They frequently indicate the approach of a warm front and may thicken and lower into stratus, then nimbostratus resulting in rain or snow. 2005)Īltostratus clouds are "strato" type clouds (see below) that possess a flat and uniform type texture in the mid levels. Īltostratus clouds over Kentucky Altocumulus (sunset 23 Nov. The two main type of mid-level clouds are altostratus and altocumulus. Depending on the altitude, time of year, and vertical temperature structure of the troposphere, these clouds may be composed of liquid water droplets, ice crystals, or a combination of the two, including supercooled droplets (i.e., liquid droplets whose temperatures are below freezing). The bases of clouds in the middle level of the troposphere, given the prefix "alto-", appear between 6,500 and 20,000 feet. They also may line up in streets or rows of clouds across the sky denoting localized areas of ascent (cloud axes) and descent (cloud-free channels).
As a warm front approaches, cirrus clouds tend to thicken into cirrostratus, which may, in turn, thicken and lower into altostratus, stratus, and even nimbostratus.įinally, cirrocumulus clouds are layered clouds permeated with small cumuliform lumpiness. When sunlight or moonlight passes through the hexagonal-shaped ice crystals of cirrostratus clouds, the light is dispersed or refracted (similar to light passing through a prism) in such a way that a familiar ring or halo may form. Unlike cirrus, cirrostratus clouds form more of a widespread, veil-like layer (similar to what stratus clouds do in low levels). They often are the first sign of an approaching warm front or upper-level jet streak. Cirrus clouds are wispy, feathery, and composed entirely of ice crystals.
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