How to Kill Zombie Hurricanes

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Twice now in recent weeks a hurricane has died and then come back to life. It’s unfair, but not particularly unnatural.

To understand these Franken-storms, however, it’s helpful to forget about hurricanes for a minute and consider the conditions that create them: Intense low pressure areas over warm ocean waters, which supply the storm energy, and lower the pressure more, by way of warm water vapor evaporating and rising from the sea surface.

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You also need winds to be pretty much aligned at low and high elevation so that wind shear doesn’t lop off the tops of thunderheads. If all this is in place, the intensifying low pressure area causes air to spiral in towards the center, which makes it windy and can intensify the evaporation even more.

There’s a lot more to it, but these are the basic ingredients.

Tropical Storm Humberto, which was briefly the first hurricane of the season last week,  died on Sept. 15 when its high cloud tops were ripped off by wind shear. That reduced the storm to little more than a low pressure area. But a day later the wind shear had abated and all the other conditions — warm ocean waters and low pressure — were still ripe for a storm, and so Humberto sprang back life and is now continuing west across the Atlantic towards the United States.

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The zombie sibling of Humberto is Gabrielle from earlier this month. Gabrielle was a Tropical Storm in the eastern Atlantic until she too was ripped apart, only to be resurrected into a tropical storm, which headed northwest, She eventually died in the northern Atlantic without hitting land.

This time she’ll not be reviving, since she has no more warm waters to keep her alive.

Photo: Will they live? On Sept. 15, Manuel, left, was a tropical storm with winds of 55 knots (102 km/hr or 63 miles per hour). Ingrid, right, was a Category 1 hurricane with winds of 65 knots (120 km/hr or 75 mi/hr). NASA image courtesy Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team at NASA GSFC.

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