The greatest part is steppe or desert-steppe. There the land receives enough seasonal rainfall to produce scattered patches of coarse grass and drought-resistant shrubs for grazing. In the An Nafud desert of northern Saudi Arabia, high parallel sand dunes roll like billows. Between the billows are troughs where pebbles, stones, and rocks are bared. The extensive arid portions of the peninsula are an extension of the Sahara's desert region.
Little rain falls on the region, and there are no permanent rivers or lakes.
Along the coastal areas the humidity may reach 90 to 100 percent on both the Red Sea and Persian Gulf shores in summer. Juniper trees grow in the mountainous regions of Asir Province in southwestern Saudi Arabia and Yemen. The mountainous regions of Oman also receive more adequate rainfall and consequently are used for agriculture. The animals found in Arabia include the now-rare oryx, camels, hyenas, wolves, and jackals. The oryx with its two parallel horns is believed to have inspired the stories of the mythical unicorn.