta and 440 feet in
Denver! And if the game were played in a vacuum with no air resistance, that 400-foot ``tater'' would go about 750 feet.

Q.  Do these things affect the pitcher?

A.  Yes, but not quite as dramatically. Again, it's a matter of air density and how much friction the ball encounters. When the air is less dense, a pitcher's fastball would be slightly faster but his curveball wouldn't bite into the air as much and would be less effective.

Q.  Well, how does the wind affect the ball?

A.  As you would expect, it depends on whether the ball is traveling with or against the wind. A fastball thrown at 90 mph into a 10 mph wind would have a ground speed of about 89.3 mph; if it's thrown downwind, it would be about 90.7 mph. However, a 10 mph crosswind will blow a fastball off its path by about three inches, or the diameter of the ball. This may not seem like much, but even this small deflection can mean  the difference between a ball and a strike, or a strike and a home run. A crosswind has an even greater effect on slower pitches -- curveballs and knuckleballs.   Remember, a gust of wind blew pitcher Stu Miller off the
mound during the 1961 All-Star game at Candlestick Park.

Q.  How different is the weather at Pacific Bell Park than at 3Com (Candlestick) Park?

A.  For a number of reasons, the weather should be more benign at Pac Bell than at the 'Stick. Although air temperatures are much the same, wind speed and thus wind chill are less at the new ballpark.  Candlestick's wind problems were twofold. First, the wind
funneled over the coastal hills from Daly City and down Visitacion Valley, where it picked up speed. This was further exacerbated by the 400-foot-high Bay View Hill directly behind the ballpark. Much like a rock in the middle of a river creates rapids and rough water downstream, the hill makes the wind very gusty and changeable. The terrain upwind from Pac Bell is less rough and not as channeled, so the wind generally will not only be slower but also less gusty. And the third-base stands at the new ballpark were oriented to partially block the wind.

Q.  Are games rained out very often in the Bay Area?

A.  Other than the Southern California stadiums, the Bay Area probably has the fewest rainouts of any outdoor baseball park.  In the 42-plus seasons the Giants have played in San Francisco, there have been only 29 rainouts, of which only five have been in June, July and August. And from April 10, 1978, through Oct. 2, 1981, the Giants played 454 home games without a weather cancellation. The Oakland A's record is just as enviable with only 19 rainouts in the 32-plus seasons the team has been at the Coliseum.

Q.  Where can I find out more about baseball and the weather?

A.  An excellent book on the topic is ``The Physics of Baseball,'' by Robert K. Adair (Harper & Row, 1990). The San Francisco Exploratorium has an excellent interactive Web site devoted to the science of baseball (

Jan Null, founder of Golden Gate Weather Services and director of meteorology for, is a retired lead forecaster with the National Weather Service. Send questions to him C/O Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, e-mail them to or fax them to (510) 315-3015.