[Illustration: FIG. 41.——Weather. Second Day.] [Illustration: FIG. 42.——Weather. Third Day.] [Illustration: FIG. 43.——Weather. Fourth Day.] [Illustration: FIG. 44.——Weather. Fifth Day.] [Illustration: FIG. 45.——Weather. Sixth Day.] =The Weather of Temperate and Torrid Zones.=——The facts of the presence of clear weather in one region while snow is falling in another, and of the variability of our weather from day to day in different parts of the United States, are emphasized by these charts of weather conditions. This changeableness of weather is a marked characteristic of the greater portion of the Temperate Zones, especially in winter. The weather maps for successive days do not, as a rule, show a repetition of the same conditions over extended regions. In the Torrid Zone it is different. Over the greater part of that zone the regularity of the weather conditions is such that, day after day, for weeks and months, the same features are repeated. There monotony, here variety, is the dominant characteristic of the weather. PART IV.——THE CORRELATIONS OF THE WEATHER ELEMENTS AND WEATHER FORECASTING. CHAPTER IX. CORRELATION OF THE DIRECTION OF THE WIND AND THE PRESSURE. The study of the series of weather maps in Chapters V-VIII has made it clear that some fairly definite relation exists between the general flow of the winds and the distribution of pressure. We now wish to obtain some more definite result as to the relation of the direction of the wind and the pressure. In doing this it is convenient to refer the wind direction to the _barometric_ or _pressure gradient_ at the station at which the observation is made. The barometric gradient, it will be remembered, is the line along which there is the most rapid change of pressure, and lies at right angles to the isobars (Chapter VII). [Illustration: FIG. 46.] Take a small piece of tracing paper, about 3 inches square, and draw upon it a diagram similar to the one here shown. Select the station (between two isobars on any weather map) at which you intend to make your observation. Place the center of the tracing paper diagram over the station, with the dotted line along the barometric gradient, the minus end of the line being towards the area of low pressure. Observe into which of the four sectors (marked _right_, _left_, _with_, _against_) the wind arrow at the station points. Keep a record of the observation. Repeat the observation at least 100 times, using different stations, on the same map or on different maps. Tabulate your results according to the following scheme, noting in the first column the date of the map, in the second, third, fourth, and fifth columns the number of winds found blowing _with_, to the _right_ or _left_ of, and _against_, the gradient. TABLE I.——CORRELATION OF THE DIRECTION OF THE WIND AND THE PRESSURE.