Page:Handbook of Meteorology.djvu/47

Rh years but normals once established seldom change materially.

It is the custom of many observers to note, as a part of the daily record, the number of degrees above or below the daily normal; this is the “departure from the normal.” If below the normal the number is prefixed by a minus sign. It is an excellent plan to carry the algebraic sum of the daily departures to the end of the year. Many of the daily neswpapers desire these figures as a matter of public interest.

The monthly normals, by comparison, furnish the most instructive data concerning the temperature conditions of a given locality. Thus the January mean at Devils Lake, North Dakota, is 0°; at New Orleans it is 53°. The one is an inland station in comparatively high latitude; the other is practically a coast station in much lower latitude. The January mean of San Francisco is 50°; that of New York is 30°. Both are coast stations, but San Francisco is warmed by ocean winds. For the eastern part of the United States, the coast stations excepted, January normals are not far from 30°, and July normals range from 70° to 75°. At Moorhead, Minnesota, the summer mean is 67°; at San Francisco, 58°; at Seattle, 63°; at New Orleans, 81°; at Key West, 83°; at Yuma, 90°. A few stations excepted, January is the coldest and July the warmest month.

Temperature and Prevailing Winds.—Land winds are marked by great ranges in temperature. In regions far from the sea, changing winds are far more frequent than in maritime regions. Some of these winds, like the anticyclones which bring cold waves, are widespread in prevalence; others, like the simoon, an intensely hot and dry wind, are confined mainly to desert regions.

Throughout the greater part of Europe and the United States, westerly winds prevail; in summer they are frequently from the southwest, and in winter mainly from the northwest. The Pacific Coast of the United States receives ocean winds, and the winters are mild; west of the high mountain ranges zero temperatures rarely if ever occur. Along the coast, summer