Sal, Boa Vista and Maio have a flat landscape and arid climate, whilst the other islands are generally rockier and have more vegetation. Because of the infrequent occurrence of rainfall, where not mountainous, the landscape is so arid that less than two per cent of it is arable. The archipelago can be divided into four broad ecological zones — arid, semiarid, subhumid and humid, according to altitude and average annual rainfall ranging from less than  in the arid areas of the coast as in the Deserto de Viana ( in Sal Rei) to more than  in the humid mountain. Most rainfall precipitation is due to condensation of the ocean mist.