The Sahara is the world's largest hot desert and third largest desert, after Antarctica and Arctic. it covers most of Northern Africa, making it almost as large as China or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean. Some of the sand dunes can reach 180 metres (590 ft) in height. The Sahara covers large parts of Algeria, Chad, Egypt, Libya, Mali, Mauritania, Morocco, Niger, Western Sahara, Sudan and Tunisia. The climate of the Sahara has undergone enormous variation between wet and dry over the last few hundred thousand years. The Sahara has one of the harshest climates in the world. The prevailing north-easterly wind often causes sand storms and dust devils. When this wind reaches the Mediterranean, it is known as sirocco and often reaches hurricane speeds in North Africa and southern Europe. Half of the Sahara receives less than 20 mm (0.79 in) of rain per year, and the rest receives up to 100 mm (3.9 in) per year. The rainfall happens very rarely, but when it does it is usually torrential when it occurs after long dry periods.