A broad band of heated air perpetually rises from the sweltering equatorial belt of lands and seas. The supply never ceases, the warming process goes on night and day, and to a great height the light warm incense mounts. Then, cooling, from this altitude it begins to run down hill toward the poles. This is happening all the way around the globe. So naturally the common centers, the poles, cannot accommodate all this downrush of air. Therefore as it approaches the goal it falls into a majestic file about the center, very much as water does in running out of a hole in the center of a circular basin. The nearer north, the cooler this vast maelstrom grows and the nearer has it sunk to the earth. It descends circuitously and, by the force arising from the earth's rotation, is sheered to the right in the northern hemisphere, to the left in the southern. Watching the water circle out of the basin you will notice the outside whirl is in no hurry to get to the center. This corresponds to the easterly trades of commerce, geography, and fiction. The direction of the upper currents flowing back to the poles is from southwest to northeast; but in our middle zones this becomes almost from west to east, is constant and is known to the profession as the prevailing westerlies. Look up some day when wisps of clouds are floating very high. You will notice that their port is in the east, mattering not what wind may be blowing where you are. They are above the petty disturbances of the shallow surface winds. They follow a Gulf Stream of immeasurable grandeur. Onward, always onward, they sail, emblems of a great serenity. Beneath this vast drift of air, which increases in velocity as it nears the pole, is an undertow from two to three miles thick. It is the movements of this undertow that affect our lives. These movements are influenced by all the changes of temperature and by the configurations of land. They take the form of whirls. These whirls may be small eddies, local in effect, or vast cyclones with diameters of fifteen hundred miles. Small or large they roll along under the Westerlies, translated by friction, and invariably moving for most of their course in an easterly direction, like their tractor above. They circle across the United States every few days. Their courses do not vary a great deal, and yet enough to make each one a matter for conjecture. And all the conjecturing centers upon the condition of the atmosphere,--the changing atmosphere which is yet so dependable. The weather we are used to, the daily weather that catches us unprepared, and yet that does not mistreat us all the time is the product of these little whirls, which are so remotely connected with the grander atmospheric movements of our planet. Remembering this, we can at last come back to earth and set about our real business which is to see why certain kinds of weather come at such uncertain times and how to tell when they will arrive. CHAPTER II THE CLEAR DAY