Weather Study 



879 



HOW TO READ WEATHER MAPS 



Teacher's Story 



Weather maps may be obtained by 

 writing to the nearest Weather Station, 

 or by writing to the Chief of the Weather 

 Bureau, Dr. Willis L. Moore, Washing- 

 ton, D. C., stating that you wish to post 

 the maps in a public place. A supply 

 of maps for three successive days for use 

 in these lessons may be obtained at 20 

 cents per hundred. Sometimes they are 

 sent free, if it is stated that they are 

 to be used for school purposes. 



The words isobar and isotherm have 

 been bogies which have frightened 

 many a teacher from undertaking to 

 teach about weather maps, and yet how 

 simple are the meaning of these two 

 words. Isobar is made up of two 

 Greek words, isos meaning equal and 

 baros meaning weight. Therefore, an 

 isobar means equal weight, and on a map 

 one of these continuous lines means that, 

 wherever it passes, the atmosphere there 

 has equal weight and the barometer 

 stands at equal height. The isobar of 

 30 means that the mercury in the bar- 

 ometer stands 30 inches in height in 

 all the regions where that line passes. 

 "Isotherm" comes from the two Greek words, isos meaning equal and 

 therme meaning heat. Therefore, on the map the dotted lines show the 

 region where the temperature is the same. If at the end of the dotted line 

 you find 60 it means that, wherever that line passes, the thermometer 

 stands at 60 degrees. 



Many of the "highs" and "lows" enter the United States from the 

 Pacific Ocean about the latitude of Washington State or southwest British 

 Columbia. They follow one another alternately, crossing the continent in 

 the general direction of west to east in a path which curves somewhat to the 

 north, and they leave the United States in the latitude of Maine or New 

 Brunswick. If they enter by way of lower California, they pass over to the 

 Atlantic Ocean farther south. The time for the passage of a high or low 

 across the continent averages about three and one-half days, sometimes a 

 little more. These areas are usually more marked in winter, and wind 

 storms are more marked and more regular. 



A low area is called a cyclone and a high area an anti-cyclone. The 

 destructive winds, popularly called cyclones, which occur in certain regions, 

 should be called tornadoes instead, although in fact they are simply small 

 and violent cyclones. But a cyclone, when used in a meteorological sense, 

 extends over thousands of square miles and is not violent ; while a tornado 

 may be only a few rods in diameter and be very destructive. The little 

 whirlwinds which lift the dust in the roads are rotary winds also, but merely 

 the eddies of a gentle wind. 



Dew on clover- leaf. Hoarfrost on 



clover leaf. 

 Photo by W. A. Bentley. 



