a year, or several years, may be constructed as shown in Fig. 51.
A certain convenient scale is adopted as representing a frequency of 10%, and a circle is drawn with this unit as a radius. A second circle, with a radius twice as long, represents a frequency of 20%, and a third circle, with a radius three times as long, represents a frequency of 30%. Additional circles may be added if necessary. Distances corresponding to the different percentages of frequency of the eight wind directions are then laid off along the eight radii of the circles, and the points thus fixed are joined by a line.
The results asked for under question B may be plotted as a weather rose on a diagram similar to that above figured. In this case the percentages of frequency of the different varieties of weather (clear, fair, cloudy, stormy) may be indicated on the same figure by using different kinds of lines. Thus, a solid line may be employed to represent clear weather; a broken line for fair; a broken and dotted line for cloudy; and a dotted line for stormy weather.
CHAPTER XVI.
CORRELATION OF CYCLONES AND ANTICYCLONES AND THE
WEATHER.
A. Cyclones.—Prepare a piece of tracing paper as shown in Fig. 52, making the diameter of the outer circle about 1000 miles[6] and of the inner circle 500 miles. Place this diagram over a cyclone on any weather map, centering and orienting it carefully. Trace off the weather signs (indicating clear, fair, cloudy, rain or snow) around the cyclonic center from the map on to the tracing paper, taking only observations which are not more than halfway from the cyclonic center to the neighboring
anticyclonic center. Repeat this process with successive weather maps until the diagram is well filled in all its different divisions.
[6] Use the scale of miles given on the weather map.
A. Draw a line on the tracing paper enclosing the average area of cloud (including rain and snow), and a second line enclosing the average area of precipitation (rain or snow).