Clouds are just water vapor, so why do they move?

Category: Earth Science
Published: December 16, 2012

Clouds are not water vapor. Water vapor is the gas state of H₂O and is invisible. The air around you on a humid summer day is chock full of water vapor, but you don't see any of it. On the other hand, there is very little water vapor in the air during the cold of winter, yet you can easily make clouds with your breath. Clouds are collections of liquid water droplets or ice that are small enough to float. When the water vapor in the air gets cold enough, it condenses back into liquid in the forms of droplets. But the condensation is not automatic. It takes a bit of matter – a condensation nucleus – in order to jump start the process. Dust, salt, and ice in the air do the trick by providing a surface for the water to condense on to, according to the book "Cloud Physics" by Louis J. Battan. Clouds are white because the water droplets making the cloud are the right size to scatter light resonantly according to Mie scattering. Mie scattering does not depend on wavelength, so all colors get reflects. Humans experience a mix of all colors as white. When the water droplets of the cloud get too big, their weight overcomes the cloud's buoyancy and the drops begin falling as rain. Drops that are big enough to fall as rain are also big enough to scatter light geometrically and not according to Mie scattering. We see such drops as clear. Returning back the original question, clouds can move because wind can blow water droplets around.

Clouds are collections of liquid water droplets or ice particles, not water vapor. Public Domain Image, source: Christopher S. Baird.

Christopher S. Baird is a physics professor at West Texas A&M University and author of the book, The Top 50 Science Questions with Surprising Answers.

Topics: cloud, clouds, condensation, gas, humidity, ice, steam, water droplets, water vapor