Why does it have to rain?

So we’re all in the car yesterday, and it is pouring down rain. Buckets of it. And my son asks “why does it have to rain now?”

“Because the clouds are full,” my wife answers. 

“That’s no fair,” replies my son.

This got me thinking. I’m familiar with the basics of the water cycle – I had to learn it in second grade. But, I was a little fuzzy. What actually causes rain?  And so, invoking the power of the Internet, I went to NOAA.

If you don’t recognize the name, NOAA is the National Oceanic and Atmospheric Administration. They’re a federal agency tasked with, among other things, weather forecasting and climate monitoring.   And on a SciJinks page titled What makes it rain?, they reminded me of some stuff I’d learned back in second grade and forgot.

See, we all know that heat makes water evaporate.  And warm air can hold more water than cool air. This warm air condenses around dirt and dust and pollen in the air, making clouds (or fog, depending on altitude).  

How long those clouds last is a function of air temperature, because cooler air will make water condense faster. When too much water has condensed to remain airborne, the water falls.

Voila!  Rain!

So why did it have to rain yesterday?  Because there was a lot of water in the air, and it was cold.