Answered by Dennis Finn, Research Meteorologist, NOAA, Idaho Falls
Most water in the atmosphere is in the form of water vapor, a gas. Warm air holds more water vapor than cold air. For a climate like Idaho Falls, water vapor content is typically about 0.4-0.5% in the summer and less than half of that during the winter. As air is cooled, part of the water vapor condenses to form water droplets or freezes to form ice crystals. Typical percentages of actual liquid water in clouds (or the equivalent amount of ice) are only about 0.02-0.04%. This is true for puffy white (cumulus) clouds and gray, overcast (stratus) clouds. The thin, hazy clouds high in the sky are composed of ice crystals, are called cirrus, and only have about 0.003% or less. The huge towering cloud formations associated with thunderstorms can have water plus ice contents of up to about 0.3%. Higher percentages are usually limited by the loss of water through precipitation as rain, \snow, or hail or mixing with drier air.
Only a very small amount of clouds consists of dust and other very fine material, much less than the solid, liquid, and gas components of water. However, dust and other particles are very important in cloud formation because they provide surfaces onto which water vapor molecules can easily condense to form water droplets or freeze to form ice crystals when air is cooled. Over land areas, there are usually several hundred to thousands of dust or other tiny particles in every cubic inch of air. Another way to think about it is that fog is really just a cloud that is close to the ground and composed of many tiny water droplets in every cubic inch but that the percentage of actual liquid water in the air is still very small.
Make a Cloud
CAUTION: YOU MUST HAVE HELP FROM AN ADULT
Materials: A clear plastic bottle with a cap, cold water, matches (handled by a grown-up)
- Pour about 1/3 cup of water in the bottle and put the cap on. Shake for 30 seconds. Uncap the bottle.
- Have your adult helper light the match and hold it over the bottle’s mouth.
- Blow out the match by quickly squeezing the bottle. Slowly release the pressure on the bottle to draw the smoke into it.
- Replace the cap on the bottle. Squeeze and release it several times. Do you see a cloud forming in the bottle as you release the squeeze?
Clouds need three things to form. They need water. They also need smoke or salt or dust in the air for the water molecules to condense around. And they need cooler air. When you squeezed the sides of the bottle, you forced the molecules to squeeze together or compress. Releasing the pressure allowed the air to expand, which cooled the air and helped the water molecules stick together more easily.