CoachA cloud is a great number of tiny droplets of water, or tiny pieces of ice, floating together in the air. Fog is a cloud that touches the ground.
示例
Example 7.2 (Your breath on a cold morning)
On a cold morning, your breath makes a little white cloud in front of your mouth. The air you breathe out is warm and holds water vapour. The cold air cools it at once, the vapour condenses into tiny droplets — and a miniature cloud appears, then vanishes as the droplets evaporate again.