Page:The National geographic magazine, volume 1.djvu/81





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:

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.