METEOROLOGY 311 



than at the earth's surface, but this is explaiued by the fact that gener- 

 ally the fog did uot reach up to the higher poiut. {Z. 0. G. M., xix, p. 84.) 



187. Professor Wollny, investigating the influence of the ground upon 

 the temperature of the air, especially for altitudes up to 2 meters, shows 

 Ihat the air above a field covered with vegetation is materially cooler than 

 that over one lying fallow, and again that the variations of temperature 

 above the former are less than above the latter. This he explains as due 

 to the fact that the vegetation prevents the direct warming of the soil 

 by the sun; and even more important is the consumption of heat in the 

 evaporation of water at the surface of the leaves. Similarly at night 

 time the plants prevent the cooling of the earth's surface by radiation 

 during clear weather, and on the other hand themselves give out an ex- 

 traordinary amount of heat, whereby the cooling of the air is made still 

 less. Even this cooling process is, however, checked by the formation of 

 dew and the evolution of latent heat. The difference in temperature 

 over cultivated and uncultivated land diminishes in proportion as the 

 evaporation by day and the radiation by night are modified; therefore 

 it almost disappears in the winter season or in severe droughts, and es- 

 pecially in cloudy weather. {Z. 0. G. M., xix, p. 539.) 



188. Dr. E. Lamp, of the Kiel Observatory, quotes observations show- 

 ing that it is not at all necessary, as frequently implied, that the tem- 

 l)erature should be uniform in different portions of a fog. Thus he ob- 

 served in a thermometer shelter -f-2o.2 C. ; in the free air near by + 1°.8 ; 

 at the summit of the wind tower [for anemometer exposure ?J, -f 4^,2 ; at 

 its bottom, +'^°G; at the base of the observatory hill, — 0o.2; on the 

 side of the hill rapid variations of temperature within five minutes 

 from +2O.0 to +0^.5. The observations were not taken with a sling- 

 thermometer, as would have been preferable. (i>. 31. Z., i, p. 474.) 



189. [These, like the numermis observations made by 'Prof, H. A. 

 Hazen in various parts of Washington, show that great local tempera- 

 ture differences invariably exist, so that the meteorologist, in choosing 

 the exposure for his thermometer, must be guided by the object of in- 

 vestigation or the use to which his figures will probably be i)ut. At 

 Columbus, Ohio, Louisville, Ky., and Davenport, Iowa, differences, on 

 cold, clear nights, of 25° V. have been observed. The Signal Service 

 rule (partly necessity and partly justified by the results) is to place tbe 

 thermometer shelter on the highest available jioint, like the roofs of 

 houses or towers, in the cities where the stations must do tbeir work. 

 The local difference of temperature of the air over cities and forests and 

 prairies is in this way reduced to a minimum in the daily telegraphic 

 work, and we attain for general weather i)redictious the average tern 

 perature of the mass of air blowing past the station. Were any other, 

 especially any very low-ground exposure adopted, it would be proper 

 and necessary to reduce such observations up to the uniform height of 

 about 100 feet above the surface of the ground.] 



190. A. Buchan has i)ublished in the Journal of the Scottish Meteoro- 



