In general, Saudi Arabia is high and mountainous along the Red Sea, in the west, and slopes gradually eastward to the Persian Gulf. The mountainous region, which is known as Hejaz in the north and Asir in the south, rises abruptly from a narrow coastal plain to heights of 8,000 to 10,000 feet (2,440 to 3,050 m). In many areas the terrain is deeply eroded and rugged; some sections are covered by extensive lava flows. East of the mountains lies the Nejd, a high central plateau with a relatively level surface. In some areas long clifflike hills rise abruptly from the Nejd.
Bordering the plateau are three of Saudi Arabia's largest and most barren deserts---the Nefud in the north, the Dahna in the east, and the Rub al Khali (Empty Quarter) in the south. The Rub al Khali, covering an area about the size of Texas, is an almost continuous body of sand. It is little explored and rarely crossed. Along the Persian Gulf stretches the low-lying, oil-rich Eastern Province, also known as Hasa.
Situated in one of the most arid regions on earth, Saudi Arabia suffers from a severe lack of water. There are no lakes or rivers---only wide gullies, called wadis, that flow briefly after rains and end in the desert. Normally, water is available only at oases---for centuries the basis of almost all of Saudi Arabia's settled life. Also important are deep underground sources of water in some parts of the country and desalinization projects, which provide water for some of the large cities.
Summers are intensely hot, especially in the interior. Here temperatures average above 90º F. (32º C.) and daytime highs sometimes reach more than 120º F. (49º C.). Nights usually bring a drop of 30 to 40 degrees Fahrenheit (17 to 22 degrees Celsius). Summers are extremely oppressive along the coasts, where high humidity accompanies the intense heat Winter is the most pleasant time of the year; temperatures in most areas average between 55º and 75º F. (13º and 24º C.). Daytime winter readings are occasionally high, but nights are invariably cool. In some places, particularly the high inland areas, frost occurs.
Except in the highlands of Asir, where the annual rainfall ranges from 12 to 20 inches (300 to 500 mm) Saudi Arabia receives very little rain---in most places less than 4 inches (100 mm) annually. Some areas go rainless for years. Most of the rain falls during winter.