IN RAIN AND SNOW, IN THE UNITED STATES. 9 



location with reference to level ground and surrounding objects, as trees, houses, 

 or hills being left to the judgment of observers, the rule has not always been 

 observed to place the gauge horizontally at a distance from any such object of at 

 least twice the amount of its elevation above the location of the gauge. The effect 

 on the wind of elevated objects is to produce eddies, often felt at great distances, as 

 may frequently be seen by tlie whirling motion of dust. The rule generally 

 adopted of allowing one-tenth of the depth of snow as its equivalent amount in 

 water, is, of course, a very rough one, as the allowance must depend upon the 

 temperature of the snow, the form and size of the snowflakes, and the depth fallen, 

 since the snow will be compressed by its own weight. For precise measures, the 

 water resulting from the melted snow should always be recorded. The loss by 

 evaporation of the water collected in the gauge, or of the snow when melting, 

 can easily be guarded against. Respecting the elevation of the gauges above the 

 general level of the place, so very little information could be obtained that no 

 special mention of it is made in the tables ; they were generally on the ground, 

 or, at most, but a foot above. There can hardly be too many observers even 

 within quite a limited area, for instance, such as is covered by a city of moderate 

 extent ; the results here collected give abundant proof, not only of large differ- 

 ences in the amount collected in single showers, in consequence of their not unfrc- 

 quently sharp definition of extent, but also in the aggregate monthly and annual 

 amounts. There are a few records of western stations which, in some parts, 

 leave it doubtful whether a blank indicates that no rain fell or whether no obser- 

 vations were made ; not to introduce any uncertainty in the deductions, and in 

 the absence of better information in such cases, it was assumed that no observa- 

 tions were taken. A more embarrassing case to the computer was the uncer- 

 tainty, in many instances, whether melted snow had been included by the observer 

 in his result or not. This question was not unfrequently difficult to decide, but, 

 upon the whole, the discussion of the annual periodicity of the rain gave sufficient 

 proof that no serious error has crept into the results from that source of defective 

 information. No results at stations, when such omissions were known to exist, 

 were admitted in the discussion. 



The following tables, A and B, show the average amount of precipitation for 

 each month, season, and year, and the amounts for a series of years, in the United 

 States, some adjacent parts of North America, and in Central and South America. 

 They have been, as previously stated, deduced from the manuscript tables retained 

 in the Smithsonian Institution for future use and reference. 



December 1971. 



