In the Lows the conditions of the air and its movements are exactly the reverse of what they are in the Highs; the air is warmer and moister, it is drawn spirally inward from all directions instead of being forced outward as in the High, and it ascends as it approaches the center of depression, sometimes causing rain or snow as it cools by expansion during its ascent. While the air cools with ascent in the Low at the same rate that it warms with descent in the High, the earth experiences a general warming effect with the passage of the Lows, because the air falls but little in temperature as it rises before it reaches its dew point, and then there is a liberation of the latent heat of condensation (see Chapter V); and what is more important, there is formed a covering of clouds that checks or wholly stops radiation outward from the lower air. However there are times when the passage of Lows produces a cooling effect. This is when abnormally hot weather has prevailed for some days; then the air may be mixed, washed, and cooled by thunder-showers. [Illustration: CHART 10.—STORM TRACKS FOR AUGUST FOR TEN YEARS.] Highs and Lows alternately drift across the continent in periods of about three days each. They are a part of the divine economy that provides for the seedtime and the harvest, for, as previously stated, the Lows draw the warm, vapor-bearing currents inland from the Gulf and the ocean and cause them to deposit their moisture far to the north and west. Four sevenths of all our storms come from the middle or the north plateau regions of the Rocky Mountains, or at least enter our field of observation from those regions, and pass from this arid or sub-arid section of the continent easterly over the Lakes and New England, producing but little rainfall. The greater part of the remaining three sevenths are first observed in the arid regions of our southwestern States; they always move northeastward and can be depended on to give bountiful rainfall so soon as or a little before they reach the Mississippi River. Some of them cross the Atlantic and affect the continent of Europe. Charts 10 and 11 show the courses of storms in this country, and where they originate, or are first brought under the survey of our system of observation. [Illustration: CHART 11.—STORM TRACKS FOR FEBRUARY FOR TEN YEARS.]