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Cirrostratus cloud

earth science Maturity 5-7 Next article →

Cirrostratus is a specific genus of high-altitude clouds. These clouds are very thin and usually form wide, uniform sheets. Because they are composed of ice crystals, they are made of frozen water particles. This composition makes them translucent rather than opaque.

MoonHaloDonnellyMillsWA 2005 SeanMcClean
MoonHaloDonnellyMillsWA 2005 SeanMcClean

When these clouds are present, the sky may take on a milky appearance. People often use the phrase "milky sunshine" to describe this effect. These clouds are important because they indicate a large amount of moisture in the upper troposphere.

The appearance of cirrostratus depends on its specific structure. One form is called cirrostratus nebulosus, which acts like a featureless, thin veil. This nebulous form can cause optical effects like halos. A halo is a bright ring of light that appears around the moon or sun. Another form is cirrostratus fibratus, which has a distinct fibrous texture. This type looks like many small threads and does not produce halos. Within the fibratus species, you might also see pattern-based varieties. These include the closely spaced duplicatus type or the wavy undulatus type.

Highcloudsymbols
Highcloudsymbols

Scientists use the SYNOP code to identify and track these clouds. If the cloud is a high, fibrous sheet, it is identified as cirrostratus fibratus, or Cs fib. This is coded as CH8, or sometimes CH5 or CH6 depending on the sky coverage. If the cloud covers the entire sky as a featureless veil, it is classified as cirrostratus nebulosus, or Cs neb. This specific species is assigned the code CH7. These precise classifications allow meteorologists to monitor changes in the atmosphere accurately.

Cirrostratus clouds are often reliable indicators of approaching weather systems. They frequently signal the approach of a warm front. This happens when cirrus clouds appear first and then spread across the sky. If this occurs, precipitation might follow within 12 to 24 hours. If the frontal system is moving quickly, precipitation could arrive in just 6 to 8 hours. However, if the cirrostratus appears as fragmented clouds, the front may be weak. In such cases, the resulting precipitation might be light drizzle or snow grains rather than moderate rain or snow.

These clouds can also form through the transformation of other cloud types. This process is known as being a "mutatus" or "genitus" cloud. For example, cirrostratus fibratus cumulonimbogenitus forms when the top of a mature thundercloud spreads out and flattens. Cirrostratus fibratus cirrocumulogenitus occurs when cirrocumulus clouds flatten and lose their structure. There are also clouds that change completely, such as cirromutatus. This happens when cirrus or cirrocumulus clouds undergo a total transformation into cirrostratus. Even altostratus can become cirrostratus if a grey layer thins out into a white, featureless layer.

Cirrostratus also interacts with other cloud layers in the atmosphere. You will often find cumulus humilis or stratocumulus clouds sitting below cirrostratus formations. This happens because the presence of cirrostratus is associated with stable air. This stability creates an atmospheric inversion that restricts convection. This restriction causes the lower cumuliform clouds to become flattened. Additionally, man-made contrails from airplanes tend to spread out in these clouds. These trails can remain visible for up to one hour when they are within a cirrostratus layer.

While most cirrostratus form in the upper troposphere, there are rare exceptions. Similar clouds can occasionally form in the lower stratosphere over the polar regions of Earth. These are known as polar stratospheric clouds. They can look like cirrostratus because they are composed of tiny supercooled droplets of water or nitric acid. While most clouds produce precipitation, cirrostratus clouds themselves produce no precipitation or virga. They also do not appear with any accessory clouds, maintaining their distinct, high-altitude identity.

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