The Rocky Mountains form the eastern barrier which the storms encounter. Their summits are very high and are covered with deep snow during the winter. East of these mountains lie the Great Plains, where the precipitation is light until we go far enough toward the Mississippi Valley to reach the influence of the moist air currents from the Gulf of Mexico. Many storms originate over the region of the Gulf of California, particularly in the late summer, and supplement to some extent the light winter storms of Arizona and New Mexico. The storms of which we have been speaking are known as cyclones. This term does not refer to the local storms which occur in the Mississippi Valley and are frequently so destructive, but to great disturbances of the air. Sometimes the column of whirling air is more than a thousand miles in diameter. The air in a cyclone is circling and at the same time rising, so that the motion is spiral. If you will study an eddy in a stream of water, you will get an idea of the nature of the motion, except that in the case of the water eddy the movement is downward. The motion of the particles in the dust-whirls which all have seen moving across the fields near noon on warm summer days illustrate the movement of the air in one of these great storms. The direction of the air in a cyclone is opposite to that of the hands of a clock. When the wind comes up from a southerly point, when high, thin clouds, gradually growing thicken, spread over the sky, and the barometer begins to fall, then it is known that a storm is corning. If one will learn to watch the clouds and the winds carefully he may become able to predict a storm with almost as much certainty as if he had a barometer. This instrument registers the pressure of the air, which is always less within the area of a storm, because then the air is rising. So when the barometer falls we may always know that a storm is approaching. The greater number of the storms which occur in the central and northern United States come in from the Pacific Ocean in the latitude of Washington. Continuing east or southeast they reach the Mississippi Valley, and then turn northeastward toward the St. Lawrence Valley. In the summer months there are few storms, and they very rarely reach as far south as California. As winter approaches the storms become more frequent and severe, and move farther and farther south until the whole land as far as Mexico receives a wetting. Upon the Pacific coast there is often very little warning of the coming of a storm, but in the Middle and Eastern States they may frequently be predicted several days in advance. With the passing of one of these storms the temperature falls rapidly, and this lowering of temperature, together with the fierce wind, gives rise upon the Great Plains to "blizzards" or "northers." These storms endanger the lives of both men and animals.