Thanks to warm air being less dense than cold air, baseballs will travel further on warm days than cold days. Thus, it's much easier to hit a home run here during a (rare) warm day game than a (frequent) cool night game.
Air pressure also affects air density. The higher pressures at parks near sea level (like Seattle) are tougher to hit homers in than parks at higher elevations (like Denver -- where the mile-high altitude can add as much as 20-40 feet to a would-be homer)
Humidity doesnt have a real affect on baseball. Humid air is less dense than dry air, but the added water weight on a baseball counter-balances any additional distance from the lighter air.
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