While relative humidity is high, rainfall is very low due to strong atmospheric stability. The severely low rainfall impacts the city's water supply, which originates from wells and from rivers that flow from the Andes. Inland districts receive anywhere between  of rainfall per year, which accumulates mainly during the winter months. Coastal districts receive only . As previously mentioned, winter precipitation occurs in the form of persistent morning drizzle events. These are locally called 'garúa', 'llovizna' or 'camanchacas'. On the other hand, summer rain is infrequent and occurs in the form of isolated light and brief showers. These generally occur during afternoons and evenings when leftovers from Andean storms arrive from the east. The lack of heavy rainfall arises from high atmospheric stability caused, in turn, by the combination of cool waters from semi-permanent coastal upwelling and the presence of the cold Humboldt Current and warm air aloft associated with the South Pacific anticyclone.