Explain the different types of clouds and how they are formed

Graphic by: Yiqi Shao

A cloud is a visible aggregate of tiny water droplets and/or ice crystals suspended in the atmosphere and can exist in a variety of shapes and sizes. Some clouds are accompanied by precipitation; rain, snow, hail, sleet, even freezing rain. The purpose of this module is to introduce a number of cloud classifications, different types of precipitation, and the mechanisms responsible for producing them. The Clouds and Precipitation module has been organized into the following sections:

Sections

Development The importance of rising motion and the mechanisms responsible for lifting the air. The States of Water solid, liquid, gas Water is known to exist in three different states; as a solid, liquid or gas. Clouds, snow, and rain are all made of up of some form of water. A cloud is comprised of tiny water droplets and/or ice crystals, a snowflake is an aggregate of many ice crystals, and rain is just liquid water. Water existing as a gas is called water vapor. When referring to the amount of moisture in the air, we are actually referring to the amount of water vapor. If the air is described as "moist", that means the air contains large amounts of water vapor. Common sources of moisture for the United States are the warm moist air masses that flow northward from the Gulf of Mexico and western Atlantic Ocean as well as the moist Pacific air masses brought onshore by the westerlies. As cyclones move eastward from the Rocky Mountains, southerly winds ahead of these storm systems transport the warm moist air northward. Moisture is a necessary ingredient for the production of clouds and precipitation. Cloud Types High, middle and low-level clouds, vertically developed clouds, plus some less common cloud types. Clouds are classified into a system that uses Latin words to describe the appearance of clouds as seen by an observer on the ground. The table below summarizes the four principal components of this classification system (Ahrens, 1994). Latin Root Translation Example cumulus stratus cirrus nimbus heap layer curl of hair rain fair weather cumulus altostratus cirrus cumulonimbus Further classification identifies clouds by height of cloud base. For example, cloud names containing the prefix "cirr-", as in cirrus clouds, are located at high levels while cloud names with the prefix "alto-", as in altostratus, are found at middle levels. This module introduces several cloud groups. The first three groups are identified based upon their height above the ground. The fourth group consists of vertically developed clouds, while the final group consists of a collection of miscellaneous cloud types. Photograph by: Knupp High-Level Clouds High-level clouds form above 20,000 feet (6,000 meters) and since the temperatures are so cold at such high elevations, these clouds are primarily composed of ice crystals. High-level clouds are typically thin and white in appearance, but can appear in a magnificent array of colors when the sun is low on the horizon. Photograph by: Holle Mid-Level Clouds The bases of mid-level clouds typically appear between 6,500 to 20,000 feet (2,000 to 6,000 meters). Because of their lower altitudes, they are composed primarily of water droplets, however, they can also be composed of ice crystals when temperatures are cold enough. Low-level Clouds Low clouds are of mostly composed of water droplets since their bases generally lie below 6,500 feet (2,000 meters). However, when temperatures are cold enough, these clouds may also contain ice particles and snow. Photograph by: Holle Vertically Developed Clouds Probably the most familiar of the classified clouds is the cumulus cloud. Generated most commonly through either thermal convection or frontal lifting, these clouds can grow to heights in excess of 39,000 feet (12,000 meters), releasing incredible amounts of energy through the condensation of water vapor within the cloud itself. Photograph by: Holle Other Cloud Types Finally, we will introduce a collection of miscellaneous cloud types which do not fit into the previous four groups. Classifications Last Update: 07/09/97 High-Level Clouds Cloud types include: cirrus and cirrostratus. Mid-Level Clouds Cloud types include: altocumulus, altostratus. Low-Level Clouds Cloud types include: nimbostratus and stratocumulus. Clouds with Vertical Development Cloud types include: fair weather cumulus and cumulonimbus. Other Cloud Types Cloud types include: contrails, billow clouds, mammatus, orographic and pileus clouds. Precipitation Rain, snow, hail, sleet and freezing rain. Photograph by: Norene McGhiey When cloud particles become too heavy to remain suspended in the air, they fall to the earth as precipitation. Precipitation occurs in a variety of forms; hail, rain, freezing rain, sleet or snow. This portion of the Clouds and Precipitation module focuses on precipitation and has been organized into the following sections. Sections Latest Update: 07/21/97 Rain and Hail Atmospheric conditions that lead to the development of rain and hail. Freezing Rain A detailed look at freezing rain, associated dangers and the conditions that lead to its development. Sleet Atmospheric conditions that lead to the development of sleet. Snow Atmospheric conditions that lead to the development of snow. Acknowledgments Those who contributed to the Precipitation sections of the Clouds and Precipitation module. The navigation menu (left) for this section is called "Precipitation" and the menu items are arranged in a recommended sequence, beginning with this introduction. In addition, this entire web server is accessible in both "graphics" and "text"-based modes, a feature controlled from the blue "User Interface" menu (located beneath the black navigation menus). More information about the user interface options, the navigation system, or WW2010 in general is accessible from About This Server. Acknowledgments Those who contributed to the development of this module. WW2010 Personnel (Clouds): Steven E. Hall - Content Developer and Editor - Scanned in slides, constructed original text and diagrams. Responsible for new layout, organization and cross linking of helper pages. Implemented text and graphics modifications as recommended by the Content Reviewers. John Walsh - Content Reviewer - Professor of Atmospheric Sciences who edited module text and diagrams for scientific accuracy. Ken Beard - Content Reviewer - Professor of Atmospheric Sciences who edited module text and diagrams for scientific accuracy. Mythili Sridhar - Graphics Assistant- Graphically enhanced previous version of this module. Yiqi Shao - HTML Programmer and Graphics Assistant - Helped with the integration of this module into the WW2010 format and constructed home page graphic.

The navigation menu (left) for this module is called "Clouds, Precipitation" and the menu items are arranged in a recommended sequence, beginning with this introduction. In addition, this entire web server is accessible in both "graphics" and "text"-based modes, a feature controlled from the blue "User Interface" menu (located beneath the black navigation menus). More information about the user interface options, the navigation system, or WW2010 in general is accessible from About This Server.