E\\~.\IZ'l-, B.SC. I Received August !2l--He;id December 16, 1325.1Thc measurement of rainfall is probably the simplest of all quantitative meteorological observations so far as the skill demanded of the observer is concerned, but it has long been realised that the accurate reading of a raingauge is by no means the same thing as the accurate measurement of rainfall. It may safely be asserted that, except on occasions of zero wind speed, any raingauge as ordinarily mounted and exposed will register less rain than actually falls; but in the absence of any suitable standard of comparison it is impossible t o say by how much the catch in any particular instance falls short of the full amount.The chief cause of deficiency, the only one, in fact, of which account will be taken in the present discussion, is uind. Eddies set up k-q. the projccting portions oi the gauge a r c sufficient t o deflect froni the gauge drops which nloilld normally be caught, and at times these may even be strong enough to eject drops which have already entered the funnel. On the other hand the gauge gains ;I certain amount from the fine spray caused by thc splashing of drops o i i the ground round about, and in soilit' situations spray from adj;iccnt water surfaces and drops of water blown from trees may introduce an element of uncertainty into thc records.The muses of error in raingauges hii\-C bcen discussed by several writers and a convenient summary of the phenomena may be obt;iiiicrl by reference to the I;'ncyc.hpct.di,c Britccnnicn (10th Edition), where the subject is referred to in the articles on Meteorology and Water Supply. ' Long experience has prored that, other things remaining the same, a raingauge at a height will register less than one lower down, owing to the greater wind speed usually found a t the higher altitude; the differences in some cases are very striking. Even with the raingauge rim a t a height of only one foot, as in the standard British exposure, the effect of wind is still appreciable. It is hardly practicable to reduce the height of the gauge for normal use, so that the only possible measure t o be taken t o minimise wind effects further is t o shelter the gauge in sonie way from the wind without a t the s:ime time guarding it cram the rain. W e have n o means of knowing precisely how near we may be approaching to thc true rainfall with any system of shielding, natural or artificial, but we may by experiment succeed in effecting definite improvement in rainfall measurements even though we still leave them imperfect.I t is sufficiently evident that proximity to buildings or hedges will result in a certain amount of shielding for a gauge, but it is equally obvious that for some wind directions the gain by shielding may be more than counterbalanced by rain being shut off directly from the gauge unless the distance is properly selected. It has been laid down, therefore, that for the exposute of a gauge to be 1 See also an article by E. Gold in the Meteorological Bfngasinc for October, 1 9~2 .
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