registering gauge is a most excellent feature of equipment. H. J. Green makes an indoor dial that may be attached readily to any tilting-bucket gauge. Such a device is very convenient.
Intensity of Precipitation.—The intensity of rainfall may be of greater importance than the gross amount. The Marvin and the Friez gauges record intensity graphically. The observer with the ordinary gauge can find the intensity in one way only—by making measurements at regular intervals. During ordinary rainstorms measurements made at half-hour intervals will suffice, and these need be continued for not more than two hours. During heavy summer downpours, however, the measurements should be made at five-minute intervals.
To the farmer, 2 inches of rain distributed over the greater part of the day means a thorough soaking of the ground; but if concentrated within half an hour it means beaten-down grain and washed-out ditches. Such a rainfall, to the engineer, means washouts all along the track; to the city engineer it means flooded sewers and excavations. The engineer who takes care of drainage must know how to guard against phenomenal rainfalls by building so as to take care of them.
Observers will make their work more helpful by noting not only the fact of excessive rainfall but also its rate at five-minute intervals. In Weather Bureau practise, the term excessive has a technical application, and the tabulation of excessive amounts during such intervals is required. The following table shows the intensity of precipitation that technically is excessive. It is based on the experience of many years.
| [1]Duration in minutes |
Duration in inches |
Duration in minutes |
Duration in inches |
| 5 | 0.25 | 35 | 0.55 |
| 10 | 0.30 | 40 | 0.60 |
| 15 | 0.35 | 45 | 0.65 |
| 20 | 0.40 | 50 | 0.70 |
| 25 | 0.45 | 60 | 0.80 |
| 30 | 0.50 | ·· | ···· |
- ↑ At Porto Bello, Panama, 2.48 inches were reported during a period of five minutes, Nov. 29, 1911, at 2:07 a.m. At Curtea de Arges, Rumania, 8.07 inches fell in twenty minutes, July 7, 1889. These are the heaviest rainfalls of record, but they may have been exceeded by cloudbursts in which measurements were not made.
