When you see cirrus you can be sure the air is very dry; if the air were moist other types of cloud would form at lower levels. In an otherwise clear sky, cirrus may mean fine weather will continue, but cirrus can also appear at the leading edge of a warm front. If cirrus increases to cover most of the sky, and especially if it forms a continuous sheet called cirrostratus, wind and rain may soon follow, although you may be out of the direct line of advance of the front (and escape the worst of the weather). Cirrus is made up of falling ice crystals drawn out by the wind into filaments. In this picture (left), crystals falling from a patch of cirrus are being drawn into filaments by the wind. The longer the filaments, the stronger the wind. Sailors once used cirrus clouds as a "wind warning."
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