阅读理解 (4)The winds of the earth behave in accordance with one simple pattern: equalize the surface temperatures by carrying heat away from the equator and moving it towards the poles. Because the process is enormous in scale, it involves complex intricacies which account for the unpredictability of weather despite the modern tools available to the forecaster. However, there are two observable features of global atmospheric circulation which are particularly applicable to North America. One is the gentle, yet persistent, sinking current which falls from the upper atmosphere. Such subsidence is the end of the global convection cycle which begins with updrafts of warm air near the equator; the air loses moisture as it rises and begins to migrate towards the poles. At approximately thirty degrees latitude the air begins to sink, creating the hot, dry conditions common in the southwestern U. S. throughout the year, but especially during the summer. The other significant part of the worldwide phenomena is known as the jet stream, a shifting belt of high-speed winds blowing from the west across the width of the continent at high latitudes. The path of these rapid breezes on any given day is a key to predicting the surface conditions. During the summer months, the winds are generally along the Canadian border, although they may alter their course as high as Alaska or as low as Louisiana. The jet stream generally pulls wet air into Washington, Oregon and Canada from the Pacific Ocean. In the lower regions of North America, its effect is almost the opposite; the warm air from the deserts of Mexico and the southwestern U. S. is pulled towards the jet stream causing higher temperatures throughout the area. In the East, however, the cool air from the Gulf of Mexico is drawn up by the jet creating a cooling effect for the South and Midwest. In the winter, the jet moves to lower latitudes resulting in greater rainfall for southwestern states while the cool Canadian air causes winter storms in the northern areas.