Play Ball!

NatCast
Nationals vs. Arizona Diamondbacks, 7:05pm on April 14, RFK
Winds will be northeasterly from 10 to 15 mph, diminishing during the game. Temperatures will be about 65 when the gates open, but expect them to drop steadily after sunset at 7:44 from the game-time level of 58 down to the lower 50's by the 7th-inning stretch and into the 40's on the way home. Go Nats!

Currently

Today's weather map shows a weak low pressure area moving away from the North Carolina coast and high pressure dominating virtually the entire country east of the Rockies. The only precipitation showing on national radar in the eastern two-thirds of the U.S. is an area of lingering showers in the eastern Carolinas. Under brilliantly sunny skies, temperatures are rising from an official low of 42 through the 50's at mid-day toward highs in the mid 60's. Some of the outlying areas were down into the 30's last night with Frederick at 37, Dulles at 39, Culpeper at 33.

Under clear skies, temperatures should drop to near 40 tonight. Tomorrow should be slightly cooler with highs around 60.

Baseball and the atmosphere

Images (click to enlarge): Magnus force, from American Scientist

The May-June issue of American Scientist has an article on "Predicting a Baseball's Path". It explains how the spin of a baseball pitch interacts with the air through the "Magnus force" to affect the ball's motion. A ball moving through air has a turbulent wake behind it. The direction of spin affects the location of the "separation points" where the air flow becomes turbulent. In the case of topspin (movement of the top of the ball is in the direction of the pitch), the upper separation point is deflected upward, so the air in the wake has upward momentum. By the law of conservation of momentum, the ball must move downward. Conversely, a ball with backspin will move upward. By controlling the axis of rotation of the pitch, the pitcher can determine the direction of deflection of the ball.