Page:Handbook of Meteorology.djvu/251

 Beaufort wind scale affords a very good and practical method of approximate determination of wind velocity. The force numbers of the scale adopted by the Weather Bureau are the same as those of the British scale; the velocity in miles per hour corresponding to the force numbers differs considerably.

The table, p. 240, gives the Beaufort number, designation of wind and velocity as adopted by the Weather Bureau. The physical effects are those of the British scale. Cooperative and volunteer observers report merely the prevailing direction of the surface' winds, except as specifically directed. At the regular Weather Bureau stations the direction and velocity of upper winds are noted, a necessary step for the information of the rapidly growing air service. More information concerning the times of the daily maxima and minima of wind velocity is needed for the benefit of air service, and volunteer observers may be very helpful in obtaining this information.

The Anemometer.—For all ordinary purposes in the measurement of wind velocity the Robinson anemometer is almost universally used. For determining the mean velocity of the wind it is the best instrument in meteorological service. Inasmuch as it fails to record momentary gusts of wind perfectly, a Dines pressure anemometer is usually added to the equipment of regular stations. A wind meter of the Biram type is also useful in the measurement of wind gusts; it merely registers wind velocities without recording them.

The Robinson type of anemometer is simple in construction and does not easily get out of order. Four hemispherical cups fastened to arms 6.72 inches from axis to center, made fast to a spindle, communicate their motion to the measuring mechanism. The upper end of the spindle revolves in a sleeve; the lower end rests in an oil cup which also is a bearing. A worm screw thread near the lower end engages a train of wheels. Two of these are registering disks turning on the same axis.