The New Student's Reference Work/Cold Wave

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Cold Wave. From time to time great masses of air that have been chilled by the cold soil of northern Canada flow southward over the United States. Such cold waves, as they are called by the United States weather-bureau, usually cause a sudden fall in the temperature to the extent of at least 20°. The mass of cold air, being dense, usually lies near to the surface of the ground. It pushes its way beneath the less dense air of the south, which curls and rises away before it. The clouds which mark the advance of a cold wave, and often bring snow, seem to be caused by the cooling of the southern air, so that it can no longer contain so much water-vapor. A cold wave will sometimes reach even Mexico, where it is known as the Norte.