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.
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.