iSTORMS AND WEATHER FORECASTS 7:i 



no longer questioned. The forecaster at each river center con- 

 siders the rainfall, the temperature, tlie melting of snow, if tliere 

 be any, the area and slope of the watershed, and the permea- 

 bility of tile soil. From a study of floods in former years, he 

 knows the time necessary for the flow of the water from the 

 tributaries to the main stream and the time required for the 

 ])assage of the flood-crests from one city to another. 'I'he fore- 

 casts are, of course, empirically made, but still they are sufli- 

 ciently accurate to possess great value to the people of the river 

 districts. Some idea of the vast destruction of i)roperty due to 

 floods may be gathered from the statement that the floods of 

 1881 and 1882 caused a loss of not less than $15,000,000 to the 

 proi)erty interests of the Ohio and ]\Iississi])pi valleys. There 

 was also a loss of 188 lives. In 1884 the region about Cincinnati 

 alone suff'ered a loss of over $10,000,000 in property. 



Chart No. II shows a winter storm central in Iowa at 8 a. m., 

 December 15, 189o. The word "low " marks the storm center. 

 It is the one place in all the United States where the barometer 

 reading is the lowest. The heavy, black lines, oval and nearly 

 concentric about the low, show the gradation of air-pressure as it 

 increases quite unifornil}' in all directions from the storm center 

 outward. 



The arrows fly with the wind, and, as will be seen, are almost 

 without exception moving toward the low or storm center, 

 clearly demonstrating the effect of gravity in causing the air to 

 flow from the several regions marked high, where the air is ab- 

 normally heavv, toward the low, where the air is lighter. As 

 the velocity of water flowing down an inclined plane depends 

 both on the slope of the plane and on the roughness of its sur- 

 face, so the velocity of the wind as it blows along the surface 

 of the earth toward the storm center depends on the amount of 

 the depression of the barometer at the centen- and the resistance 

 offered by surfaces of varying degrees of roughness. The small 

 figures i)laced at the end of the arrows indicate high wind 

 velocities. At Chicago, where the wind is blowing at the rate of 

 40 miles per hour, the anemometer is 270 feet high, while at 

 Minneapolis, where the instrument is so low as to be in the 

 stratum whose velocity is restricted by the resistance encountered 

 in flowing over forests to the northward, the rate is not great 

 enough to be marked by a sj^ecial figure. 



Now picture in your mind the fact that all the air inside the 

 isobar (heavy black line) marked 30.2, as it moves inward is ro- 



