The prevailing westerly winds blow in both the north and the south Pacific beyond the latitudes of about 30° in winter and 40° in summer of the respective hemispheres. The weather in the zones of prevailing westerlies is highly changeable because of the prevalence of cyclones, named for the spiraling of the winds around the storm center, counterclockwise in the northern and clockwise in the southern hemisphere. These extratropical disturbances are far more numerous than the tropical cyclones and are the chief cause for the characteristic variable weather, but the climate of the prevailing westerly zones in the Pacific is largely determined by the ocean itself. Between the latitudes of 30° and 60° the mean annual air temperatures range from about 65° F or 60° F to 30° F in both hemispheres, with summer temperatures about 10° F higher and winter temperatures 10° F colder. The moderate annual range shows the marine influence of the Pacific Ocean. The mean annual rainfall over the Pacific in the zone of the prevailing westerlies is not known but probably is between 30 and 50 inches, the least precipitation towards the trade winds and the greatest in higher latitudes.
Storms of the local thunderstorm type are rare in the westerly winds except in the interior of continents during the warm season.
Cyclones of the intermediate and high latitudes develop at the contact between cold air masses from polar regions and warm air masses from the tropics. This gives to the cyclones of these latitudes a cold and a warm front. In the northern hemisphere the warm front is to the east and south of the storm center, and the cold front to the west and north. In the southern hemisphere the warm front is to the east and north, and the cold front to the west and south. In both hemispheres the storms move in general to the eastward within the prevailing westerlies at a usual speed of several hundred miles a day. The diameter of a cyclone is from a few hundred to over 1000 miles, but the thickness is only 5 to 7 miles, the storm being restricted to the lower air (troposphere). Cyclones are most numerous and strongest during the winter in their respective hemispheres. These storms account for most of the rain and snow that falls over the ocean and along the coasts in the latitudes frequented by them.
The edge of the cold air of the Arctic region is called the polar front, and from this at intervals extend tongues of southward-moving cold air. Sooner or later the cold air mass comes in contact with northward-moving warm air, which is filled with moisture because it has come from the tropics where high temperatures have encouraged large evaporation of water. The cold air mass, being heavier than the warm moist air, underrides it like a wedge and forces the warm air mass to rise. Adiabatic cooling causes the moisture in the rising warm air mass to condense and makes rain or snow, depending upon the temperature. The condensation of moisture liberates energy, which helps to maintain the storm.
The warm air masses associated with cyclones bring abundant precipitation to the west coast of North America when the cyclones reach those shores. The cyclones that bring rain to California usually originate well south of the Aleutians and sometimes are tropical cyclones that have veered north. Cyclones may be severe enough to endanger shipping, especially along coasts, and damage trees and buildings but are seldom as terribly destructive as hurricanes. The cyclones of the southern hemisphere are severe in the "roaring forties" and generally less severe at latitude 80° south or less. The temperatures in cyclones that reach Australia and New Zealand in the southern hemisphere are seldom so cold as those in cyclones occurring in winter in similar latitudes over the continents of the northern hemisphere because of the vast southern ocean over which the air masses moved before reaching the southern lands.