RAINFALL AND WIND. 177 



was hifhly unsatisfactory, and in fact impossible, and that the true reason of dimin- 

 ution of apparent rainfall with the height of gauge is the influence of eddies of wind 

 around the building and the mouth of the gauge. This explanation had, however, 

 been also quite clearly pointed out by Prof. Bache, who had shown that eddies around 

 the top of the tower affected the distribution of the rainfall on the tower. Alexan- 

 der Dallas Bache and Joseph Henry were intimately associated in their scientific 

 work as early as 1835 (and especially after Henry came to Washington, in 1847,) and 

 the latter had adopted that which is now called Jevon's explanation, although as 

 we have seen it was first given by Meikle, 1819, and subsequently independently 

 arrived at by many others. This theory was definitely adopted and disseminateil 

 by Henry at least as early as 1853 in connection with his instructions to Smithson- 

 ian observers. 



The essence of this explanation may be stated thus : In the case of ordinary rain- 

 falls we invariably have the air full of large and small drops, including the finer 

 particles that constitute a drizzling mist and the fragments of drops that are broken 

 up by spattering. All these are descending with various velocities which, according 

 to Stokes, depend on their size and density and the viscous resistance of the air; the 

 particles of hail descend even faster than drops of water and the flakes of snow de- 

 scend slower than ordinary drops. Now when the wind strikes an obstacle the de- 

 flected currents on all sides of the obstacle move past the latter more rapidly; 

 therefore, the open mouth of the rain gauge has above it an invisible layer of air 

 whose horizontal motion is more rapid than that of the wind a little distance higher 

 up. Of the falling raindrops the larger ones may descend with a rapidity sufficient 

 to penetrate this swiftly moving layer, but the slower falling drops will be carried 

 over to the leeward of the gauge, and failing to enter it will miss being counted as 

 rainfall, although they go on to the ground near by. Evidently the stronger the 

 wind the larger will be the proportion of small drops that are carried past the 

 gauge; or again, the larger the proportion of small drops and light flakes of snow 

 that constitute a given shower, the more a gauge will lose for a given velocity of 

 the wind. In brief, the loss will depend both upon the velocity of the wind and the 

 velocity of the descent of the precipitation ; therefore, a gauge will in general catch 

 less, in winter than in summer — less in a climate where light, fine rains occur than 

 where the rains are composed of larger, heavier drops ; less in a country or in a season 

 of strong winds than of feeble winds; less when exposed to the full force of the 

 wind by being elevated on a post than when exposed to the feebler winds near the 

 ground. 



The action of the wind in blowing the precipitation over to the leeward of the 

 gauge depends on velocity rather than on the square of the velocity of the wind and 

 of the raindrop, and it is aggravated by the formation of whirls or eddies within the 

 gauge itself by reason of which light and dry snowflakes are even whirled out of the 

 gauge after being once caught in it. Similar remarks apply to the rainfall on the 

 top of a large square building with a flat or depressed roof; not only does the top as 

 a whole receive less than an equal area at the ground, but the distribution of rain- 

 fall an the roof is such that the least rain falls on the windward portion and the 

 most on the portion to leeward, while somewhere on the roof will be found a region 

 whose average rainfall coincides with that on the ground. But the location of this 

 region will vary with the direction and strength of the wind and the <|uality of the 

 precipitation, so that we have but little assurance that any single rain gauge on the 

 roof will rex)resent the rainfall on the ground. 



An interesting illustration of this action of the wind has been noted by me in the 

 case of several gauges established in a cluster in a sandy region. Tlio gauges sat on 

 the ground; their mouths were 2 or 3 feet above the surface, aud being cylindrical 

 they offered considerable resistance to the wind. The windward gauges caught h-ss 

 rain than the leeward, but they also caught more sand, showing that t"he strong 

 winds wliich carried the light raindrops on beyond also stirred up the light surface 

 12444— No. 7 12 



