COLD WAVE. A term first applied by the United States Weather Bureau in 1872 to the areas of cold, clear, dry air that Row near the ground from Canada southward over the United States and become the so-called 'northers' when they reach the Gulf States, or 'Nodes' when they reach the Gulf coast of Mexico and Yuca tan. The northers of Colon may possibly have a different origin. The cold stratum of air, being quite shallow, keeps to the lowlands and rarely rises to the 5000-foot level; there are hut one or two cases on record in which it attained the altitude of Cheyenne or Santa F6; often it is not deep enough to overflow the 3000-foot level of the Appalachian Range. The cold wave ad vances with a well-defined front, marked by a sudden fall of temperature and an outflowing wind that undoubtedly curls upward and over flows backward, forming an advancing border of clouds with spits of rain or snow. The baro metric pressure underneath this cloud is a few hundredths of an inch higher than in front of it, and it is this difference of pressure that causes the mass of cold air to underflow and lift up the warmer air as it spreads southward toward the equator. This excess of pressure is in part caused by gravity or the hydrostatic pressure due to the weight of the air in the rear, and is also in part the result of the diurnal rota tion of the earth on its axis, giving a centrifugal force to the denser cold air greater than that of the neighboring warm air. The progress
southward or southeastward of the front of a cold wave is so steady that, having charted its position at several successive moments by means of telegraphic reports, the Weather Bu reau has almost always been able to forecast its future progress with satisfactory accuracy, thereby enabling all interested in the matter to make provision against sudden drops in tem perature, which often exceed 30° iu twenty-four hours. According to the technical definition adopted by the Weather Bureau, the forecast of a cold wave (as made by hoisting the cold-wave flag) implies that there will be a drop of at least 20° within twenty-four hours, and that the tem perature will go below freezing. Similar sudden changes in the warmer half of the year, when temperatures do not go below freezing, are simply cool waves.