水烧开了，从水壶里袅袅升起的白气是水蒸气，这是很多人都坚信的常识。但是实际上并非如此。下面前程百利小编为大家带来的托福阅读背景材料告诉我们，水蒸气和这股白气可不是一码事。大家一起来看一下吧。
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.
译文：
灌满水壶，扭开炉灶。
不一会儿，你会发现水壶向空气喷出滚滚“白气”。这是什么?水蒸气?事实上，不是。
水加温到212华氏度就变成了水蒸气。不管你信不信，水蒸气是透明的。如果你仔细观察壶嘴，会发现“白气”离壶嘴有半英尺远，不是直接从壶嘴喷出，中间还隔着透明的气体。这透明气体才是水蒸气。而翻滚的“白气”是水蒸气遇到厨房的干冷空气凝结而成。
事实上，袅袅“白气”不是水蒸气而是积云。从很多方面比较，和我们看到天空的云朵并无二致。我们会看到白色是因为是水蒸气凝结成的小水滴飘浮在空气中。
更准确地说，我们称翻滚的“白气”为“混合云”。两种温度、含水量不同的云团混合就可以产生。比如，壶嘴中喷出的高温水蒸气遇到厨房空气，迅速冷却，至少水蒸气凝结。
“混合云”很普遍，没有水蒸气也能产生。冬天天冷时，你会发现呼出的气体，变成了“混合云”。同样，热汤也会飘出“混合云”。只要有“混合云”，你就可以肯定某些高温潮湿气体和低温干燥气体相遇且混合。
以上就是小编为大家分享的托福阅读背景材料。有关托福阅读的更多内容，请关注前程百利托福考试栏目。
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