THE MODERN WEATHER BUREAU. 

 Br Prop. Cleveland Abbe, M.A. 



", 



[Read Wednesday, January 29th, 1890.] 

 This broad title may imply that I am to give a comparative study of 

 the various weather bureaus of the world ; but I shall to-night ouly 

 have time to give some account of the Signal Service at Washington. 

 It will be very gratifying to me if anything I may say or do shall 

 contribute toward the founding of a similar organisation for the 

 prediction of wind and weather in the South African States. 



The study of the weather and the efforts to predict its changes are 

 matters of most ancient and universal custom. History shows that 

 the world has passed through several stages of weatner science in its 

 progress from barbaric ignorance to our present beginnings of a 

 rational scientific meteorology. At the present stage of this science 

 we wholly disclaim any belief in the special influence of the stars, 

 planets or moon. We do not believe that plants or animals can 

 furnish indications of the future weather. We have given up en- 

 deavouring to discover recurring cycles of storms and weather. We 

 attach no especial importance to electricity or sun spots as a means of 

 prediction. The modern meteorologist defines climatology as the study 

 of averages in their relation to animal and vegetable life ; but restricts 

 meteorology to the study of the motions and phenomena of the 

 atmospheric air and its moisture ; this is therefore a study of the 

 dynamics of gases and vapours ; it requires the solution of a series of 

 complex problems in fluid motion and thermo-dynamics. The forces 

 that govern the atmosphere are numerous ; the principal ones that I 

 have been accustomed to consider in my daily predictions are as 

 follows : (1) the sun's heat ; (2) the radiation from the earth, the 

 air and the clouds ; (3) the moisture in the air ; (4) the evaporation 

 from the land and water surfaces ; (5) the differences of density of 

 hot and cold, or of dry and moist air ; (6) the horizontal flow of air 

 on a spherical earth whose diurnal revolution causes the well-known 

 deflection to the right in the Northern Hemisphere, but to the left in 

 the Southern, whence the equatorial and the polar depressions ; (7) the 

 influence of continents and mountains and especially of plateaux in 

 causing upward deflections and consequent cooling and cloud with 

 rain ; (8) the cooling of ascending expanding air up to the level of 

 cloud formations, and the influence of condensing vapour on the 

 subsequent history of the cloud ; (9) the less important influence of 

 the variations of gravity with altitude and with latitude ; (10) the 

 variations in insolation due to the diurnal and annual changes in the 

 position of the sun with reference to the zenith; (11) the less 

 important conduction of heat from below the surface of the ground ; 



