WEATHER CHART.--MARCH 13.
Meteorological conditions at noon, Greenwich mean time (7 A. M., 75th meridian time).
Barometer.—Isobars in full black lines for each tenth of an inch, reduced pressure. The trough of low barometer is shown by a line of dashes.
Temperature.—Isotherms in dotted black lines for each ten degrees Fahr. Temperatures below freezing (32° F.) in shades of blue, and above freezing in red.
Wind.—The small black arrows fly with the wind at the position where each is plotted. The force of wind is indicated in a general way by the number of feathers on the arrows, according to the scale given in the following table:
| Plotted on Chart. |
Force, by Scales in practical use. | Pounds per square foot. |
Miles per hour. | Kilometers per hour. |
Meters per second. | |||||
| 0—12 | 0—10 | 0—8 | 0—7 | 0—6 | ||||||
 Calm.
|
0 | 0 | 0 | 0 | 0 | 0. | 0. | 0. | 0. | |
 1
|
1—2 | 1—2 | 1 | 1—2 | 1 | 0.—.40 | 0.—9. | 0.—14.4 | 0.—4. | |
 2
|
3—4 | 3—4 | 2 | 3—4 | 2 | 0.41—2.53 | 9.1—22.5 | 14.5—36.2 | 4.1—10.1 | |
 3
|
5—7 | 5—6 | 3—4 | 5 | 3 | 2.54—8.20 | 22.6—40.5 | 36.3—65.2 | 10.2—18.1 | |
 4
|
8—10 | 7—8 | 5—6 | 6 | 4—5 | 8.21—22.90 | 40.6—67.5 | 65.8—108.7 | 18.2—30.1 | |
 5
|
11—12 | 9—10 | 7—8 | 7 | 6 | 22.91 and over. | 67.6 and over. | 108.8 and over. | 30.2 and over. | |
It will be noticed that the Beaufort scale (0-12), in general use at sea, has been converted into the international scale (0-10) for the sake of clearness in plotting data on the chart. The absence of arrows over large areas indicates absence of simultaneous data; at sea, however, this has been partly compensated for in the construction of the chart by information obtained from journals and special storm reports of vessels in the vicinity.
