DYNAMIC METEOROLOGY. 403 



Adiabatic or pseudo-adiabatic changes of free air that do not involve 

 evaporation either leave the potential temperature unchanged or increase 

 it, but never diminish it. Bezold uses the term to simplify the state- 

 ment of theorems in atmospheric circulation. 



v.— PREDICTION OF STORMS AND WEATHER. 



(28) Abbe. — In 1871 the present writer prepared a short sketch of 

 views held by himself and other meteorologists with reference to the 

 laws of storms and methods of weather predictions. Three editions of 

 this sketch were published by the Signal Office under the title of " Sug- 

 gestions as to the Practical Use of Meteorological Reports and Weather 

 Maps." A fourth edition, under the title of "How to Use Weather 

 Maps," was prepared in 1883 and printed without charts in 1884, but 

 has not yet been published, and will be replaced by a more recent re- 

 port compiled at the request of General Greely as chief signal officer. 

 As a popular presentation of most of the principles that underlie a 

 philosophical system of weather predictions the last v,hapter of the 

 work, as printed in 1884, is perhaps worthy of the attention of the 

 reader, and a rather full synopsis of it is here given, so that the course 

 of the entire argument maybe seen. This chapter is entitled "The 

 Prediction of Storms and the Weather by the use of daily Weather 

 Maps." 



The weather maps first prepared at the Signal Ofiice were based upon 

 observations taken simultaneously at 7:35 A. m., 4:35 and 11:35 P. M., 

 subsequently these hours were changed to 7 A. m.,3 p. m., and 11 P. M., 

 Wasbington time ; after the introduction of standard seventy-fifth meri- 

 dian time the hours were 7, 3, and 10 standard time. Since July, 1888, 

 they have been 8 A. M. and 8 p. m. standard time. The present remarks 

 refer to the earlier series, but are very nearly applicable to the others. 



The early morning map gives the condition of the atmosphere at a 

 moment of almost perfect repose, so far as concerns the disturbing in- 

 fluence of the direct radiation of the sun. 



The mid-day map gives the condition at the moment when this dis- 

 turbing influence is nearly at its maximum. The evening map is, as it 

 were, an instantaneous photograph of the condition of the atmosphere 

 while it is rapidly quieting down during the absence of the sun. 



Were there no heat received from the sun the whole atmosphere, with 

 the land and the ocean, would rapidly cool off, moisture would con- 

 dense, winds would cease, except possibly very gentle currents, and tiie 

 prediction of the weather would be re luced to the simplicity of a per- 

 fect uniformity. The first step in the problem of weather prediction 

 therefore is to determine the nature and amount of the disturbing in- 

 fluences exerted from day to day by the heat of the sun, which latter is 

 the sole ultimate cause of the origin and motion of storms. 



(I) Insolation. — The solar radiation received at any station varies in 

 intensity with the season and the hour of the day, depending slightly 



