A fastball covers the distance from the mound to the plate in less than half a second, requiring lighting reflexes and judgment from the batter to even make contact. So when the hitter does make contact, he has to try to make the most of it. As Brian Johnson describes it, a hitter has to be aware of wind and atmospheric conditions to get the most out of a hit: "Sometimes the wind might be blowing somewhat in towards the plate in left field, and slightly out of the park in right. In that case, you might take advantage by trying to hit the ball towards right." The density of the atmosphere can also play a role: "In the Colorado Rockies' new stadium, which is at altitude, they have much thinner air. So however the thickness of the oxygen molecules is calculated, the ball flies much farther." Under such conditions, hits that might be routine fly balls can sail over the fence for a home run, and hitters who might not try for home runs can suddenly think about swinging for the fences.
So what is it about traveling through air that affects a hit? Say that the ball, struck by the bat, flies into the air at 165 miles per hour, at an angle of 55 degrees. If the ball were flying through a vacuum, the distance it would travel would be determined solely by the ability to resist gravity imparted by its speed and trajectory, and it would travel 799 feet! Even in a baseball stadium twice as large as those which exist, this would still be a home run. However, since we are on earth, the ball must travel through the air. The direction and strength of the wind can alter the ball's path, as can the relative density and humidity of the air.
 The two properties of the air that affect the ball's flight are density and viscosity. In a dense gas or fluid, the molecules are closer together, and an object moving through it must push aside a larger number of molecules, which takes more energy, diminishing the distance the ball will travel. The viscosity of a substance reflects how much it resists flowing, and also how "sticky" it is. Substances like motor oil and honey have high viscosity, while gasoline and benzene are examples of low-viscosity liquids. Gases are much less viscous than fluids--about 100 times less. The viscosity of air increases slightly as temperatures increase, but not enough to make a noticeable difference in drag. The density of the air changes with variations in the temperature, pressure, and humidity of the air. As the temperature increases, the air density decreases. Air is 12 percent less dense at 95 degrees Fahrenheit than it is at 30 degrees Fahrenheit, resulting in markedly less drag. 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 15 percent less drag at the 5000 foot elevation of Denver's "Mile High" stadium than at a sea-level stadium like Boston's Fenway Park.
 "How far can you hit one?"