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crossed winds rule A paraphrase of a phrase used by meteorologist Alan Watts in his popular books on weather for sailors. It is a valuable extension of the Buys Ballot's law that relies on observation of the direction of winds aloft from waves and patterns in cirrus clouds (see under winds aloft). See also billow cloud. The Buys-Ballot law determines the direction toward lowest pressure based on the direction of the surface wind. Since the direction of winds aloft is where weather comes from, if the Low is in that direction the weather will deteriorate, if it is the opposite direction, it will improve. Intermediate cases can be analyzed by drawing a typical (idealized) frontal wave pattern (warm front followed by cold front) and labeling the winds aloft direction as following the isobars in the warm sector.

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