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Hadley Cell Helps Along our Summer Heat

(KXII)
Published: Jul. 18, 2016 at 9:49 PM CDT
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We often speak of “upper level highs” during the summer months, and the high temperatures that usually go hand-in-hand with them.

So, what makes these beasts of summer heat?

Well….we find part of the answer in the upper level wind flow between the deep tropics, say 0 to 10 degrees latitude from the Equator, and Texoma, which is in the sub-tropics around 34 degrees north of the Equator.

In the Equatorial zone, it’s hot year-round and this constant source of heat creates a rising motion which then spreads out to the north and south high above the ground. This current of air travels northeastward because of the earth's rotation, not due north, into the sub-tropics, where it gradually cools. At around 30 degrees north it begins to sink and warm up. After the air reaches the surface it tends to flow back toward the tropics in the Trade Winds. It’s known as a Hadley Cell Circulation after George Hadley, an 18th Century amateur meteorologist who most accurately proposed this flow.

Hadley Cells are huge, thousands of miles long and wide. Upper highs will often form in these descending currents of air. In fact, many areas on the globe around the 30-degree north latitude circle are deserts partly because of this descending air (mountains often play a role as well, blocking moist air from getting inland).

Smaller atmospheric features also contribute to the development of upper highs; what goes up must come down so if a large low-pressure area (rising air) has formed in one place, a zone of sinking air will show up somewhere else to compensate. The location of the “down” currents change with time due to the dynamic nature of weather with its many currents on different scales, some large like the jet stream and the Hadley Cell and some small like a thunderstorm.

The long summer days allow upper highs to perpetuate sometimes for weeks at a time; a large enough “blob” of hot sinking air once formed acts like a dome, and can deflect the jet stream to the north and feed on itself. Severe heat waves as we saw in 2011, 2006, and 2000 are often directly linked to slow-moving and intense upper highs.

Record-breaking summer temperature in Texoma (like in the year 2011) is also helped along by a southwest or west surface wind which further dries and heats the air. An upper high with a south or southeast surface wind can give us brutally humid days, but with maximum temperatures that remain around 100 or less.

The effect of the upper high diminishes over the sub-tropics in the winter/spring as a more active jet stream pushes these zones southward. The lower sun angle also makes the heating process less efficient so the high is much weaker. So while the Hadley Cells go year-round, their effect on upper highs is most noticeable in the summer.

Take Care,

Steve LaNore

Chief Meteorologist

News 12 / KXII-TV