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, 'I', -32- members. Many of the markers (or "snow stakes") have identification num- bers about a foot high, cut from sheet metal and attached to the uppermost crossbar or atop the marker pole, 'J1otalp.eight of the marker depends upon the maximum accumulation of snow expected at the site~'A Snow depths are read on these markers from low-flying aircraft. Either the observer is furnished with a stand!l.rdform on which the snow ievelis marked after visual observE\tionor, as is done commonly in Calif- ornia, the marker is photographed with an aerial camera and the snow level is read subsequently in the office, on the resulting,photograph. The markers are simple and easily installed, and a wideflung !l.rray of them caw ~e read quickly under good flying conditions. However, when flying condi- tions are adverse, the visual observer may easily misread a marker, or Wrongly identify a marker that is not clearly numbered. Observations of' snow depth at markers are converted to water equiva- lent according to snow density measured at adjacent courses. Obviously, results are approximate and should be used with discrimination. They are most useful for early-winter forecasts of runoff, when accuracy can be influenced more by future weather than by current variations in snow density. , An aerial surveyor land-surfade traverse of the snow line may be *ade where there is a potential for, melt-water floods. Such determina- tions of snow line supplement concurrent observations of aerial markers, measurements from snow courses, and ,reports from other stations to round but knowledge of snow conditions that may generate flood runoff. Snowljoards Another device for measuring Snowfall and its water equivalent is the snowboard--usually a flat board, painted white, about 3 feet square, placed on the ground or snow surfac~. Snow that collects on the board Within a given period is measured ~th a snow sampler or a standard 8-inch rain gage. Measurements are 'fairly reliable if they are taken soon after each snowfall and if the accumulation on the board has not been modified by drifting, melting, or evaporation. Usually snowboards are installed only at first-order stations of the Weather Bureau, at research stations of other agencies, orin e!l.sily accessible areas of intensive snow studies. SelectiOn of sites The amount of snow that accumulates in any area depends upon the dynamic structUre of the atmosphere 'that produces 'the snow and land-form features of the area on which the snow falls. The land forms that are significant m!l.Y range from the very ,large to the very small. Collectively they may have a pronounced effect on the degree to 'Which measurements of snow on the ground, along courses, or at aerial markers correspond to water equivalent of the snowpack over a basin or other extensive area. tor example, owing to drifts and abnormally large accumulations on the ~ownwind side of obstacles, the amount and water equivalent of snow at marry places is not representative. '