Cold-Wave Forecasts.—A severe cold wave in winter does much damage to fruit and crops growing out of doors in our Southern States, and to perishable food products in cars, on the way from the South to supply the great cities of the North. Therefore it is important that warnings should be issued giving early information of the coming cold, so that farmers and fruit growers and shippers may take every precaution to protect their crops and produce. Our Weather Bureau takes special pains to study the movements of cold waves and to make forecasts of them, and so well are the warnings distributed over the country that the fruit growers and the transportation companies, and the dealers in farm produce, are able every winter to save thousands of dollars’ worth of fruit and vegetables which would otherwise be lost. Cold-wave warnings are heeded by many persons besides those who are directly interested in fruits and farm products. The ranchmen in the West, with thousands of cattle under their charge; the trainmen in charge of cattle trains; the engineers of large buildings, such as hotels, stores, and office buildings, who must have their fires hotter in cold weather,—these and many more watch, and are governed by, the cold-wave forecasts of our Weather Bureau.
Mean Annual and Mean Monthly Isothermal Charts.—We have thus far considered isothermal charts for the United States only, based on the temperature observations made at a single moment of time. It is, of course, possible to draw isothermal charts, the data for which are not the temperatures at a given moment, but are the mean or average temperatures for a month or a year. Such charts have been constructed for other countries besides our own, as well as for the whole world. An isothermal chart based on the mean annual temperatures is known as a mean annual isothermal chart. These charts show at once the average distribution of temperature for the month or for the year, just as the ones we have drawn show the distribution of temperature over the United States at a single moment.
B. Direction and Rate of Temperature Decrease. Temperature Gradient.—Take your isothermal map for the first day and imagine yourself at Kansas City, Mo. In what direction must you go from Kansas City in order to enter most rapidly into colder weather? In what direction must you go from Kansas City in order to enter most rapidly into warmer weather? Take the case of Salt Lake City. In what direction must you go from that station in order to enter most rapidly into colder weather? Into warmer weather? What are the corresponding directions in the case of Spokane, Wash.? Of Bismarck, N. Dak.? Of Buffalo, N. Y.? Of Montreal, Que.? Of Portland, Me.? Of Sacramento, Cal.?
Draw a line from Kansas City to the nearest point at which there is a temperature 10° lower than at Kansas City. Evidently this point is on the isotherm of 0°, and will be found if a line be drawn from Kansas City towards, and at right angles to, the isotherm of 0°. Continue the line beyond the 0° isotherm in the direction of still lower temperatures, i.e., to the isotherms of -10°, -20°, and -30°. Beyond the isotherm of -30° the line must stop. Draw similar lines from Seattle, Wash.; Salt Lake City, Utah; Denver, Col.; St. Paul, Minn.; Cleveland, O.; and New York, N. Y. Prolong these lines all across the map, so that they will extend from the regions of highest temperature to those of the lowest. A number of intermediate lines may also be added. Note that the various directions followed by these lines are square to, or at right angles to, the successive isotherms, and that although the lines all run from higher to lower temperatures, they do not all trend in the same direction. These lines may be called lines of decrease of temperature. Fig. 25 shows a few of these lines of decrease of temperature drawn for the first day.
Draw similar lines on the other isothermal charts, for the same stations. Are the directions of temperature decrease the same on these charts as on the chart for the first day, for Kansas
City, Seattle, Salt Lake City, Denver, St. Paul, Cleveland, New York? Draw lines of decrease of temperature from the following additional stations: Key West, Fla.; New Orleans, La.; Charleston, S. C.; El Paso, Tex.; San Diego, Cal.; Hatteras, N. C.
Compare the directions of these lines on the different days. How do they change from one day to the next?
