Select a file of daily weather maps for some month. Commencing with the first map in the set, observe the weather and the direction of the wind at a considerable number of stations in the same general region (as, e.g., the Lake region, the lower Mississippi Valley, the Pacific Coast, etc.). Enter each case in a table, similar to Table IV below, by making a check in the column under the appropriate wind direction and on a line with the appropriate type of weather.

Count every observation of rain or snow also as cloudy, for it usually rains or snows only when the sky is cloudy. Continue your observations on all the maps for the month you have chosen. Then count up the whole number of cases of clear weather you have found with north winds, and write down this sum in the first space, in the column reserved for N. winds. Do the same with fair and cloudy weather. Add up and enter at the bottom of the column in the space marked Totals the whole number of observations of clear, fair, and cloudy weather you have observed with N. winds. Then find what percentage of the weather with N. winds was clear, and enter this percentage next to the sum of clear weather observations, in the first division in the column headed N. Do the same for fair, cloudy, and rainy or snowy weather, deriving the percentages of rain or snow from the total of clear and fair and cloudy. Repeat this process of summarizing in every column. Your results will then show the percentages of the different kinds of weather noted with the different wind directions.

The lower division of the table and the last two columns on the right are to be used for a general summary of the whole investigation. By adding together all the totals of clear, fair, and cloudy weather observed with all the eight wind directions you obtain the whole number of observations you have made. Enter this in the space marked A, at the right of the table. From this grand total and the total number of observations in each column you may find (in percentages) the relative frequencies of the different wind directions. These should be entered under the totals at the bottom of each column, in the spaces marked Percentages (spaces B, C, etc.). The total number of observations of clear, fair, cloud, and rain or snow, noted with all the wind directions, are to be entered in spaces D, F, G, and H, at the right of the table. From these totals, and from the grand total in space A, we can determine the relative frequency (in percentages) of each kind of weather during the month. These results should be entered in spaces J, K, L, and M.

Study these results carefully. Formulate them in a brief written statement. Express graphically the following:—

A. The percentages of frequency of the different wind directions during the month.

B. The percentages of the different kinds of weather noted during the whole month for all wind directions.

A wind rose, indicating the percentages of frequency of different winds during a month, or 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.