As the moist sea air flows from the Indian Ocean towards the continent it first encounters the East Usambara mountains plateau levelling off at about 1000 masl. Part of the moisture condenses to water and rain. The average annual rainfall in the plateau is about 2000 mm, which is about two times higher than in Tanga town. However, there tends to be occasional precipitation even during the dry seasons.
Relative humidity is high.
The eastern slopes of the of the East Usambaras are drained by Muzi, Semdoe and Kihuhwi rivers which flow into the Sigi-river, which itself drains most of the Amani plateau region. The total area of watershed of Sigi is about 1100 km2. The rivers flows into the sea North of Tanga town. The Sigi is the main source of the water supply for the town population. Moreover, in the mountains a population of about 100,000 utilise water from the river.