When a young boy sees his breath on a cold winter day, it is because he has made a small cloud. The air he breathes out is full of moisture, and when it leaves his mouth it cools and expands. When it does this, the water vapor in it turns to liquid water, condensing into a cloud.
The process is similar when a cloud forms in the sky. The heat of the sun causes liquid surface water to evaporate. This moisture rises to an altitude where the air pressure is less (because there is less air above it to press down), and as it does it expands and cools.
Air cools about 3 degrees Centigrade for every 1000 feet it rises. At the same time, the dew point falls by half a degree for every 1000 feet of gain in altitude. The dew point is the temperature at which water vapor can condense and become liquid again. At some point, the cooling water vapor reaches the dew point.
To condense though, the vapor must have something to condense onto, a nucleus. This may be dust, salt, smoke, or water droplets that have already formed. Given cooled water vapor and nuclei, the water vapor will become water droplets and a cloud will form. If the cloud is lifted high enough and becomes cold enough, it will be made of more ice than water.
Two main processes lift water vapor up to become clouds. One is convection. In this process, a parcel of air is heated, say by the sun beating on a southwest-facing slope, to the point that it is hotter than the air around it. Hot air, of course, rises. This convection produces the puffy piles of clouds called cumulus. Cumulus means "piled" in Latin. White cumulus clouds often predict fair weather.
Layered clouds are called stratus, from the Latin for layer. The process that forms stratus clouds often occurs when there is a cold front. A cold front is a mass of frigid air that pushes into an area. The leading edge of it is wedge shaped, and the relatively warm moist air already in the area can slide up that edge, forming flat layers of clouds as it rises. So stratus clouds may predict a change in the weather, brought by the cold front that produced them.
There is a classification system for clouds, designed by British pharmacist and amateur weather watcher Luke Howard in 1802. It categorizes clouds by their height, by which meteorologists mean the height of the bottom of the cloud.
The high clouds, above 20,000 feet, are the cirrocumulus, cirrostratus and cirrus. Cirrus is Latin for curl of hair, but as used in meteorology, it indicates the highest clouds.
*Cirrus clouds are thin wispy strands of cloud, sometimes called mare's tails. They form when water vapor freezes at altitudes above 26,000 feet.
*Cirrocumulus clouds are small high puffs of cloud, which often occur in close rows. The traditional name for grouped clouds like this is a mackerel sky. Cirrocumulus is the highest cloud that contains water droplets, though these clouds often contain ice as well.
*Cirrostratus, sometimes formed from a dense gathering of cirrus clouds, are the highest layered clouds, and are often quite thin. They are the sheets of cloud through which the sun or moon can still be seen, often with a halo.
Middle Height Clouds
*Altostratus is a widespread gray layer of cloud that forms between 6,500 and 20,000 feet. It is formed of ice crystals, and may ice up aircraft. It forms from an air mass that is lifted but then condensed.
*Altocumulus is a middle level cloud that is formed by convection. It is puffy and dense, and may be white or gray. One particular form of altocumulus, the lenticular cloud, has sometimes been mistaken for a UFO. It is shaped like the popular idea of a flying saucer, and may be created when air flowing over a mountain ridge forms a standing wave which shapes moisture caught at the peak of the wave. Commercial pilots avoid these, to avoid associated turbulence, but gliders seek them out, for long high rides.
*Fog is commonly a stratus cloud in contact with the ground, though it comes in many types.
*Cumulus clouds are low clouds formed by convection. Low varieties include
*Cumulus humilis, fair-weather clouds
*Cumulus mediocris, slightly taller and puffier.
*Stratocumulus clouds are produced when convective currents produce clouds, but dry stable air above prevents their full development. These clouds are common in the polar oceans, at the edges of the horse latitudes, and following rough weather.
*Nimbostratus takes part of its name from the Latin for rain. It is a flat gray sheet, and does bring wet weather.
Vertical Development clouds
The last category of clouds is the vertical development cloud, known for height.
*Pyrocumulus is a vertical development cloud associated with volcanoes, fires, and sometimes industrial activity. It is dense and dark, perhaps brownish from included smoke or ash, and resembles a cumulonimbus. If a pyrocumulus forms over a forest fire, it may add to firefighters' troubles by sparking more lightening-caused blazes. The convection currents associated with it may also make wind conditions more difficult.
*Cumulonimbus clouds are also classed as vertical development clouds. They contain strong updrafts, are tall, and are associated with thunderstorms and worse. They develop from cumulus clouds when conditions add extra energy to the system. They can produce heavy rain, lightening, thunder, and hail. The sharp convection currents in these towering clouds are dangerous to aircraft, and of course, these clouds may produce tornadoes. Each cloud only lasts 20 minutes at most, because they lose energy with their precipitation. However, conditions can be such that new storm cells form as the old are dying, and thus thunderstorms can go on for hours.
Sometimes we need protection from the severe weather clouds bring, and that is one of the reasons they are studied carefully. For the most part, though, clouds are a wonder that humanity can only observe, learn about, and dreamily enjoy.