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. |
|
