There are three distinct ecological systems in Peru: the coast, the highlands, and the montaña with the Amazon basin. Each of the regions has its own set of unique characteristics and is subject to its peculiar internal divisions. Common to all the Peruvian coast, and the factor that contributes to its special climate, is the Humboldt current. That broad river, flowing in a counterclockwise fashion in the eastern South Pacific, sweeps cold Antarctic water along the South American coast. The prevailing westerly winds push the air, cooled by the water, against the unbroken Andean chain, which in places surpasses 6,000 meters in elevation. The result is an area of high barometric pressure. Almost no rain falls along the mid-Chilean to north Peruvian coasts. The consequence is a narrow desert strip some 3,000 kilometers in length. As the air rises along the western Andean slopes, moisture condenses. In some areas, at roughly 600–1,500 meters in elevation, the moisture forms a fog or garua. At higher altitudes there is seasonal rainfall and at the highest points snow, which provide the water for coastal rivers and irrigation systems.