Clouds form when saturated air reaches  its dew point, the temperature at which, under a given set of barometric conditions, water  vapour condenses into liquid due to cooling air or increased saturation from an adjacent water  source.

当饱和空气达到露点时， 云就会形成。露点是在给定的气压条件下，水蒸气由于冷却空气或邻近水源饱和度增加而凝结成液体的温度。