The ranges of the Rocky Mountains in their turn receive some rainfall from the passing winds, but it is only after the westerlies are reinforced by a moist indraft from the Gulf of Mexico and the Atlanticthe result of summer or of cyclonic inflowthat rainfall increases to a sufficient measure on the lower lands to support agriculture without irrigation.
The prevailing winds respond to the stronger poleward temperature gradients of winter by rising to a higher velocity and a more frequent and severer cyclonic storminess; and to the weaker gradients of summer by relaxing to a lower velocity with fewer and weaker cyclonic storms; but furthermore the northern zone occupied by the prevailing westerlies expands as the winds strengthen in winter, and shrinks as they weaken in summer; thus the stormy westerlies, which impinge upon the north-western coast and give it plentiful rainfall all through the year, in winter reach southern California and sweep across part of the Gulf of Mexico and Florida; it is for this reason that southern California has a rainy winter season, and that the states bordering on the Gulf of Mexico are visited in winter by occasional intensified cold winds, inappropriate to their latitude.