_A._ =Cyclones.=——Prepare a piece of tracing paper as shown in Fig. 52, making the diameter of the outer circle about 1000 miles[6] and of the inner circle 500 miles. Place this diagram over a cyclone on any weather map, centering and orienting it carefully. Trace off the weather signs (indicating _clear_, _fair_, _cloudy_, _rain_ or _snow_) around the cyclonic center from the map on to the tracing paper, taking only observations which are not more than halfway from the cyclonic center to the neighboring anticyclonic center. Repeat this process with successive weather maps until the diagram is well filled in all its different divisions. [Footnote 6: Use the scale of miles given on the weather map.] _A._ Draw a line on the tracing paper enclosing the average area of _cloud_ (including _rain and snow_), and a second line enclosing the average area of _precipitation_ (_rain_ or _snow_). _B._ Determine the percentages of _clear_, _fair_, _cloudy_, and _stormy_ observations for each division of the tracing paper, _i.e._, (_a_) for the eight sectors of the large circle; (_b_) for the whole of the small circle; and (_c_) for the portion of the diagram between the circumference of the inner circle and the circumference of the outer circle. [Illustration: FIG. 52.] _C._ Write out in general terms a description of the weather distribution in cyclones as illustrated by your own investigation. _B._ =Anticyclones.=——This exercise is done in the same way as the preceding one, except that anticyclones are substituted for cyclones. =The association of fair weather with anticyclones and of stormy weather with cyclones= is one of the most important lessons learned from a study of the weather maps. The great areas of high and low pressure control our weather. On land, where daily weather maps are so easily accessible, a glance at the map serves in most cases to give a fairly accurate idea of the position and extent of cyclones and anticyclones, and hence also of the distribution of weather. At sea, on the other hand, the navigator has no daily weather maps to refer to, and his knowledge of the weather conditions which he may expect must be gained from his own observations alone. Of these local observations, the pressure readings are by far the most important. A falling barometer usually means the approach of a cyclone, with wind, or storm, or both. A rising barometer, on the other hand, is usually an indication of the fine weather associated with an anticyclone. The unsymmetrical distribution of weather, characteristic of our cyclones in the United States, and also of most cyclones in the Temperate Zone, is associated with their unsymmetrical form, and the unsymmetrical distribution of their temperature already studied. Tropical cyclones have a wonderfully uniform distribution of weather on all sides of their centers, just as they have a symmetrical form and an even temperature distribution all around them. CHAPTER XVII.