Why does warm air hold more moisture than cold air?
Many meteorologists and science educators like to put the word “hold” in quotation marks in questions like this because warm air does not hold more moisture than cool air in the sense that a big glass holds more water than a small glass. In basic terms, what happens is that the warmer the air the faster all of the molecules in it, including water vapor, are moving. As the air cools the molecules slow down and more of them are likely to be going slowly enough to be “captured” by water that is already liquid. For more on this, go to the USATODAY.com graphic on Molecular motion determines water’s state. The graphic and the links from it should help you understand what’s going on. For students who want to get into detail and especially for teachers, I recommend the link to Alistair B. Fraser’s “Bad clouds” Web pages.
