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.

為了精確地預(yù)報(bào)天氣和進(jìn)行氣候分析，研究者們需要關(guān)于地球上方大氣循環(huán)的盡可能最好的模型。 現(xiàn)在有一項(xiàng)新的研究對(duì)傳統(tǒng)的大氣運(yùn)動(dòng)理論提出了挑戰(zhàn)。以前的理論認(rèn)為有兩個(gè)大循環(huán)系統(tǒng)——在炎熱的赤道，大氣向上運(yùn)動(dòng)，然后向北極或者南極運(yùn)動(dòng)，并最終冷卻下降。但是，新的研究認(rèn)為實(shí)際存在四種不同的大氣團(tuán)，即赤道以北兩個(gè)，赤道以南兩個(gè)。這項(xiàng)研究發(fā)表在8月21日的《科學(xué)》（Science）雜志上。

按這種新觀點(diǎn)，赤道的空氣同樣地向上運(yùn)動(dòng)，但是在亞熱帶就開(kāi)始向下降，從而形成第一大氣圈。中緯度（北緯以及南緯30-60度）的空氣向上遠(yuǎn)動(dòng)，然后在南北極附近下降，這些空氣構(gòu)成第二大氣圈。這項(xiàng)研究說(shuō)向上運(yùn)行的第二大氣圈的空氣影響地球中緯度的氣溫、風(fēng)以及濕度水平。水蒸汽對(duì)赤道附近天氣起主要影響。在更遠(yuǎn)的南極和北極，水蒸汽也可能對(duì)天氣以及氣候起到同等重要作用。

Vocabulary:
Accurate: 精確的
Forecast: 預(yù)報(bào)
Climate: 氣候
Circulate：循環(huán)
Conventional：傳統(tǒng)的
Equator：赤道
Chill: 冷卻
Posit：斷定；認(rèn)為
Distinct：明顯不同的
Subtropics：亞熱帶
Latitude：緯度
Accounts for：為…負(fù)責(zé)