=Mean Annual and Mean Monthly Isobaric Charts.=——We have thus far been studying isobaric charts based on barometer readings made at a single moment of time. Just as there are mean annual and mean monthly isothermal charts, based on the mean annual and mean monthly temperatures, so there are mean annual and mean monthly isobaric charts for the different countries and for the whole world, based on the mean annual and mean monthly pressures. The mean annual and mean monthly isobaric charts of the world show the presence of great oval areas of low and high pressure covering a whole continent, or a whole ocean, and keeping about the same position for months at a time. Thus, on the isobaric chart showing the mean pressure over the world in January, there are seen immense areas of high pressure (anticyclones) over the two great continental masses of the Northern Hemisphere. These anticyclonic areas, although vastly greater in extent than the small ones seen on the weather maps of the United States, have the same system of spirally _outflowing_ winds. Over the northeastern portion of the North Pacific and the North Atlantic, in January, are seen immense areas of low pressure (cyclones) with spirally _inflowing_ winds. In July the northern continents are covered by cyclonic areas, and the central portion of the northern oceans by anticyclonic areas. _B._ =Direction and Rate of Pressure Decrease: Pressure Gradient.=——In Chapter V we studied the direction and rate of temperature decrease, or temperature gradient. We saw that the direction of this decrease varies in different parts of the map, and that the rate, which depends upon the closeness of the isotherms, also varies. An understanding of temperature gradients makes it easy to study the directions and rates of pressure decrease, or _pressure gradients_, as they are commonly called. Examine the series of isobaric charts to see how the lines of pressure decrease run. Draw lines of pressure decrease for the six isobaric charts, as you have already done on the isothermal charts. When the isobars are near together, the lines of pressure decrease may be drawn heavier, to indicate a more rapid rate of decrease of pressure. Fig. 39 shows lines of pressure decrease for the first day. Note how the arrangement and direction of these lines change from one map to the next. Compare these lines with the lines of temperature decrease. [Illustration: FIG. 39.——Pressure Gradients. First Day.] Next study the _rate_ of pressure decrease. This rate depends upon the closeness of the isobars, just as the rate of temperature decrease depends upon the closeness of the isotherms. Examine the rates of pressure decrease upon the series of isobaric charts. On which charts do you find the most rapid rate? Where? On which the slowest? Where? Do you discover any relation between rate of pressure decrease and the pressure itself? What relation?