He will find that the centers of the Lows will follow closely the direction indicated by the isotherms that lead eastward out of their centers, and that they move across the country from the west in quite regular succession, and that the frequent changes from sunshine to clouds and from warm to cold are the result of the mixing of the air by these atmospheric eddies. Experience will teach him that Lows from the southwest are reasonably sure of causing precipitation, and that if his temperature be sufficiently low—anywhere from zero to 20°—the fall will be in the shape of snow; that Lows that only skirt our northern border will be deficient in precipitation, even if they cause any at all; that the slow settling of a High over the South Atlantic States means heat for all the rest of the country east of the Rocky Mountains in degree that will be dependent upon the magnitude and the intensity of the southern High; that the heat will continue, even if temporarily interrupted by showers, so long as this High retains its location in the southeast; that tornadoes occur in the spring of the year when Lows have excessive heat and humidity in their southeast quadrants; that V-shaped Lows cause violent local storms, if not tornadoes, and often deluges of rain; and that frosts may be expected in the country when a minimum temperature of 40° is forecast for the city; and that the severity of cold waves modifies as they come eastward, and that they will only flow as far south as the area covered by the Low that preceded them,—that is to say, by that part of the Low included in the thirty-inch isobar, or by a close approximation to such area. National Forecaster E. H. Bowie, known to the writer as one of the ablest forecasters ever developed by the Weather Bureau, in a recent most valuable publication by the Bureau, entitled “Weather Forecasting in the United States”, formulates rules for forecasting as follows: 1. When there is an area of high pressure over the southeast and a cold wave in the northwest threatens, there will be a storm development in the southwest and precipitation will be general. 2. If a storm form in the southwest and be forced to the left of a normal track (Charts 10 and 11), another storm will immediately begin to develop in the southwest and it becomes a sure rain producer. Storms that develop in the southwest and move normally are quickly followed by clearing weather. 3. Troughs of low pressure moving from the west are of two types—the narrow and the wide. The former moves eastward slowly and storm centers develop in the extreme northern and the extreme southern ends. When the trough is wide, the development of an extensive storm area is not uncommon, especially if the wide intervening area between the Highs shows relatively high temperatures.