Charlie Hayes
  Charlie Hayes caption
Density also decreases with a drop in air pressure. As you move to higher altitudes, air pressure decreases significantly -- about 3 percent for every 1000 feet of elevation. So a moving baseball experiences about 16 percent less drag at the 5,000 foot elevation of Denver's Coors Field than at a sea-level stadium like Boston's Fenway Park.

Humidity is a measure of the percentage of water vapor in the air. An increase in humidity has a surprising effect on air density: As humidity increases, air density decreases. In damp air, the large, heavy oxygen and nitrogen molecules are replaced by lighter water molecules, resulting in less density -- in essence, "lighter air." Physicist Paul Doherty explains it this way: "We think of humidity as something that's added to the air on a hot, muggy day. So you might think that a ball would go farther on a dry day than on a humid day. But for every water molecule that we add to the air, we displace a heavier nitrogen or oxygen molecule. Since the addition of humidity actually makes the air less dense, a ball will go farther on a humid day than it will on a dry day." The changes in air density related to humidity are not large: Compared to dry air at the same temperature and pressure, there's only about a 1 percent reduction in density for a humidity of 80 percent.

The chart below shows a sample of balls hit at different speeds and angles under different conditions of temperature, humidity, and altitude. Notice that even a 5 percent difference in drag can make the difference between a fly ball and a home run.

Trajectory Number Speed (ft/sec) Angle (deg.) Distance in feet given standard temp (70 degree) and pressure (sea level). Vacuum -10% air density (hot and humid) -5% air density (hot and humid) +5% air density (cold and dry) +10% air density (cold and dry) 'head' wind 'tail' wind
       Range in feet    
1 161 45 400 812 419 409 391 382 363 434
2 140 35 341 577 354 348 335 329 310 370
3 120 60 236 390 245 241 232 228 186 282
4 100 25 192 239 196 194 190 188 166 216
5 75 55 133 165 136 134 131 130 91 171

Trajectory number:
1 = home run
2 = catchable flyball
3 = catchable flyball
4 =line drive
5 = pop-up
Table based on initial figures provided by:

Watts, Robert G., and A. Terry Bahill. Keep Your Eye on the Ball: The Science and Folklore of Baseball. New York: W.H. Freeman and Co., 1990.


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