Isotherms on weather maps are drawn for every even 10° of temperature. They are drawn in smooth curves and not in angular sections. Two isotherms cannot cross one another, for if they did you would have two temperatures, differing by 10°, at the point of crossing, which is obviously impossible. Complete the chart for this day by drawing the remaining isotherms, _i.e._, those for 50°, 20°, 10°, 0°, -10°, -20°, and -30°, bearing in mind what has been said in regard to the determination of the positions of isotherms when the _exact_ temperature you are seeking is not given on the map. The dotted lines in Fig. 18 show the positions of the isotherms when drawn. Notice how clearly the temperature distribution now stands out, and how simple the description of that distribution has become. Observe that the isotherms, although more or less irregular, show a good deal of uniformity in their general courses, and this uniformity is a great assistance in drawing them. Study the distribution of temperature on this map, and the positions of the isotherms, very carefully. Construct isothermal charts for the remaining days of the series. Use a new blank map for each day, and take the temperature observations from the table in Chapter VIII. Proceed as in the case of the first day. Draw the isotherms for every even 10° of temperature, taking care to study the course of each line before you begin to draw the line. The charts when completed form a series in which the temperature distribution over the United States is shown at successive intervals of 24 hours. [Illustration: FIG. 18.——Isotherms. First day.] In order to bring out the temperature distribution on the maps more clearly, color (with colored pencils or water colors) all that portion of each map which lies within the -20° isotherm a dark blue; that portion which is between the 0° isotherm and the -20° isotherm a somewhat lighter shade of blue, and those districts which are between 0° and +30° a still lighter blue. The portion of the map above 30° and below 40° may be left uncolored, while the districts having temperatures over 40° may be colored red. In the map for the third day the district which has temperatures below -50° should be colored darker blue than any shade used on the other maps, or black, in order to emphasize the extremely low temperatures there found. Figs. 19-24, on which the isotherms are shown, also illustrate the appearance of these maps when the different temperature areas are colored, as has been suggested. [Illustration: FIG. 19.——Temperature. First Day.] [Illustration: FIG. 20.——Temperature. Second Day.] [Illustration: FIG. 21.——Temperature. Third Day.] [Illustration: FIG. 22.——Temperature. Fourth Day.] [Illustration: FIG. 23.——Temperature. Fifth Day.] [Illustration: FIG. 24.——Temperature. Sixth Day.]