Types of Clouds
Formation of Clouds
Oceans, lakes, and other bodies of water on the earth's surface undergo continuous evaporation. As a result, the atmosphere always contains water vapor. If this vapor is sufficiently cooled, it condenses into countless tiny droplets of water that may form clouds.
The principal way in which water vapor is cooled is by expansion. When moist air at the surface of the earth is heated by the sun, it rises in masses like great bubbles. As these air masses rise they encounter constantly decreasing atmospheric pressure, which causes them to expand. Since all gases cool when their volume is expanded, the masses of moist air become cooler as they rise and their water vapor condenses into clouds. The rising air masses form separate clouds, not layers such as those produced by the mixing of cold and warm air. Air masses may also be forced upward by having to pass over mountain ranges; by wind convergence; or by the movement of fronts, which are the boundaries between air masses.
Why don't all clouds produce rain?
Have you ever flown through clouds in an aircraft, or perhaps been high up on a mountain where the clouds swirled all about yon? Then you must have gained a fairly good idea of what a cloud is: just an accumulation of mist.
As you know, there is always water vapor in the air. During the summer there is more of this vapor in the air because the temperature is higher. When there is so much water vapor in the air that just a small reduction in temperature will make the vapor condense (form tiny droplets of water), we say the air is saturated.
It takes only a slight drop in temperature to make water vapor condense in saturated air. So when saturated warm air rises to an altitude where the temperature is lower, condensation takes place and we have a cloud. The molecules of water have come together to form countless little droplets.
What happens if all these water droplets in a cloud meet a mass of warm air? They evaporate—and the cloud disappears! This is why clouds are constantly changing shape. The water in them is changing back and forth from vapor to liquid.
The droplets of water in a cloud have weight, so gravity gradually pulls them down and they sink lower and lower. As most of them fall, they reach a warmer layer of air, and this warmer air causes them to evaporate. So here we have clouds that don't produce rain. They evaporate before the drops can reach the earth as rain.
But suppose the air beneath a cloud is not warmer air? Suppose it's very moist air? Naturally, the droplets won't evaporate. Instead, the droplets will get bigger and bigger as more and more condensation takes place. Pretty soon, each tiny droplet has become a drop and it continues falling downward and we have rain.
How Condensation Takes Place
At any given temperature a given volume of air can hold only a certain maximum amount of water vapor. When it holds this amount the air is said to be saturated. The lower the temperature, the less water vapor it takes to saturate the air. This means that a mass of moist but unsatu-rated air may become saturated after it has become sufficiently cool. If saturated air is cooled below a certain temperature, called the dew point, its water vapor condenses.
Condensation in the atmosphere, and therefore cloud formation, begins only in the presence of tiny particles of smoke, salt, or certain other substances. Such particles are called condensation nuclei. The role of condensation nuclei in cloud formation is not entirely clear, although it is known that water droplets condense on them. It is also known that the nature and size of the particles are important factors in the condensation of water vapor.
Kinds of Clouds
Clouds are classified according to their shape and altitude. The shape of a cloud depends largely upon the way it is formed. Hundreds of different cloud shapes are recognized by meteorologists. However, all clouds that are commonly seen are variations or combinations of three basic cloud types: stratus, cumulus, and cirrus.
Stratus Clouds. Stratus clouds form flat layers that obscure the sun and therefore appear grayish. They are usually found at altitudes of anywhere from ground level up to 6,000 feet. Typically, stratus clouds occur over regions of low atmospheric pressure. Alto-stratus, or high-altitude stratus, clouds are found up to 20,000 feet.
Cumulus Clouds. Cumulus clouds are single, billowy clouds with cauliflower-shaped tops and flat bases. In sunlight they appear brilliant white. The bases of cumulus clouds may be found as low as 2,000 feet over water and as high as 8,000 feet above ground. The tops vary greatly in altitude, some extending up to 50,000 feet.
Cirrus Clouds. Cirrus clouds are white, thin, and fibrous. They may form in small patches or in bands, or streaks, and they are usually found at altitudes above 20,000 feet. Cirrus clouds are composed entirely of ice crystals because of the very low temperatures at the altitudes where they occur.
Supercooled Clouds. The water droplets in clouds do not usually freeze into ice at 0° C. (32° F.), although this is the normal melting point of ice. In many cases clouds must reach heights where the temperature is lower than -10° C. (14° F.) before their water freezes into ice. At temperatures lower than -40° C. (-40° F.) all clouds consist of ice particles. Clouds that consist of water droplets at temperatures below 0° C. are called supercooled clouds, and they are of great importance in the formation of rain.
Nacreous and Noctilucent Clouds. In addition to the common clouds that are seen in the sky almost every day, there are two other kinds of clouds that are seen very rarely. They are called nacreous clouds and noc-tilucent clouds. Nacreous clouds occur at altitudes of about 20 miles, and noctilucent clouds at altitudes of about 50 miles. Meteorologists are uncertain about the composition of these clouds and how they are formed.
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