Cumulus CloudsCredit: photo byRalph F. Kresge from the NOAA archive uploaded from Wikimedia commons


Lying on the lawn gazing up at the clouds in the sky and seeing what shape they form used to be a favorite pastime of many kids.  There was never a question of what the clouds were called, but sometimes the question arose, “just what makes up that cloud?”  This of course brought on other questions such as “how can a plane fly through it?” or “can a person stand on a cloud?”  Cloud formation actually determines the classification of cloud types.

 Cloud types are classified according to their Latin names which are attributed to the chemist Luke Howard.  The Latin words to name clouds are:

  • Cirrus which means “curl of hair,”
  • Stratus which means “layer,”
  • Cumulus which means “heap” and
  • Nimbus which means “rain.”

 In addition to their classification; clouds are divided into four groups; the first three based on their distance from the ground. These four groups are:

  1. High-level clouds are over 20,000 feet (6000 meters) above the ground
  2. Mid-level clouds are 6,500 to 20,000 feet (2,000-6,000 meters) above the ground
  3. Low-level clouds are below 6,500 feet (2,000 meters) above the ground
  4. Vertically developed clouds- these clouds can’t be classified by distance from ground because of their thickness

 It must be added there are some clouds that simply don’t seem to fit into any of the four groupings.  These cloud types include mammatus, pileus clouds, billow clouds, contrails, and orographic.

 High-Level Clouds

 High level clouds are made of tiny ice crystals rather than water droplets like the lower level clouds.  This is due to their temperature of less than -40F.  There are three types of high level clouds.

 Cirrus: These clouds are usually the first clouds to be seen in a clear blue sky.  They never produceCirrus cloudsCredit: photo by Ralph F. Kresge from NOAA archive uploaded from wikimedia commons snow or rain on earth’s surface.  They are feathery and curly and their movement can indicate the direction and strength of high altitude winds.

 Cirrocumulus:  These clouds are like small puffy white balls either singular or in a row.  When these clouds form in a row, they look like they ripple across the sky. This feature distinguishes them from cirrus and cirrostratus clouds.

 Cirrostratus: These clouds are like a sheet and form above the 20,000 feet mark. They are almost transparent in appearance which lets the sun and moon clearly show through.  They may have a halo effect produced when light passes through the ice crystals. These clouds are often predictors of approaching precipitation.

 Mid-Level Clouds

 There are two categories of mid-level clouds and are classified with the prefix “alto.”

Altostratus: These Altocumulus CloudsCredit: photo by Ralph F. Kresge from the NOAA archive uploaded from wikimedia commonsclouds cover the entire sky often extending over a hundred square miles. They contain both water droplets and ice crystals and are often the predictor of oncoming precipitation.

 Altocumulus:  These clouds are look like fuzzy, puffy bubbles in long rows. They are white or grey, or a combination of both and usually have a shadowy darkness underneath their base. Without the shadowing, these clouds may get confused with cirrocumulus, but the cirrocumulus will appear smaller than a finger width held at arm’s length.

 Low-Level Clouds

 Low-level clouds are the clouds closest to the ground and in fact can actually be on the ground in the form of fog.  There are three types of low-level clouds.

Stratus:  The lower layer of these stratus clouds look like a blanket across the sky. It is these clouds that float at ground level as fog. Unlike the cumulus cloud, the stratus develops horizontally rather than vertically.

 Stratocumulus:  The stratocumulus cloud is like a puffy layer in the sky. They are grey with dark shading.   ThougNimbostratus CloudsCredit: photo courtesy of German Wikipedia, original upload 26. Sep 2002 by Ben-Zin NOAAh they do not produce rain, they often form after a rainstorm.

 Nimbostratus:  These clouds obscure the sun and look dark and wet.  They are associated with rain or snow and are sometimes considered mid-level clouds because of their immense thickness.


Vertically Developed Clouds

 The vertically developed clouds are the clouds that cause people to take notice perhaps more than others.  These clouds are the ones children find shapes in and are the bringers of severe weather conditions.  If clouds could be popular, these clouds most likely would top the list. There are two types of vertically developed clouds.

 CumulonimbusCredit: photo by Ralph F. Kresge from the NOAA archive uploaded from Wikimedia commonsCumulus:  These clouds look like huge white ball of cotton.  Most of the time, they are singular with blue sky in between.  They have flat bases and uneven, fluffy, lumpy tops.

 Cumulonimbus:  This is the granddaddy of all clouds.  The top of this cloud is often shaped like an anvil and can reach approximately seven and half miles.  On rare occasions, these clouds can penetrate the stratosphere.  The lower portion of the cloud consists of mostly water droplets and the higher portion of ice crystals.  Winds inside the clouds can reach more than sixty miles per hour.  These clouds are the storm clouds; they produce severe weather conditions such as thunder and lightning and tornadoes.  

 How Clouds are Formed

 As to be expected, the different types of clouds are formed by different processes. However, all clouds have in common the basic rules of science in their formation.  All processes involve cooling the air.  When air cools, it no longer can hold the same amount of water vapor as it did when it was warm. The extra water vapor begins to condense into liquid water droplets.

 The water vapor droplets need something on which to condense, dust, pollen or some other sort of particle called condensation nuclei.   When enough condensation takes place on the condensation nuclei, clouds are formed.  Some of the water droplets may fall to earth as rain, snow, hail or sleet.  There are four main ways clouds form.

 Surface heating:  Cumulus, Cumulonimbus, and stratocumulus are formed by this method.   The earth’s surface is heated by the sun which in turn heats the air.  The warm lighter air begins to rise and the lower pressures at the higher altitudes causes the air to expand.  As the air expands, its pressure drops and the air cools. As it continues to rise, it continues to cool.  As the cooling air becomes unable to hold the water vapor, the extra vapor condenses. As it rises higher and higher, the more it expands, cools, and condenses.  Eventually, enough condensation is formed to create a cloud.  When the earth’s surface is cooled instead of heated, fog and stratus clouds are formed.

 Mountains and Terrain:  Stratus clouds are also formed by mountains and other terrain.  In this formation, the air is hits the mountain range or other terrain and rises higher in the atmosphere. As the air rises, it processes the same as previously described.  Mountains can also cause cloud formation because they are warmer than the surrounding air and this causes the air to rise.  Cumulus and cumulonimbus clouds can be formed in this manner.

 Air Masses Being Forced to Rise:  All types of clouds can be formed by this process.  Air is forced to rise by three methods; 1) a low pressure system in which wind blows into the center from all directions causing the air to rise; 2) an upward slope in land,; and 3) weather fronts which cause lighter warmer air to flow over heavier colder air.

 Weather Fronts:   Both cold and warm weather fronts can produce clouds.  WhStorm CloudsCredit: photo courtesy of NOAA uploaded from wikimeida commonsen two large masses of air collide at the surface of the earth it creates a front.   Most cloud types are produced when a warm front moves in and replaces cold air by sliding its warm ir above it.  When a cold front moves in, the heavier cold air pushes the lighter warm air upward.  Cold fronts can produce stratus clouds, nimbostratus and stratocumulus. The most common cloud formation by cold fronts is the storm clouds, the cumulonimbus.





The copyright of the article “Types of Clouds and How They Form”  is owned by Cheryl Weldon and permission to republish in print or online must be granted by the author in writing.