Fill a kettle with water, then turn on the burner.
灌满水壶，扭开炉灶。
In a while, your kettle will start belching white billowy stuff into the air.
不一会儿，你会发现水壶向空气喷出滚滚“白气”。
What is this stuff? Steam?
这是什么？水蒸气？
Actually, no.
事实上，不是。
Steam is water that's heated to two hundred twelve degrees Fahrenheit.
水加温到212华氏度就变成了水蒸气。
Believe it or not, steam is invisible–you can seeright through it!
不管你信不信，水蒸气是透明的。
If you look closely at the end of your kettle's spout, you'll notice that the white stuff doesn't start right away.
如果你仔细观察壶嘴，会发现“白气”离壶嘴有半英尺远,
It begins billowing about half aninch away from the nozzle, with clear gas in between.
不是直接从壶嘴喷出，中间还隔着透明的气体。
This clear gas is the actual steam.
这透明气体才是水蒸气。
The billowy white stuff is what the steam turns into when it hits the drier, cooler air of your kitchen.
而翻滚的“白气”是水蒸气遇到厨房的干冷空气凝结而成。
Those white billows are, in fact, clouds, not steam.
事实上，袅袅“白气”不是水蒸气而是积云。
In many ways, they are identical to the cloudsyou can see in the sky.
从很多方面比较，和我们看到天空的云朵并无二致。
The white color comes from tiny liquid water droplets that have condensed from the steam.
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。
More accurately, these billows are a type of cloud called a “mixing cloud.”
更准确地说，我们称翻滚的“白气”为“混合云”。
These can form when two separate air masses–with different temperatures and different amounts of water in them–mix together.
两种温度、含水量不同的云团混合就可以产生。
In the case of your kettle, the hot, steamy gas cools rapidly in the kitchen air, and thissudden coolness is what makes some of the vapor condense.
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。
Mixing clouds are pretty common, and they don't need to start with steam.
“混合云”很普遍，没有水蒸气也能产生。
You see mixingclouds when you “see your breath” on a cold winter day.
冬天天冷时，你会发现呼出的气体，变成了“混合云”。
You'll find them rising from a bowl of warm soup.
同样，热汤也会飘出“混合云”。
Wherever there's a mixing cloud, you can bet some warm, moist air is mixing with air that's cooler and drier.
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。
