MISCONCEPTIONS CONCERNING THE WEATHER 139 



that of the surroiinding country, while partially true, is often exag- 

 gerated in the public mind. According to Professor J. Hann, unques- 

 tionably the leading authority on climate, city temperatures differ from 

 those of the open country nearby in the following respects: The mean 

 annual temperature of the air in places where there are many buildings 

 in from 1° to 2° F. too high, the differences being greatest in the 

 morning and evening, and least at noon. The diurnal range is smaller 

 in cities, especially in summer. The cooling by radiation, at night, is 

 much greater in the open than in places which are built up. The cool- 

 ing due to evaporation probably also plays a part. While it has been 

 calculated that the burning of gas and coal in London develops suffi- 

 cient heat to have an appreciable effect upon the temperature of the air 

 in a stratum 100 feet thick over that city, no progressive increase in the 

 mean temperatures of New York City and Boston can be found to form 

 a parallel with the growth of those cities. The absolute winter minima 

 are much less marked in- the interior of cities than in the surrounding 

 open country. A study of certain cold waves showed that the absolute 

 minimum temperatures recorded in the cities of Toledo, Cleveland, 

 Columbus, and Cincinnati, Ohio, were 20° to 25° F. higher than those 

 noted in the country surrounding these cities. From this the investi- 

 gator concluded that it would be well to put weather stations near, 

 rather than in, large cities, and at a sufficient distance from them to be 

 free from purely local conditions. It should be added that the tem- 

 perature felt in the city, under the influence of the radiation from the 

 heated walls of buildings and the reflection from the bare ground, is 

 very different from that felt in the country. Other meteorological ele- 

 ments which show difference between city and country are sunshine, 

 which, on account of smoke, is somewhat less in cities than in the sur- 

 rounding country, and wind velocity. Every large city has one or more 

 tall buildings about which the wind blows with frequent and violent 

 gusts, even on comparatively calm days. As there is everywhere a rapid 

 increase in wind velocity with height, the taller buildings tend to bring 

 down the higher velocities from aloft. It is thus apparent that there is 

 some degree of difference between the climate of city and country, but 

 when due allowance is made for actual and sensible differences, the effect 

 of the local control upon climate is seen to be small. 



Concerning the course followed by a thunderstorm, there are many 

 and varied misconceptions. It is often remarked that a thunder- 

 storm, upon coming to a river valley or a mountain gorge, will divide 

 into two parts, one moving up and the other down the valley, in other 

 words, that thunderstorms tend to follow valleys. Another statement 

 is to the effect that the center or most severe part of the storm passes 

 not over the point of observation, but at some distance away. Instru- 

 mental observations fail to verify these and similar generalizations. 



