Varieties of evergreens, privets, babuls, wild olives, jasmines, camel thorn, salt cedars and an abundance of wild flowers and grasses provide ample grazing during the summer for herds of dairy cattle, camels and goats numbering many hundreds. The uplands rise gently to a height of between 3,000 and 4,000 feet at the top of the watershed and thence slope away to the north draining into Wadi Muqshin on the southern edge of the Rub' al Khali (Empty Quarter). Cloud formations are entrapped over the enclosed plain and on southern slopes of the mountains, the reverse slopes draining to the north being practically free of clouds and devoid of moisture. It is in these wadis that the frankincense, for which the mountains have been famous through the ages, grows wild. The dryness of the air determines the quality of the frankincense, the resin of similar trees growing on the southern slopes being spoilt by the rain. Although the southern slopes enjoy a good rainfall the ground soil does not retain the water. Underground rivers drain the waters of Jabal Aram into Wadi Darbat, the mouth of which is sealed by a sheer limestone cliff 500 feet high. The accumulated waters fill a lake two miles long, situated at a height of 1,000 feet, and overflow during the rains to form a picturesque waterfall. Other underground rivers feed perennial springs at the foot of the hills. There are six of these springs; Jarziz, the best of them, producing an estimated flow of 40,000 gallons of water an hour. These waters, if unharnessed, again disappear underground in the foothills and reappear to feed extensive fresh water creeks in the coastal belt divided from the sea only by narrow sandbars. Fresh water is easily obtainable from shallow wells at a distance of a hundred yards from high water mark and up to a distance of one mile inland, beyond which the increased depth of the water discourages prospective cultivators. Dhofar has a tropical climate. Dhofar and a small portion of the northern tip of Yemen are directly exposed to the South East monsoon from mid-June to mid-September; this is known as the Khareef. Monsoon clouds keep the summer cool and humid, while the winter months are warm except for periods when cold winds from the northern deserts cause heavy and prolonged dust storms and a sharp fall in temperature. Dhofar's heavy seasonal rainfall contrasts sharply with the neighboring barren Empty Quarter Desert.