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Rh factors, appearance and altitude, aid the observer in determining the name and character of a cloud. For all practical purposes the physical form and appearance must always be the chief feature in cloud determination; experience will teach the observer to determine whether the cloud in question is to be classed as “upper,” “intermediate,” or “lower”; a distinction which is sometimes essential. The following is the classification elaborated by Abercromby and Hildebrandsson and adopted by the International Meteorological Congress:

1. (Ci). —Detached clouds of delicate and fibrous appearance, often showing a featherlike structure, generally of a whitish color. Cirrus clouds take the most varied shapes, such as isolated tufts, thin filaments on a blue sky, threads spreading out in the form of feathers, curved filaments ending in tufts, sometimes called cirrus uncinus, etc.; they are sometimes arranged in parallel belts which cross a portion of the sky in a great circle and, by an effect of perspective, appear to converge toward a point on the horizon, or, if sufficiently extended, towards the opposite point also (Ci-St and Cu-Ci, etc., are also sometimes arranged in similar bands).

Cirrus clouds moving from the southwest indicate falling temperature; moving from the northwest they indicate the probability of rising temperature. They are the mares’ tails and cattails of sailors’ cant. Near the horizon, cirrus clouds may have a stratiform appearance.