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Wind direction, wind velocity and the duration of sunshine are usually recorded on the same sheet, each by a separate magneto apparatus. The revolving drum is driven by a powerful clock.

A record of wind direction is required at all observation stations controlled by the various departments of the government. At seaports wind observations are made the subject of public bulletins and shipping interests are furnished not only with information to date but also with forecasts of expected changes. Warnings of dangerous winds are sent broadcast by wireless for the benefit of all vessels that may encounter them.

The requirements of air navigation are even more exacting than those of marine navigation; for, while the marine pilot needs to know the conditions of wind and weather at sea level, the airman must know them from sea level to an altitude of 10,000 feet or more.

Until recently Weather Bureau stations have been equipped with instruments for the study of surface winds only. The research laboratories, however, have made great advances in the study of air conditions at considerable heights, using kites and pilot balloons carrying recording instruments. Ordinarily the observer must depend on wind vanes, smoke columns, flags, dust movements and clouds for the determination of wind conditions.

Wind Direction.—The prevailing directions of the planetary winds are discussed in another chapter. Observers are concerned chiefly with the direction of the wind at the surface. Ordinarily this does not vary materially from the direction of lower cloud movement; sometimes it does vary, however, and when it does the fact should be recorded as cross-winds,