DYNAMIC METEOROLOGY. 415 



A philosophical prediction as to the origin, development, and move- 

 ment of a storm must take account of the three items above, bat in the 

 rapid approximate -work of the Signal Service indication-room there is 

 need to avoid tables and calculations as much as possible and to seek 

 to obtain from direct observations such indications as they will give. 



The first item is already i)rovided for in the maps showing dew-point 

 and relative humidity. With regard to the second item, the study of the 

 clouds and fog, the past and the prevalent rainfall, the weather, and 

 daily range of temperature from maximum to minimum give indications 

 as to the extent of the process of condensation at any moment, and the 

 evolution of heat dependent thereon, and the student is guided by con- 

 siderations such as the following : 



(a) Fog at the earth's surface bespeaks clear dry air above, free from 

 cloud or haze, through which radiation is taking place rapidly ; hence it 

 implies the absence of uprising convective currents ; hence also the gen- 

 eral jirevaleuce of a stable equilibrium as regards vertical temperature 

 gradient. The time of day at which the fog breaks up under the solar 

 radiation indicates the thickness of the layer. 



(&) Clear weather followed by cirrus clouds, increasing in quantity at 

 the successive reports, and these again followed by small cumuli, indi- 

 cate the advance of overlying areas of moist air or a steadily increas- 

 ing amount of moisture that is being locally carried up into the air. 

 These clouds are soon followed by larger cumuli, and these by rain and 

 storm centers. 



(c) The range of temperature between 11 p. m. and the morning mini- 

 mum (or in lieu thereof an early observation) has its maximum when 

 radiation takes place uninterruptedly through a clear dry atmosphere, 

 and diminishes in proportion as moisture, haze, and cloudiness increase. 

 If therefore we make allowance for the effect of the wind in bringing 

 to the thermometer warm or cold air we shall find this range of tem- 

 perature a valuable index to the condition of the mass of air overhead. 

 In place of this indirect process of reasoning, it is probable that direct 

 observations, with proper apparatus, of the intensity of radiation would 

 give better results. 



(d) The slightest foilnation of haze in a clear sky in the evening 

 proves that the upper layers are cooling to their dew-point. Fre- 

 quently the 11 p. M. observations will give such indications as show that 

 before 7 A. M. the sky will probably be covered with cloud, and possibly 

 followed by rain or snow. This cooling is partly due to radiation, as 

 the vapor is a good radiator, but it is also frequently due to a dynamical 

 cooling of rising air currents flowing toward a distant area of low press- 

 ure. As soon as clouds form, the air beneath ceases to cool by radia- 

 tion and the area that was clear at 11 P. M. is, by sunrise, covered by 

 a layer of cloud that protects the warm air, whose buoyancy increases 

 during the day and specially favors the formation of a local new storm 

 center. In this way many centers originate within a few hours in Mary- 



