Introduction

Clouds are usually the most obvious feature of the sky. They both reflect weather patterns and play a role in what the weather does. In addition to their obvious role as sources of precipitation, clouds also can affect the temperatures of the places below them. Clouds not only block incoming sunlight during the day, which cools the air, but they can also block outgoing radiation from the Earth, which can warm temperatures. Many unanswered questions about Earth's climate revolve around the roles of clouds. Clouds also create some patterns of light in the sky such as halos.

Locations of Clouds

• Low-level clouds: (generally found below 6,500 feet, or 2,000 meters) Low-level clouds are usually composed of liquid water droplets, but they can have snow and ice crystals in cold weather
• Mid-level clouds: (generally found between 6,500 and 23,000 feet, or 2,000 and 7,000 meters) Most mid-level clouds are composed of liquid water droplets during summer and a liquid droplet-ice crystal mix during winter. Mid-level cloud names are preceded by an "alto" prefix.
• High-level clouds: (generally found above 20,000 feet, or 6,000 meters) High-level clouds are composed of ice crystals and tend to be very thin and wispy. High-level cloud names are preceded by a "cirro" index

Types of Clouds

The names of clouds usually indicate both the cloud's location in the sky, as noted above, and its type, as listed below. The listings below give several examples of cloud names.

• Stratus clouds are a uniform gray and usually cover most of the sky.
• Cirrus clouds are thin and high in the sky.
• Cumulus clouds are lumpy and can stretch high into the sky.

	Stratus Clouds Stratus clouds are uniform gray clouds that usually cover the entire sky. They can form when very weak, upward vertical air currents lift a thin layer of air high enough to initiate condensation. Stratus clouds also form when a layer of air is cooled from below to its dewpoint temperature and water vapor condenses into liquid droplets. Stratus clouds look like a layer of fog that never reaches the ground. In fact, fog that "lifts" off the ground forms a layer of low stratus clouds. Precipitation rarely falls from true stratus clouds since the upward vertical motion needed for precipitation is very weak, but light mist and drizzle can sometimes accompany stratus clouds.
	Cirrus Clouds Cirrus clouds are thin, wispy clouds that usually form above 18,000 feet. These clouds are blown by strong westerly winds aloft into streamers known as "mares' tails" Cirrus clouds generally move from west to east across the sky and usually "point" to fair weather. Cirrus clouds form when water vapor undergoes deposition and forms ice crystals. Cirrus clouds are thin because they form in the higher levels of the atmosphere where little water vapor is present.
	Cumulus Clouds Cumulus clouds form as water vapor condenses in strong, upward air currents above the earth's surface. These clouds usually have flat bases and lumpy tops. Cumulus clouds are usually very isolated with large areas of blue sky in between the clouds. Most cumulus clouds form below 6,000 feet and are relatively thin and associated with fair weather. However, when the atmosphere becomes unstable and very strong, upward air currents form, cumulus clouds can grow into cumulus congestus, or towering cumulus. If the atmosphere is unstable enough, cumulonimbus clouds, better known as thunderstorms, form. Cumulus congestus and cumulonimbus clouds can tower from below 6,000 feet to greater than 50,000 feet.

Additionally, a prefix is frequently given to the cloud name to indicate what level of the atmosphere it is in.

• Cirro is the prefix given to high clouds, those with bases above 20,000 feet.
• Alto is the prefix given to mid-level clouds, those between 6,000 and 20,000 feet.
• Nimbo added to the beginning, or nimbus added to the end of a cloud name means the cloud is producing precipiation.

The system is by no means uniform. There is no term for low clouds, and there are some odd joinings, such as stratocumulus, which is a cloud with two different shapes.

How are clouds formed?

Water evaporates from land, lakes, and sea and is carried by the air as water vapour. Warm air can hold more water vapour than cold air. As warm air rises and cools, for example over a mountain, the water vapour condenses to water, forming clouds. Eventually, the water falls from the clouds as rain. The rainwater runs back into the rivers and lakes.

Source: http://www.gridclub.com/fact_gadget/best_ever_qa/our_world/environment/1432.html