Humidity is a measure of the percentage of water vapor in the air. 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". The changes in air density related to humidity are not large: only about a 1 percent reduction in density for humidity of 80 percent compared to dry air at the same temperature and pressure.

The chart below shows a sample of balls hit at different speeds and angles, under different atmospheric 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. Obviously, Brian Johnson is right when he says that hitters can gain an advantage by taking wind and weather conditions into account.

 Trajectory #  Speed (ft/s)  Angle (deg.) standard temp and pressure 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) Result if hit at standard temp and pressure
       Range in Feet  
 1  165  55  401  799  441  421 381 361 home run
 2  140  50  351  603  386  369 333 316 catchable fly ball
 3  120  60  250  390  275  263  238 225 catchable fly ball
 4  100  30  209  271  230  219  199  188  line drive hit
 5  80  60  137  173  151  144  130  123 infield pop fly

But what else happens when the batter makes contact with a pitch? What factors besides aerodynamic drag determine how far and how fast the ball will travel? What makes one hit a home run, another a weak fly ball, another a hard grounder?

The first thing affecting a hit is the angle and location of contact between the bat and the ball. There is a perfect location for contact along the length of the bat, which batters refer to as the "sweet spot," or "good wood." This is the point scientists call the "center of percussion," the place on the bat which, when hit by an impulse (the ball) produces no impulsive reaction at the point of suspension (your hands holding the bat). When a hitter hits a ball at the sweet spot, she feels no wobbling or twisting of the bat, and all of the swing's power is transferred to the ball. On a baseball bat, the sweet spot is usually located in the middle of a six-inch area around the maker's label, extending up towards the tip of the bat. A hit outside of the sweet spot results in vibration of the bat and a weak hit. It can even break the bat.

The vertical location of the contact matters too. The baseball is round, and the bat is cylindrical, or barrel-shaped. If the batter's swing is off-center by more than a few millimeters vertically, the hit will be a fly ball or a grounder. A dead-center hit will be a line drive, and a hit a few millimeters below center could be a home run. But it could also result in a deep fly ball, easily caught for an out by the fielders. What's the difference? The determining factor lies in the physical properties of the bat and the ball, the moment of contact between bat and ball, and the interaction of the ball with the air as the ball flies towards the outfield.

"How far can you hit one?"


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