The stratosphere is the second atmospheric layer above the Earth’s surface. This layer is characterized by both vertical stability and dry air, allowing little cloud development. While almost all weather occurs in the troposphere below the stratosphere, there are still times when certain types of clouds can be found in the stratosphere. Examples of these stratospheric clouds include polar stratospheric clouds, intense cumulonimbus clouds, and wind-related clouds.
Atmospheric layers
The Earth’s atmosphere is divided into five distinct layers. The lowest level of these is called the troposphere, from the surface of the Earth to a height of about ten thousand miles. The troposphere is the atmospheric field in which almost all of Earth’s weather and clouds occur. The next layer of the atmosphere is the stratosphere, extending from about ten to 31 miles in height. Above the stratosphere is the mesosphere, extending from 31 to 50 miles in height. The mesosphere is the atmospheric layer responsible for consuming meteors. The next layer is the thermosphere, which extends in height from 50 to 600 miles. Finally, the exosphere refers to the point where the Earth’s atmosphere sinks into space.
Of the Stratosphere
The stratosphere is a highly stratified atmospheric region with little vertical motion. Unlike the troposphere where temperature decreases with height, temperatures in the stratosphere actually increase with height. This temperature inversion is caused by solar radiation (the sun) heating the ozone molecules found in the stratosphere. Atmospheric forcing is the result of warm air rising in cooler air. The temperature inversion of the stratosphere produces the opposite result – very stable and stratified atmospheric conditions. These conditions are clearly adverse to cloud development. In addition, the air within the stratosphere is very dry, containing little water vapor, which forms clouds.
The height at which the stratosphere begins is not so consistent as in the rest, varying with latitude and time. The base is usually situated about ten miles from the equator, and the poles are lowered up to five miles. The base is higher during the summer, while it goes down during the winter.
Cloud Genera
Clouds are typically divided into three categories, based on the height of the base. Low clouds form between the surface and 6,500 feet. Medium level clouds, used in high altitude, range in altitude from 6,500 to 23,000 feet. High clouds, using the prefix cirrus, range in altitude from 16,000 to 42,000. The fourth kind, which some say is a vertical cloud, which is called a cumulus; but they can still be distinguished by the height from which the base begins. These three levels (or four) contain almost all the common types of clouds that all occur in the troposphere.
Stratospheric Clouds
Despite the difficulties presented by the stratosphere, there are some instances where clouds can be found in this atmosphere. In winter, polar stratospheric clouds (PSCs) can form in the lower stratosphere, around the poles. These clouds are composed entirely of ice crystals. Seasonally mature cumulonimbus cells, such as supercells, can sometimes reach heights of 60,000 feet. However, it requires an immense amount of upward elevators for vertical cloud development to reach a stable stratosphere. In addition, these storm cells must also overcome horizontal shear by the jet stream, which flows just below the stratosphere. These things conspire against the clouds trying to reach the stratosphere, creating the anvil seen from the top of the thunderbolt. Concurrently unable to support further vertical development in the stratosphere, the jet streams are sheared off by upper-level winds. Finally, intense hurricanes can foster cloud development, which has the ability to reach up to 60,000 feet, knocking as much as 10,000 feet into the stratosphere.
Sources:
“Stratosphere” Windows to the Universe
“The Troposphere” Windows Universe
Michael Pidwirny, Ph.D., “Atmospheric Layers,” The Encyclopedia of Earth Michael Pidwirny, Ph.D.
“Cloud Genres” Windows to the Universe
“The Clouds” Aeronautical Learning Laboratory for Science, Technology, and Research
Clare Averill and Greg McFarquhar, “Hurricane Wilma: A Natural Disaster,” NASA: Earth Observatory
