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Hurricane Irene: Why Aug. 20 is an important date for hurricane watchers

Hurricane Irene: Why do hurricane watchers circle Aug. 20 on their calendars each year?

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An image released by the NOAA made from the GEOS East satellite shows Hurricane Irene on Wednesday as it moves northwest from the Dominican Republic.

NOAA/AP

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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.

"Now, storms can get going before Aug. 20, but this is typically about when they start," said Phil Klotzbach, a hurricane forecaster at Colorado State University in Fort Collins.

This year was typical in saving its biggest storm so far for after Aug. 20, but it was unprecedented in the number of tropical storms that fizzled before reaching hurricane strength. The 2011 hurricane season began with eight tropical storms that all failed to become hurricanes before Irene broke the streak. One reason for the flurry of tropical storms is that scientists are naming more tropical storms than in past years, Klotzbach said.

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