If you live anywhere in the world (except Antarctica), you're likely to see cumulus clouds. They drift lazily along like floats in a heavenly parade. They are the fair-weather friends of the sky, although they can grow into thunderstorms at times.
They will generally have a well-defined underside section that is quite flat. Usually, they begin white as cotton but may darken as they grow larger.
A cumulus cloud is ground zero for all thunderstorms.
Air currents are "bubbling" up inside of it in the same way that air pockets rise from the bottom of a heated pan on a stove.
The air cools and condenses inside the cloud to form billions of visible water droplets. It may stop there, due to too much dry air or too little lift, or it may go on to become a cumulonimbus, or thunderhead cloud. Cumulus clouds can form as low as 1200 feet in a very moist atmosphere to as high as 6,000 feet in dry regions. This difference is due to how much the air must cool before you get condensation.
Cumulus clouds offer a visual clue as to the "bumpiness" (turbulence) in the lower atmosphere. Pilots know to use them as a rough gauge of where not to fly. If there is a field of these clouds on a typical spring or summer day, then it will likely be a rougher ride below the cloud bases than above.
Below the base, rising "thermals' of warm air create the towering clouds, while in between the thermal, the air is gently sinking. This makes for upward and downward forces upon the aircraft as it flies along; much like driving down a bumpy road.
A meteorologist can forecast whether or not cumulus clouds will develop by looking at a vertical temperature and moisture profile of the atmosphere, called a sounding.
Air cools as it rises; if it cools more than the surrounding air at that level, then it will lose lift (being heavier) and begin to sink. Clouds will not form on such a day. If, on the other hand, the rising air cools less than the environmental air it will be lighter and continue to rise. This upward momentum is the same force you witness with a hot air balloon: the less dense air within the balloon displaces the heavier air around and it is pressed upward.
Besides the science, there's a soothing appearance to these cotton balls in the sky. The next time you see cumulus clouds in the sky, it might be fun to get a blanket, lie out and gaze up at them in all of their magnificence. See how many animals and people you can find!