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

The Manta Air Base in Ecuador is currently used by the United States Air Force (USAF) division of the United States Southern Command for operations against illegal cocaine trafficking in northwestern South America.

CENTERFORSECURITYPOLICY: China displaces the U.S. at Ecuador's Manta Base

It was also responsible for the creation of a separate Department of the Air Force from the existing United States Army Air Forces.

CENTERFORSECURITYPOLICY: The National Security Act of 1947

When completed, the merger will put the vast majority of air travel in the United States under the control of just four companies: American, Delta Air Lines, United Continental and Southwest Airlines.

FORBES: American Airlines And US Airways Finally Do It