Geography of Clouds

| |

Clouds are a visible mass of small water drops or ice crystals formed by the condensation of water vapor in the atmosphere, usually at a considerable height above the earth’s surface.  Clouds are caused mainly by the adiabatic cooling of air below, its dew point.

Clouds are grouped by whether they are high, medium, or low clouds.  There are ten basic cloud types. A couple of example of clouds are:

Cirrus clouds

These clouds are high, white and thin. They are composed of ice crystals. They form delicate patches and give a fibrous and feathery appearance.  Other high clouds include cirrostratus and cirrocumulus.

Cirrus clouds.  Photo: Caitlin Dempsey.
Cirrus clouds. Photo: Caitlin Dempsey.

Cumulus clouds

These low clouds have a flat base and have the appearance of rising domes. These clouds are often described as having cauliflower structure. Other types of low clouds are:  stratocumulus, stratus, nimbostratus, and cumulo nimbus.

Cumulus clouds.  Photo: Caitlin Dempsey.
Cumulus clouds. Photo: Caitlin Dempsey.

Stratus clouds

These low clouds are fairly thin and blanket like. Subdivided into high clouds, medium clouds and low clouds.

Types of medium clouds include: altocumulus, nimbostratus, and altostratus. Alto and Nimbo are the two prefixes which refer to middle level clouds and low clouds of considerable thickness with dark grey appearance respectively.

Are Contrails Clouds?

Contrails are narrow, elongated clouds that form when exhaust from airplanes crystalizes at high altitudes. Contrails appear as narrow, white bands in the sky.

The Jemez Red Rocks at Walatowa, along with several contrails evident in the sky. Photo: Annette Olson, USGS. Public domain.
The Jemez Red Rocks at Walatowa, along with several contrails evident in the sky. Photo: Annette Olson, USGS. Public domain.

Precipitation and Clouds

Precipitation is the formation of water particles or ice within the cloud that falls towards the earth’s surface. It occurs when condensation takes place rapidly within the cloud.

A localized heavy summer rainstorm in Colorado. Photo: Howard Perlman, USGS. Public domain.
A localized heavy summer rainstorm in Colorado. Photo: Howard Perlman, USGS. Public domain.

Precipitation in the form of ice pellets (hail stones) that develops in and fall from cumulo nimbus clouds, either at a cold front or where intense heating of the surface causes rapidly ascending convection currents called Hails.

Where Do Clouds Occur on Earth?

Clouds can be found all around the world. One study looked at almost a decade of satellite imagery to calculate that clouds cover about 67% of the Earth.

The amount of cloud cover in any given located varies and is influenced by a factors such as humidity, landscapes, and wind. Cloud cover tends to be higher over oceans where only about ten percent of oceans are cloud-free. Over land, that percentage rises to about 30% cloud free.

NASA created a map that shows average cloudiness around the world. Cloud cover from satellite imagery collected between July 2002 and April 2015 was averaged for this cloud fraction map. Areas that are white indicate a higher cloud cover while darker blue areas indicate no clouds.

World map of cloud fraction based on satellite imagery from July 2002 to April 2015.  Map: NASA.
World map of cloud fraction based on satellite imagery from July 2002 to April 2015. Map: NASA.

Desert regions like the Sahara Desert and the Atacama Desert have very low humidity and subsequently very little to no cloud cover. Regions with high humidity like the Amazon also have a high cloud cover.

This article was originally published on March 19, 2011 and has been updated with new information

Related

Share:


Enter your email to receive the Geography Realm newsletter: