Quirks & Quarks

As water covers most of the Earth, why isn't it completely shrouded in clouds?

Most water in the atmosphere is in the form of invisible vapour, not clouds, which only appear when cooling causes vapour to condense into water or ice.

Most water in the atmosphere is in the form of invisible vapour, not clouds

Lake Ontario water evaporation forms clouds over the Toronto skyline (Patrick Morrell/CBC)

Originally published on December 14, 2019.

This week's question comes to us from Jeffrey Smith in Saskatoon. He asks: 

The Earth's surface is primarily covered with water, so why isn't it completely shrouded in clouds?

Shannon Sterling, an associate professor of Earth and Environmental Sciences at Dalhousie University in Halifax, explains that in order to answer this question, it is important to understand the water cycle. Water is delivered to the atmosphere by evaporation of surface water from lakes and oceans and transpiration by plants.

This water is in the form of vapour, which is invisible in the atmosphere. In order for the vapour to form clouds, it has to be cooled to condense into liquid water droplets or freeze into ice particles.

This cooling most commonly occurs where air is pushed up higher in the atmosphere. This uplift can happen where surface winds or air masses collide, or where air is pushed up over a mountain top. Because it does not happen everywhere, clouds do not occur everywhere. 

Another reason why clouds do not occur everywhere is that water vapour, the essential ingredient for their formation, is not evenly distributed in the atmosphere. 

Climate change will play a key role in the future of the water cycle. A warmer atmosphere can hold much more water vapour. This is one reason why rain storms are predicted to become more extreme in the future.  


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