422 



RFX'ORD OF SCIENCE f^OR 1887 AND 1888. 



supply of moist air aud by local topogiapliy ; tliey die out wlieu friction 

 or diy air cuts off tlie moisture or wLeu the supply wiud blows down 

 grade. They apparently owe their characteristics to au upward draft 

 of air toward the dark masses of cloud that are overhead, aud within 

 which the processes of condensation of moisture, evolution of latent 

 heat, and uprising of lighter air are going on with intensity. The de- 

 tails of the mechanism of these storms are fully given by Ferrel in his 

 "Meteorological Researches, part ii," Washington, 1880 (and in his 

 "Recent Advances," Washington, 1886). 



The relative frequency of occurrence of tornadoes in the United States 

 for each month is shown by the following table, as given by Finley, who 

 has however possibly included a few storms that should more properly 

 have ))een classed as destructive thunder storms: 



Monthlg frequency of tornadoes. 



(3) Special local predictions. — Beside the predictions of storms the 

 next most important item is the prediction of the separate elements of 

 the weather that affect local human industries; especially hoar-frost, 

 heat, wiud, and rain. 



(a) Frost. — Severe frosts occur only when the atmosphere is clear ; 

 they are in great part the effect of radiation of heat from the surfaces 

 of plants and the earth. Their prediction depends principally upon 

 the questions, how many degrees will the temperature of the surface of 

 leaves of plants fall by radiation ; if it falls to 32° or less, then will the 

 atmospheric moisture condense into fog or cloud before or after the 

 radiating surface of the plants cools down to that freezing temperature ? 

 If on the evening map there are entered, first, a series of relative num- 

 bers showing the i)rospective clearnessof the sky; second, a seriesshow- 

 ing the time that will elapse before sunrise; then a series showing the 

 probable diurnal range of temperature, an estimate can be made of the fall 

 in temperature for each section of the country before the next sunrise. 

 If now these latter figures are compared with the map showing the 

 depression of the dew-point at 11 P. M., those regions will be perceived 

 over which fog and clouds are likely to be formed; the remaining or 

 clear regions are the only ones in which dew is to be feared, and of 

 these only those are in imminent danger of frost at which the dew- 

 point is below 32°. 



The probable diurnal range of temperature is best found by study- 

 ing the actual fall between 11 p. m. and 7 A. M., as reported during the 

 preceding few clear days from the stations in the neighborliood in 

 question. The preceding rule takes no account of the wind, which if 



