Unlike most other greenhouse gases, water vapour cycles fairly rapidly through our atmosphere. On average a molecule of water vapour will spend 8-11 days in our atmosphere before entering another reservoir in the water cycle.
Click the play button on the video clip above to watch a simplified version of the hydrological cycle.
Your Turn
Question: How might water vapour in the atmosphere result in a negative forcing on Earth's radiation balance in addition to the positive forcing that results from the fact that water vapour is a greenhouse gas?
Hint: Think about how clouds are made and the substances that are necessary for their formation.