云是由大气中的水蒸气凝结成的。
A cloud is a condensation of water vapour in the atmosphere.
蒸气接触冷的表面时，即凝结为水。
Steam is condensed into water when it touches a cold surface.
干燥的空气吸收看不见的水蒸气并且马上就凝结了。
Dry air reabsorbs the water as invisible vapor almost as soon as it condenses.
水蒸气一接触冷的一面立即凝结成水。
蒸气已被凝结成数滴水。
蒸气已被凝结成数滴水。
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