英語聽力文摘 被誤解的水蒸氣

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廚房飄著的云朵

Fill a kettle with water, then turn on the burner.

In a while, your kettle will start belching white billowy stuff into the air. What is this stuff? Steam? Actually, no.

Steam is water that’s heated to two hundred twelve degrees Fahrenheit. Believe it or not, steam is invisible–you can see right through it! If you look closely at the end of your kettle’s spout, you’ll notice that the white stuff doesn’t start right away. It begins billowing about half an inch away from the nozzle, with clear gas in between. This clear gas is the actual steam. The billowy white stuff is what the steam turns into when it hits the drier, cooler air of your kitchen.

Those white billows are, in fact, clouds, not steam. In many ways, they are identical to the clouds you can see in the sky. The white color comes from tiny liquid water droplets that have condensed from the steam.

More accurately, these billows are a type of cloud called a “mixing cloud.” These can form when two separate air masses–with different temperatures and different amounts of water in them–mix together. In the case of your kettle, the hot, steamy gas cools rapidly in the kitchen air, and this sudden coolness is what makes some of the vapor condense.

Mixing clouds are pretty common, and they don’t need to start with steam. You see mixing clouds when you “see your breath” on a cold winter day. You’ll find them rising from a bowl of warm soup. Wherever there’s a mixing cloud, you can bet some warm, moist air is mixing with air that’s cooler and drier.

譯文：

灌滿水壺，扭開爐灶。

不一會兒，你會發(fā)現(xiàn)水壺向空氣噴出滾滾“白氣”。這是什么?水蒸氣?事實上，不是。

水加溫到212華氏度就變成了水蒸氣。不管你信不信，水蒸氣是透明的。如果你仔細觀察壺嘴，會發(fā)現(xiàn)“白氣”離壺嘴有半英尺遠，不是直接從壺嘴噴出，中間還隔著透明的氣體。這透明氣體才是水蒸氣。而翻滾的“白氣”是水蒸氣遇到廚房的干冷空氣凝結(jié)而成。

事實上，裊裊“白氣”不是水蒸氣而是積云。從很多方面比較，和我們看到天空的云朵并無二致。我們會看到白色是因為是水蒸氣凝結(jié)成的小水滴飄浮在空氣中。

更準(zhǔn)確地說，我們稱翻滾的“白氣”為“混合云”。兩種溫度、含水量不同的云團混合就可以產(chǎn)生。比如，壺嘴中噴出的高溫水蒸氣遇到廚房空氣，迅速冷卻，至少水蒸氣凝結(jié)。

“混合云”很普遍，沒有水蒸氣也能產(chǎn)生。冬天天冷時，你會發(fā)現(xiàn)呼出的氣體，變成了“混合云”。同樣，熱湯也會飄出“混合云”。只要有“混合云”，你就可以肯定某些高溫潮濕氣體和低溫干燥氣體相遇且混合。