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 the landscape is arid. the archipelago can be divided into four broad ecological zones — arid, semiarid, subhumid and humid, according to altitude and average annual rainfall ranging from in the arid areas of the coast to more than in the humid mountain. most rainfall precipitation is due to condensation of the ocean mist.