Page:Physical Geography of the Sea and its Meteorology.djvu/120

94 the Gulf of Mexico, lat. 27°.5. They are often felt to the west in Mexico, but rarely in eastern or northern Texas. The fact that they are not known in northern Texas goes to show that the cold they bring is not translated by the surface winds from the north.

243. Their severe cold.—A correspondent in Nueces, lat. 27° 36' N., long. 97° 27' W., has described these winds there during the winter of 1859-60: They prevail from November to March, and commence with the thermometer at about 80° or 85°. A calm ensues on the coast; black clouds roll up from the north; the wind is heard several minutes before it is felt; the thermometer begins to fall; the cold norther bursts upon the people, bringing the temperature down to 28°, and sometimes even to 25°, before the inhabitants have time to change clothing and make fires. So severe is the cold, so dry the air, that men and cattle have been known to perish in them. These are the winds which, entering the Gulf and sucking up heat and moisture therefrom, still retain enough of strength to make themselves terrible to mariners—they are the far-famed northers of Vera Cruz.

244. "Cold Snaps."—The temperature of the atmosphere at the height of three or four miles is variable — observations and balloonists tell us so. Air may be brought below the normal temperature due the height at which it may be, by radiation and other processes. It may also be raised above that normal temperature by the setting free there of the latent heat of vapour or by the action of the solar ray upon the cloud stratum. When this upper air is brought to the surface in this abnormal condition, the people of the district upon which it descends find themselves in a "cold snap" or "hot term," as the case may be. 245. Anemometers to determine the inclination of the wind wanted.—That our climates, especially the continental, are affected by, and that many of the changes in the weather are due to, the vertical circulation of the atmosphere, seems clear. We have