First, a recap on density: warm air is less dense than cold air. For a pitcher, the lesser density means a fastball will go slightly faster on warm days than on cold days.
However, with less drag, it also means their curveballs won't curve as much and their sinkers won't sink as much. We're not talking a lot, but it could be the difference between a home run and a pop up to second base.
Knuckleball pitchers, who heavily rely on the dragging effects of air to alter the ball's trajectory, usually fare much better on cold nights than during warm days.
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