1148 Home Nature-Study Course. 



(4). Examine the scale at the bottom of the map marked statute miles. 

 How many miles are represented by one inch on the scale ? 



(5). With your ruler find out how many miles one area of high or 

 low has moved in 24 hours ; in 3 days. Divide the distance which the 

 area has moved in 3 days by 3 and this will give the average velocity 

 for one day. 



(6). In the same way find the average velocity of each of the areas 

 on your map for 3 days and write down all your answers. From all 

 your results find the average velocity, that is, how many miles per hour 

 and the general direction which has characterized the movement of the 

 high and low areas. 



Facts for teachers. — 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, Washington, D. C, stating that you wish to post the maps in a public place. 

 A supply of the maps for three successive days for use in these lessons may be 

 obtained in the same way. Blank maps for use in telling a forecast 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 the 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 barometer stands at 30 inches in 

 height in all the regions where that line passes. 



Isotherm comes from two Greek words, isos meaning equal and thertne meaning 

 heat. Therefore, on the map the dotted lines show the region where the tempera- 

 ture 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°. 



Many of the highs and lows enter the U. S. from the Pacific Ocean about the 

 latitude of Washington State or southwest British America. 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 when they leave the U. S. in the 

 latitude of Maine or New Brunswick. If they enter by way of lower California 

 '.hey pass over to the Atlantic, Ocean farther south. The time for the passage 

 of a high or low across the continent averages about ^iVs days, sometimes a little 

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

 marked and more regular. 



The cause of these alternating areas is not yet definitely known. For fuller 

 information than is practicable for us to give here, consult any recent physical 

 geography or an elementary book on meteorology. Knowledge of the atmosphere 

 and the laws which control its phenomena is still imperfect, but enough is known 

 so that many lives and many thousands of dollars worth of property have been 

 saved by the forecasts made by the Weather Bureau. 



