Earth's Air in Four Big Cells

For accurate weather forecasting and climate analysis, researchers need the best models possible about how the air circulates above the earth. And a new study is challenging the conventional picture of the planet's air movements. Previous theories pointed to two large circular systems-air rises at the warm equator and then travels toward either pole, where it chills and falls. But the new study posits that there are actually four distinct air masses, two north of the equator and two south. The work appears in the August 21st issue of the journal Science.

In this new view, air again rises at the equator, but falls in the subtropics, making the first cell. The second cell consists of air rising in the middle latitudes-about 30 to 60 degrees north and south-and then falling again near the poles. The study says that this second cell of rising air accounts for temperatures, winds and moisture levels in the mid-latitudes. Water vapor is a major driver of weather events in the tropics. Turns out it may be just as important for weather-and climate-farther north and south.

地球的大气层有4个大气圈

为了精确地预报天气和进行气候分析，研究者们需要关于地球上方大气循环的尽可能最好的模型。 现在有一项新的研究对传统的大气运动理论提出了挑战。以前的理论认为有两个大循环系统--在炎热的赤道，大气向上运动，然后向北极或者南极运动，并最终冷却下降。但是，新的研究认为实际存在四种不同的大气团，即赤道以北两个，赤道以南两个。这项研究发表在8月21日的《科学》（Science）杂志上。

按这种新观点，赤道的空气同样地向上运动，但是在亚热带就开始向下降，从而形成第一大气圈。中纬度（北纬以及南纬30-60度）的空气向上远动，然后在南北极附近下降，这些空气构成第二大气圈。这项研究说向上运行的第二大气圈的空气影响地球中纬度的气温、风以及湿度水平。水蒸汽对赤道附近天气起主要影响。在更远的南极和北极，水蒸汽也可能对天气以及气候起到同等重要作用。

Vocabulary:

Accurate: 精确的

Forecast: 预报

Climate: 气候

Circulate：循环

Conventional：传统的

Equator：赤道

Chill: 冷却

Posit：断定；认为

Distinct：明显不同的

Subtropics：亚热带

Latitude：纬度

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