In the north it is due to the fact that the winds from the Pacific lose most of their moisture, especially in winter, on the western slopes of the Sierra Nevada; in the south it is due to the fact that the region lies in a zone of calms, and light, variable winds.
The belt of equatorial minimum salinity corresponds with the excessively rainy belt of calms and of the equatorial countercurrent, the salinity diminishing towards the east.
The trade-winds are generally weaker and less persistent in the Pacific than in the Atlantic, and the intervening belt of equatorial calms is broader.
Except in the east of the Pacific, the south-east trade is only fully developed during the southern winter; at other seasons the regular trade-belt is cut across from north-west to south-east by a band twenty to thirty degrees wide, in which the trades alternate with winds from north-east and north, and with calms, the calms prevailing chiefly at the boundary of the monsoon region (5° N.-15° S., 160°-185° E.).
On the Pacific coast the belt of calms, known as the northern horse latitudes, crosses the northern parts of Lower California and Sonora, which accounts for their extreme aridity.
The climate of the coast district is hot, moist and unhealthy, with a season of heavy rain lasting from May to November, during which time variable winds, calms and tornadoes succeed one another.
This effect is well seen in the way in which the wind blowing directly up the Severn estuary is directed along the edges of the Oolitic escarpment north-eastward, thus displacing the centre of cold in winter to the east coast, and the centre of heat in summer to the lower Thames, from the position which both centres would occupy, if calms prevailed, in a beit running from Birmingham to Buckingham.
Though there are things in it difficult for the feeble human mind to grasp, it is an admirable book which calms and elevates the soul.