Alan Stahler: Where will the tempest hit? |

Alan Stahler: Where will the tempest hit?

Hurricane Irma, image from space in infrared, indicating temperatures. The ring of black around the central eye indicates the clouds measure a hundred-plus degrees below zero, Fahrenheit. Super-cold clouds are super-high, indicating an intense storm.
Courtesy of NASA |

Draw your hand through the water in the tub, or in the sink, and you create vortices (VOR-tih-seez) — tiny whirlpools — that drift about slowly.

Do the same as you sit on a rock in the river, and the river’s flow quickly carries the vortices downstream.

A hurricane is a humongous atmospheric vortex. Like vortices in the river, it goes with the flow, allowing the hurricane’s track to be forecast. But the flow can be tweaked, tweaking the forecast.

The sun beams down on the tropics, warming the ground. The ground re-emits some of that energy, as heat. Greenhouse gasses absorb some of that heat, warming the air.

Warm air rises, carrying tropical moisture upward. As the rising air cools, its moisture condenses to form clouds. The clouds drop rain on the rainforests.

Having dropped its moisture, the air continues to rise, then spreads out, north and south, carrying tropical heat toward the poles.

The air moving toward the poles radiates heat to space; it cools and sinks. The sinking air piles up in the sub-tropics, north and south of the tropics.

A pile of air presses down on the air below — the air at the surface exhibits high pressure. A high pressure system in the North Pacific blocks storms from reaching California in summer. Another high pressure system — the Bermuda High — sits over the Atlantic.

Squeezed out by the air above, air in the Bermuda High blows outward. The Coriolis effect puts a spin on the air, turning the Bermuda High into a pinwheel, spinning clockwise over the Atlantic.

In the eastern part of the pinwheel, off the East Coast, air moves from south to north. The south-to-north wind blows the waters along the eastern seaboard from south to north, creating the Gulf Stream.

Farther north, roughly off Newfoundland, air in the pinwheel blows west to east.

Along the European coast, and down to Africa, the air in the pinwheel blows north to south.

In the southern part of the pinwheel, the pinwheel’s winds blow east to west, merging with the Trade winds. These winds blow hurricanes — Irma, for instance — across the Atlantic, from Africa to the new world.

If and when and where a hurricane makes landfall depends on how long it remains in the east-to-west flow of the pinwheel, how soon (if ever) it gets caught up in the south-to-north flow up the east coast.

Landfall also depends on what else is happening in the atmosphere. Earth’s geography — mountains and valleys, deserts and ice fields — creates regions of high pressure and low, generating winds that lend their push to driving the storm.

The tempest itself can set the air around into motion, further affecting the motions of the storm … and making its track that much harder to forecast.

Al Stahler enjoys sharing nature with students of all ages. His science stories can be heard on radio station KVMR (89.5 FM), and he may be reached at

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