Records of wind velocity (in miles per hour) are to be made at each regular observation hour, and are to be entered in the proper column of the table in your record book. The total wind movement in each 24 hours is to be observed once a day, always at the same hour, and is to be entered in its proper column in the record book.
The total wind movement for 24 hours is obtained as follows: Subtract the reading of the anemometer at 12 noon (or 8 A.M., or any other hour) of the preceding day from the reading taken at 12 noon or the corresponding hour of the current day, and the difference will be the total movement of the wind. When the reading of the anemometer is less than the reading of the preceding day, 990 miles should be added to it; and the remainder, after subtracting the reading of the preceding day, will be the total wind movement for the 24 hours. Thus: To-day’s reading = 91 miles; yesterday’s reading = 950 miles.
Hence 91 + 990 = 1081 miles, 1081 - 950 = 131 miles = total wind movement for the current day.
By means of an electrical attachment the anemometer may be arranged so as to record continuously on a cylinder rotating by clock-work, a pen making a mark on the paper for every mile traveled by the wind. The anemometer should be exposed on top of a building where there is as little obstruction as possible by tall chimneys, higher buildings, and the like.
The nephoscope (Greek: cloud observer) is an instrument used in determining the directions of movement of clouds. These directions, if determined by ordinary eye observation of the clouds as they drift across the sky, are apt to be quite inaccurate. The best method of observing directions of cloud movement is to note the path of the reflection of the cloud in a horizontal mirror, the observer looking at this reflection through an eyepiece which remains fixed during the operation. Such a horizontal mirror, adapted to measure the direction of motion of clouds, is known as a nephoscope. A form of nephoscope devised by Mr. H. H. Clayton, of Blue Hill Observatory, Hyde Park, Mass., is shown in Fig. 17.
Fig. 17.
This instrument consists of a circular mirror, 13 inches in
diameter, sunk in a narrow circular wooden frame, on top of which is fastened a brass circle, S.W.N.E., divided to 5° of arc. Inside of this fixed circle is a movable brass one, to which is attached a brass arc, BD, rising above the mirror and bearing a movable eyepiece, C. This arc forms the quadrant of a circle whose center is the center of the mirror, and is divided to 5° of arc. Its top is held vertically over the center of the mirror by two rods fastened to the movable circle. The center of the mirror A is marked by cross lines on the reflecting surface, the glass of which is thin. In order to determine the motion of a cloud, the movable circle and tripod are revolved until the arc BD is in the vertical plane formed by the cloud, the center of the mirror, and the eye. The eyepiece C is then shifted until some point of the cloud image, as seen through the eyepiece, is projected on the intersection of the cross lines on the glass. The cloud image soon changes its position, and while the eye is still held at the eyepiece, a small index is placed on the part of the cloud image which previously appeared on the center of the mirror. If now a ruler be placed on the index and the center of the mirror and extended backward, its intersection with the divided scale will give the direction from which the cloud came to the nearest degree, if all the measurements have been accurately made. The height of the cloud above the horizon is found by reading the position of the eyepiece on the divided quadrant.
The nephoscope may be placed on a table, out of doors in fine weather, or close to a window from which the clouds to be observed can be seen. The instrument must be properly oriented, so that the four points marked N., E., S., and W. on the frame shall correspond to the four chief compass directions. The zero (0°) of the movable brass scale is usually put at the S. Hence, if a cloud is found moving from exactly SW., the angular measurement of its direction of motion will be 45°. If a cloud is moving from