Definition
Marked on charts by red semicircles pointing in the direction of travel. Its shallow slope, around 1 in 150, produces a long lead of cloud: cirrus, then cirrostratus, altostratus and finally nimbostratus with continuous rain over many hours. Pressure falls steadily, wind backs and freshens ahead of it, then veers as it passes, and visibility often deteriorates markedly in the warm sector behind, sometimes to sea fog.
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