The term blue norther denotes a weather phenomenon common to large areas of the world's temperate zones—a rapidly moving autumnal cold front that causes temperatures to drop quickly and that often brings with it precipitation followed by a period of blue skies and cold weather. What is peculiar to Texas is the term itself. The derivation of blue norther is unclear; at least three folk attributions exist. The term refers, some say, to a norther that sweeps "out of the Panhandle under a blue-black sky"—that is, to a cold front named for the appearance of its leading edge. Another account states that the term refers to the appearance of the sky after the front has blown through, as the mid-nineteenth-century variant blew-tailed norther illustrates. Yet another derives the term from the fact that one supposedly turns blue from the cold brought by the front. Variants include blue whistler, used by J. Frank Dobie, and, in Oklahoma, blue darter and blue blizzard. Though the latter two phrases are found out-of-state, blue norther itself is a pure Texasism. The dramatic effects of the blue norther have been noted and exaggerated since Spanish times in Texas. But that the blue norther is unique to Texas is folklore.