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Fill a kettle with water, then turn on the burner.
灌滿水壺，扭開爐灶。
In a while, your kettle will start belching white billowy stuff into the air.
不一會兒，你會發現水壺向空氣噴出滾滾“白氣”。
What is this stuff? Steam?
這是什么？水蒸氣？
Actually, no.
事實上，不是。
Steam is water that's heated to two hundred twelve degrees Fahrenheit.
水加溫到212華氏度就變成了水蒸氣。
Believe it or not, steam is invisible–you can seeright through it!
不管你信不信，水蒸氣是透明的。
If you look closely at the end of your kettle's spout, you'll notice that the white stuff doesn't start right away.
如果你仔細觀察壺嘴，會發現“白氣”離壺嘴有半英尺遠,
It begins billowing about half aninch away from the nozzle, with clear gas in between.
不是直接從壺嘴噴出，中間還隔著透明的氣體。
This clear gas is the actual steam.
這透明氣體才是水蒸氣。
The billowy white stuff is what the steam turns into when it hits the drier, cooler air of your kitchen.
而翻滾的“白氣”是水蒸氣遇到廚房的干冷空氣凝結而成。
Those white billows are, in fact, clouds, not steam.
事實上，裊裊“白氣”不是水蒸氣而是積云。
In many ways, they are identical to the cloudsyou can see in the sky.
從很多方面比較，和我們看到天空的云朵并無二致。
The white color comes from tiny liquid water droplets that have condensed from the steam.
我們會看到白色是因為是水蒸氣凝結成的小水滴飄浮在空氣中。
More accurately, these billows are a type of cloud called a “mixing cloud.”
更準確地說，我們稱翻滾的“白氣”為“混合云”。
These can form when two separate air masses–with different temperatures and different amounts of water in them–mix together.
兩種溫度、含水量不同的云團混合就可以產生。
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，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。
Mixing clouds are pretty common, and they don't need to start with steam.
“混合云”很普遍，沒有水蒸氣也能產生。
You see mixingclouds when you “see your breath” on a cold winter day.
冬天天冷時，你會發現呼出的氣體，變成了“混合云”。
You'll find them rising from a bowl of warm soup.
同樣，熱湯也會飄出“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溫潮濕氣體和低溫干燥氣體相遇且混合。
