ITCZ
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The Southern writer Flannery O'Connor called one of her short stories "Everything That Rises Must Converge" - but had she been writing about weather the title would have to be: everything that converges must rise.

Hi, I'm Bryan Yeaton and this is The Weather Notebook.

The most dramatic illustration of this meteorological law is a phenomenon known as the Intertropical Convergence Zone or ITCZ. In the tropics, air flow is dominated by the trade winds - strong steady winds that blow from the southeast in the southern hemisphere and from the northeast in the northern hemisphere. When the two opposing trades converge near the equator, the air they carry is forced to rise - and because this air is so warm and humid it rises miles into the atmosphere, billowing out into some of the most spectacular cumulonimbus clouds on the planet. When these clouds burst, they dump down torrents of rain, usually accompanied by prolonged drumrolls of thunder. And the ITCZ makes this happens day after day.

Atmospheric scientist Nick Bond, who just returned from a field study of the ITCZ, notes that it tends to shift with the seasons, moving north in our summer and south during the winter. And sometimes at winter's end you'll get a double convergence zone, one north and the other south of the equator. The ITCZ also cranks up hurricanes by funneling enough energy into clusters of thunderstorms to start them rotating in that ominous maelstrom pattern.

If you happen to be in the tropics when the ITCZ is active, forget the umbrella and GoreTex, advises Bond. You're just going to get wet anyway, so you might as well enjoy it.

Thanks today to David Laskin who, as a resident of Seattle, has extensive experience with rain. The Weather Notebook is a production of the Mount Washington Observatory and is sponsored in part by the National Science Foundation.