It's a phenomenon that occurs anywhere you have a large lake that doesn't freeze and have cold air flowing over it, mostly in the Northern Hemisphere Like the great lakes in the United States.

这种现象发生在不结冰的大湖泊附近，这些湖通常被冷空气包围着，并且它们大多数坐落在北半球，就像美国的五大湖一样。

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The immensity of the United States makes air travel common place.

因为美国幅员辽阔，乘飞机旅行很常见。

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In the twenty years that the Clean Air Act has mandated unleaded gas use in the United States, the lead accumulation worldwide has decreased significantly.

在《清洁空气法案》强制要求美国使用无铅气体的二十年里，铅在世界范围内的累积量显著下降。

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Another reason for the increase in bird strikes is the growing popularity of air travel in the United States.

VOA: special.2009.04.01

One of the largest air shows in the United States is also one of the largest trade shows in the aircraft industry.

VOA: special.2010.07.30

We attended this year's United States Air Guitar Championships in Washington,D.C.to learn more about this unusual activity.

VOA: special.2009.08.14

Earlier this month, Delta's shares soared on a rumored union with Chicago-based United Air Lines.

FORBES: Magazine Article

's American Airlines and United Air Lines offer a combined 84 nonstop passenger flights to China per week.

FORBES: Magazine Article

Marwan al-Shehhi is thought to have piloted United Air Lines Flight 175 into the World Trade Center's south tower.

CNN: Uncle calls hijack suspect 'innocent passenger'