Created: 11/30/2017 Updated: 12/5/2017
In this series, we'll be addressing some common questions from visitors and readers. Do you have a science question that has you stumped? Ask our museum scientists via our form here and we'll answer it on our blog!
How do clouds float?
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.
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
Do you have a science question that has you stumped? Ask our museum scientists via our form here and we'll answer it on our blog!View Comments
Created: 11/21/2017 Updated: 11/22/2017
I’m never more grateful for the gifts of nature than during the approach to the Thanksgiving holiday.
Our Midwest Thanksgivings are accentuated by the profound season change to autumn, featuring abundant harvests, wildlife preparing for winter and leaves changing color then dropping along with the temperature. The holiday brings our relationships into focus through nature’s lens and through family gatherings.
Anticipating family time and pausing to express gratitude for one another and our blessings affirms our feelings of connectedness. In our family, it also affirms a connection to nature. Even with our children now all adults, our family gatherings and adventures usually find us outdoors in nature -- playing, exploring, watching, reflecting, or running in a Turkey Trot. It just feels good to be outdoors.
Our Thanksgiving celebration and meal is generally followed by a stroll outdoors, much as we did when I was growing up in Chicago’s west side where my police officer father connected to nature through his garden. He loved growing and harvesting the vegetables, and faithfully prepared the soil for its dormant, winter rest. He did so with appreciation and anticipation of spring planting.
Wherever you spend the holiday, a nature connection is not far away. Take a stroll outdoors. You may be rewarded by hearing the distinctive trill of sandhill cranes on their migration south. This week is expected to be the peak time for these large, highflyers to pass over Chicago. Many other birds, such as robins, are gathering in groups in preparation to migrate or are busy eating as many berries as seeds as possible to “bulk up” for winter.
Notice nature. For example, watch squirrels scurrying around lawns and parks, busily stocking up on fallen seeds and nuts. Pause to notice the trees in your neighborhood. Most are nearly leafless and reveal the tangles of branches and limbs reaching skyward. Watch and listen with appreciation for nature’s stories.
Like the connections we foster with our family and friends by spending time together, our connection to nature grows stronger through the time we spend in it.
May you and your family deepen your connections and share in the abundance of nature’s gifts that we gratefully share. Happy Thanksgiving.