Published Tuesday, Apr 6, 2004 in the San Jose Mercury News

Weather Corner

Weather affects flight of thrown, hit baseballs

Special to the Mercury News

It's springtime, the weather is warming and there are the sounds of ash (or maple or aluminum) hitting horsehide in the air. And with major league baseball's season getting under way in earnest this week, it's interesting to see what role the weather plays in the game of baseball.

The laws of physics determine how the air will move, how warm it will be and whether it is dry or damp. Likewise the flight of a baseball, whether it is batted or thrown, is governed by these principles. The combination of the two adds to baseball's unpredictability.

In the Bay Area, many night games are played when the air is damp. While it might seem that the air would be heavier when moist, the opposite is actually true. When the air becomes more humid, its density lessens, so it creates less drag or friction. Friction is what slows a ball and makes it curve.

A baseball can absorb a small amount of water from the atmosphere and become slightly heavier. But the effect of humidity is negligible.

There are more noticeable effects from variations in temperature and barometric pressure. As air warms it becomes less dense, which translates into less wind resistance to a moving ball. A decrease in barometric pressure also makes the air less dense. For example a home run would go about 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.

But the single biggest factor in the flight of a baseball is altitude. At higher elevations, the atmospheric pressure and the density of the air decrease and so does the amount of drag on the ball. Turner Field in Atlanta (elevation 1,000 feet) is the second-highest ballpark in the major leagues and the barometric pressure is about 4 percent less than sea level pressure. By contrast, Coors Field in Denver's thin, mile-high air has only 83 percent of sea-level air pressure.

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, that 400-foot ``tater'' would go about 750 feet!

A pitched ball can be impacted by the weather, though somewhat less 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.

The prevailing winds also have an influence on the flight of a ball. 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. A 10 mph crosswind will blow a fastball off its path by about three inches.

This may not seem like much, but even a 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. Not to mention the pitcher himself. Remember when Giants pitcher Stu Miller was supposedly ``blown off'' the mound during the 1961 All-Star game at Candlestick Park? When he moved, the umpire called a balk, allowing the runners to advance in the ninth inning and tie the score. Did the weather score a run?

There are slight differences in the weather at SBC Park in San Francisco and Network Associates Coliseum in Oakland, but both are less windy than the Giants' previous home at Candlestick. Although the temperatures are about the same at all three stadiums, the wind speed and thus the wind chill were significantly greater at the Stick.

The problems there were twofold. First, the wind funneled over the coastal hills from Daly City and down Visitacion Valley, where it picked up speed. The winds were further accelerated by the 400-foot-high Bay View Hill directly behind the ballpark. Much as a rock in the middle of a river creates rapids and rough water downstream, the hill makes the wind notoriously gusty and changeable.

One weather effect that local teams seldom have to deal with is rain. In the 45 seasons the Giants have played since moving to San Francisco in 1958, there have been 29 rainouts, of which five have been in June, July and August. The Oakland A's have had 19 rainouts in the 35 seasons since moving from Kansas City to the East Bay in 1968.

The Exploratorium in San Francisco has a fascinating exhibit online about the science behind baseball at .

Q I've read that a byproduct of global warming is extreme weather, such as the recent spell of record-breaking high temperatures for 12 consecutive days. As a Bay Area resident for all of my 50-plus years, I cannot remember any more than four to five straight days of record highs. Do you believe the hot spell could be part of global warming, and that more occurrences of this type can be expected in the future? Jack Dickinson - Milpitas

A No, I do not think the two are related. We tend to think locally and forget that while we had an extended period of heat, the eastern portion of the nation was seeing snow and record cold temperatures.

In general it is ill-advised to attribute single events to large-scale climatic events. A storm's impact might be exacerbated by global factors, but it might have happened anyway.


Jan Null, founder of Golden Gate Weather Services, 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, fax them to (510) 315-3015 or e-mail them to or fill out a form online at