Altitude is one of a number of factors that affects the speed of a pitched baseball. Coors Field in Denver is the highest-altitude park in the major leagues --- the home of the Colorado Rockies sits about 5,280 feet above sea level. Because the air is less dense at higher attitudes, pitches travel from the mound to the plate more quickly. Yet Coors Field is a much better ballpark for hitters than for pitchers.
Forces
A pitched baseball is subject to a number of forces --- beyond the sheer strength of the person throwing the ball --- that can affect its speed. A report on the Hesston College website entitled "The Physics of Pitching a Baseball" explains that wind, temperature, humidity, air resistance and altitude all act upon a thrown ball. Pitches will get to the plate a bit more quickly on a hot day, for example. Wind at a pitcher's back will also speed up the ball's velocity.
Altitude
Since the air is less dense in a stadium such as Coors Field, there is less drag on the ball when it is hit or pitched --- meaning pitched balls go faster and batted balls go farther. A pitched baseball at a stadium at sea level, such as Boston's Fenway Park, will lose 10 percent of its speed on the way to the plate. According to research by Alan Nathan on the University of Illinois' Department of Physics website, the ball will lose only 8 percent of its speed at Coors Field, so it will get to the plate more quickly --- but only about 1 mile per hour more quickly.
Effect
Since the pitcher may be throwing 1 mph faster at Coors Field, you might think he has the advantage over the hitter. In fact, hitters have the edge at high altitude, because the ball doesn't break as much. This is due to the Magnus effect --- the force that causes a curveball to bend and a fastball to hop. The Magnus effect is an atmospheric effect that works on a ball thrown with backspin or sidespin --- every pitch except a knuckleball --- and creates movement on that pitch. In air that is less dense, the Magnus effect is less pronounced, and the ball curves and hops less. It has been estimated that a fastball will hop about an inch less, a curveball will bend about 20 percent less and a knuckleball will dive about 20 percent less at high altitude.
Advantage
While it is relatively easy for a batter to adjust to pitches that travel 1 mph faster, it is harder for a pitcher to overcome the loss of some of the hop off his fastball and some of the break off his curveball or slider. In addition, a fly-ball out at a sea-level park may be a home run at altitude. Nathan estimates that a 380-foot fly ball at sea level would fly almost 400 feet at Coors Field. Only a pure power pitcher, who should see his 99-mph fastball top out at 100 mph at Coors Field, may derive a bit of an advantage in Denver. But in general, hitters have a big edge.
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