Clouds are made of water. Water is denser than air. Water doesn’t float in the air. Therefore, clouds can’t exist.
Clearly, that’s not true. Clouds do exist, and they do float in the air. How? Why do clouds form? Do clouds fall to the ground? Why do clouds sometimes disappear?
Clouds are created from water vapor that condenses into water droplets, and warm air and water vapor will rise above the cold air around it1. Your breath on a chilly winter day or the steam from a tea kettle are examples of water vapor that rises. Are clouds warmer than the surrounding air, and if so, what makes clouds warm?
Clouds form when the sun creates warm, moist air by heating and evaporating water on the earth’s surface. The warm, moist air is less dense than the cold air above it, so that warm air rises2. The warm air cools as it comes into contact with the cooler air above. Cold air cannot hold as much moisture as warm air: the vapor has to condense into a liquid. This is the beginning of a cloud.
The sun heats the earth, and causes water on the ground to evaporate3. The water rises, cools, and condenses. A cloud is formed!
Clouds form when warm wet air rises and condenses in cold air. This explains why clouds exist, but now how do clouds stay in the air? Once the cloud forms in the cold air, why doesn’t the cloud cool down and sink back to the ground?
There are several reasons clouds float: first, the droplets in a cloud are small. Very small. An average water droplet in a cloud may only be 20 micrometers across4. That is half as wide as a typical human hair, and about the same size across as a particle of dust5. Even though dust is heavier than the air around it, a dust particle is so small that it can float in the air for a long time before falling. Water droplets in air behave the same way as dust6.
The second reason that clouds can float in the air is that there is a constant flow of warm air rising to meet the cloud: the warm air pushes up on the cloud and keeps it afloat.
Third, clouds stay warmer than the air around them because they absorb the sun’s energy better than the surrounding air7.
Clouds don’t float forever—if the surrounding air warms up, then the air is able to contain the cloud’s moisture as vapor, and the cloud will disappear. And sometimes, the cloud becomes so large and moist that the water droplets in the cloud begin sticking to each other, and grow bigger and bigger. The water droplets become so big that they no longer behave like dust particles. The droplets begin to fall. If you look up at this kind of cloud, your face will be wet: those droplets are now called “rain” and you really ought to go inside.
This map shows how much cloud has fallen to the ground in 2017. More green means more rain8.
By Kyle Schiber, Nature Museum Volunteer
 Khan, Sal. “Ideal Gas Equation: PV=nRT” Khan Academy. 2017. Retrieved November 27, 2017 from https://www.khanacademy.org/science/chemistry/gases-and-kinetic-molecular-theory/v/ideal-gas-equation-pv-nrt
 NC State University Climate Education for K-12. “How Clouds Form”. (August 13, 2013). Retrieved November 27, 2017, from http://climate.ncsu.edu/edu/k12/.cloudformation
 Precipitation Measurement Missions, NASA. “The Water Cycle” diagram by NOAA National Weather Service Jetstream. (June 8, 2011). Retrieved November 27, 2017, from https://pmm.nasa.gov/image-gallery/water-cycle
 Space Math, NASA S’COOL Team. “Cloud Droplets and Rain Drops”. Retrieved November 27, 2017, from https://scool.larc.nasa.gov/lesson_plans/CloudDropletsRainDrops.pdf
 Engineering Toolbox. “Particle Sizes”. Retrieved November 27, 2017, from https://www.engineeringtoolbox.com/particle-sizes-d_934.html
 Scientific American. “Why do clouds float when they have tons of water in them?” (May 31, 1999). Retrieved November 27, 2017, from https://www.scientificamerican.com/article/why-do-clouds-float-when/
 My NASA Data. “Measuring the Temperature of the Sky and Clouds”. (2017). Retrieved November 27, 2017, from https://mynasadata.larc.nasa.gov/science_projects/measuring-the-temperature-of-the-sky-and-clouds/
 National Temperature and Precipitation Maps, NOAA. “Total Precipitation, January-October 2017, CONUS”. Retrieved November 27, 2017, from https://www.ncdc.noaa.gov/temp-and-precip/us-maps/ytd/201710?products=prcp-total#us-maps-select