If You Like Home Runs, Root for Hot, Humid Weather

(July 12) -- Whether it's a statement about the All-Star game or simply a tribute to the part of the game that fans enjoy most, many of us are more interested in the home run hitting contest on each Monday of the All-Star break than in the game itself on Tuesday. And fans who want to see home runs should root for hot and humid weather.

Heat and humidity are the steroids of the atmosphere. When all other factors are equal, both heat and humidity allow a batted baseball to travel farther than if the air were cool and dry.

Neither heat nor humidity will be extreme for this evening's home run hitting contest in Anaheim, Calif., home of the 2010 All-Star festivities. With temperatures in the upper 70s and moderate humidity levels, baseball players will have to generate most of their own power. Granted, the players might not need much help from the atmosphere, with pitchers deliberately feeding them perfect pitches.

When the meaningful games resume, though, players will be able to count on the weather to provide a little pop to their bats. Heat and humidity is rampant in the eastern part of the country during July and August, the so-called dog days of summer, and the Southwest will continue to bake under an unrelenting sun. In addition, the humidity typically increases in the Southwest during the second half of the summer.

The widespread heat and humidity will be favorable for home runs.

The fact that hot air is lighter than cool air is no surprise, since the statement "hot air rises" is probably the most repeated of all weather statements. But the fact that humid air is lighter than dry air is not as easily understood, because the reverse feels more correct.

Sweat doesn't evaporate from our skin well when it's humid, leaving us feeling sweaty, clammy and overheated. In addition, humid, often stagnant air masses often hang with haze, ozone and pollution, making breathing harder. None of this feels light in the way that a crisp autumn day does.

Regardless of how it feels, though, humid air is lighter than dry air.

Liquid water is heavier than air; otherwise, rain would not fall from the clouds. Water in a vapor form, though, is lighter than the main components of the atmosphere. As a result, when the amount of water vapor in the atmosphere is increased (the air is more humid), the density of the air overall is lowered.

Just as you would expect in those conditions, a batted baseball will travel farther through less dense humid air than it would through more dense dry air.

This would not be the case if the air were so humid that water droplets condensed onto the baseball, adding weight. This generally would happen only in a humidor (like one that's been used in Colorado to counteract the light air of the higher elevation), not normal atmospheric conditions.