![]() ![]() An exception to this is at the moment of formation when a spec or small, hard looking tuft of cirrus can have many thousands per liter of tiny quasispherical ice crystals which then gradually disperse after the moment of formation. The coldest cirriform cloud tops (i.e., cirrus and cirrostratus) can be −80 ☌ or lower in deep storms with high cloud tops such as in anvils associated with exceptional thunderstorms.Ĭirrus and cirrostratus clouds are fibrous, wispy, and diffuse because the concentrations of ice crystals that comprise them are relatively low (from less than 1 per liter to tens per liter) compared with particle concentrations in other clouds. The ‘bases’ or visual bottoms of cirrus and cirrostratus clouds are composed of generally low concentrations of ice crystals that are about to evaporate. In these cases, droplets may be briefly present at the instant of formation. However, many such clouds so described would actually be classified as ‘altostratus’ clouds by ground observers due to the gray shading they produced.) Cirrus ( Figures 1 and 2) and cirrostratus ( Figure 3) clouds are composed of ice crystals with, perhaps, a few momentary exceptions at formation when the temperature is higher than −40 ☌. ![]() (Many users of satellite data refer to ‘cirrus’ or ‘cirriform’ those clouds with cold tops in the upper troposphere without regard to whether they produce shading as seen from below. By WMO definition, they are not dense enough to produce shading except when the sun is near the horizon, with the single exception of a thick patchy cirrus species called cirrus spissatus ( Figure 2) in which gray shading is allowable. Rangno (Retiree), in Encyclopedia of Atmospheric Sciences (Second Edition), 2015 High CloudsĬirrus, cirrostratus, and cirrocumulus clouds ( Figures 1–5, respectively) comprise ‘high’ clouds. ![]()
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