Hurricane Irene as a Category 2 hurricane on Aug. 23. CREDIT: NOAA/NASA. View full size image

Hurricane watchers circle Aug. 20 on their calendars every year. This is the “magic” date when hurricane season seems to kick into high gear.

Like clockwork, Hurricane Irene — the Atlantic’s first hurricane of 2011 — was born on Aug. 22, later strengthening to a Category 2 hurricane. Last year was another good example of an active storm season ramping up after Aug. 20. All of the 2010’s major hurricanes (those of Category 3 or higher) formed after Aug. 20, starting with Danielle on Aug. 21.

Aug. 20 seems to be special because around this time, the air and ocean are in just the right state to foster and feed the monster storms. In climate-speak, this time of year is when vertical shear (a change in wind directions with height) in the atmosphere is low enough and sea surface temperatures are warm enough to create big storms.