– to around in the Orinoco Delta and or more in Los Llanos and Amazonas. Rainfall in mountainous areas varies considerably; sheltered valleys receive little rain, but slopes exposed to the northeast trade winds experience heavy rainfall. Caracas averages of precipitation annually, with very little rain falling from December to April. The country falls into four temperature zones based primarily on elevation. In the tropical zone—below —temperatures are hot, with yearly averages ranging between . The temperate zone ranges between with averages from ; many of Venezuela's cities, including the capital, lie in this region. Colder conditions with temperatures from are found in the cool zone between . Pastureland and permanent snowfield with yearly averages below cover land above in the high mountain areas known as the páramos. Climate data Hydrography The Orinoco is by far the most important of the more than 1,000 rivers in the country. Flowing more than 2,500 kilometers to the Atlantic from its source in the Guiana highlands at the Brazilian border, the Orinoco is the world's eighth largest river and the largest in South America after the Amazon. Its flow varies substantially by season, with the high water level in August exceeding by as much as thirteen meters the low levels of March and April. During low water periods, the river experiences high and low tides for more than 100 kilometers upstream from Ciudad Guayana. For most of the river's course, the gradient is slight. Downstream from its headwaters, it splits into two; one-third of its flow passes through the Brazo Casiquiare (Casiquiare Channel) into a tributary of the Amazon, and the remainder passes into the main Orinoco channel. This passageway allows vessels with shallow drafts to navigate from the lower Orinoco to the Amazon River system after unloading and reloading on either side of two falls on the Orinoco along the Colombian border. Most of the rivers rising in the northern mountains flow southeastward to the Río Apure, a tributary of the Orinoco. From its headwater, the Apure crosses the llanos in a generally eastward direction. Few rivers flow into it from the poorly drained region south of the river and much of this area near the Colombian border is swampland. The other major Venezuelan river is the fast-flowing Caroní, which originates in the Guiana highlands and flows northward into the Orinoco upstream from Ciudad Guyana. The Caroní is capable of producing as