Lines of equal pressure are called isobars, a word derived from two Greek words meaning equal pressure.
Describe the distribution of pressure as shown by the arrangement of the isobars. Note the differences in form between the isotherms and the isobars. The words high and low are printed on weather maps to mark the regions where the pressure is highest and lowest.
Draw isobars for the other days, using the barometer readings given in the table in Chapter VIII. Figs. 33-38 show the arrangement of the isobars on these days.
The pressure charts may be colored, as indicated by the shading in these figures, in order to bring out more clearly the distribution of pressure, according to the same general scheme as that adopted in the temperature charts. Color brown all parts of your six isobaric charts over which the pressures are below 29.50 inches; color red all parts with pressure above 30.00 inches. Use a faint shade of brown for pressures between 29.50 inches and 29.00 inches, and a darker shade for pressures below 29.00 inches. In the case of pressures over 30.00 inches, use a pale red for pressures between 30.00 and 30.50 inches, and a darker shade of red for pressures above 30.50 inches. By means of these colors the pressure distribution will stand out very clearly. The scheme of color and shading may, of course, be varied to suit the individual fancy.
Study the isobaric chart of each day of the series by itself at first. Describe the pressure distribution on each chart.
Then compare the successive charts. Note what changes have taken place in the interval between each chart and the one preceding; where the pressures have risen; where they have fallen, and where they have remained stationary. Write
a brief account of the facts of pressure change illustrated on the whole series of six charts.
Fig. 33.—Pressure. First Day.
