Page:Handbook of Meteorology.djvu/244

 is about the most expeditious method of measuring snowfall yet devised.

In the western slope of the Sierra Nevada Mountains “seasonal gauges,” with collectors large enough to hold the accumulation of snow and rain for several weeks, are employed. The catch is weighed at convenient times.

When deep snowfalls occur the Marvin snow tube has been found a most convenient device. This tube, as improved by Church, is made of galvanized iron and is 2.75 inches in diameter. The upper end is left open; the lower end is reinforced by a piece of tubing forced inside the measuring tube. The lower edge of the tube is serrated with teeth like those of a cross-cut saw in order to facilitate boring through crusted snow and sheets of ice.

Tube and core are weighed by a spring balance that records inches and hundredths. The tube has also an engraved scale to show the depth of snow. Church, working in the Sierra Nevada ranges, used the tube in snow accumulations 30 feet thick.

The Marvin density bucket provides a quick and accurate method of obtaining the rain equivalent. The copper bucket is inverted and pressed lightly into the snow until the top of the snow touches the bottom of the bucket. The bucket, even full of snow, is then weighed on the accompanying scales, which are graduated to per cent of weight of an equal volume of water. Thus, if the net weight—the total weight minus that of the bucket—is 0.12 on the scales, it means that the snow is 12 per cent of an equal volume of water. Assuming that the depth of snow is 20 inches, 0.12 × 20, or 2.40 inches is the equivalent depth of rainfall.

In mountain regions the intensity of precipitation varies with altitude. On the western slopes of the high cordillera of the Pacific coast, McAdie found the greatest intensity of precipitation between 4000 and 5000 feet above the valley floor. On the eastern slope the intensity decreased irregularly with decreasing altitude. It may be incautious to assume this to be true elsewhere, but; as a general truth, the basis of assumption is not unreasonable. It is safe to assume that the measurement of precipitation of the montane part of a watershed must extend from its upper limit to the valley floor.