One-third of the total land surface on earth is semi-arid or arid. This includes the polar areas which are known as cold or polar deserts. Deserts are barren regions which experience little precipitation and have harsh living conditions. There are four types of deserts in the world; chilly coastal, cold winter, sub-tropical, and polar deserts. These deserts are classified by their location, the cause of the desertification, temperature, and precipitation. Antarctica is the world's largest cold desert while the Sahara is the world's largest hot desert.
Antarctica is the world’s south-most continent in the Arctic region and the home to the geographical south-pole. With an area of approximately 5,400,000 square miles, this is the 5th largest continent and the largest cold desert in the world. 6,200 feet of thick ice covers about 98% of this desert. The ice covers the entire area except for the northern parts of the desert. Antarctica is the windiest, driest, and coldest continent on earth. Antarctica is the last area on earth to be discovered; in fact, it was found by the Russian expedition of Mikhail Lazarev and Gottlieb Bellingshausen in 1820.
Numerous governments have permanently manned research stations in Antarctica. The number of individuals supporting and conducting research on the continent varies from 1,000 during the cold season to 5,000 in summer. There are between 180-900 inhabitants per million square miles. The stations are usually staffed throughout the year. The coldest natural temperature ever recorded in the world was -89.2°C in the Vostok station on July 21, 1983. The temperature here can reach a low of between -80°C to -89.2°C at the interior during winter and a maximum of 5°C to 15°C at the coast in the summer.
Sahara is the earth’s biggest hot desert which occupies an area of about 3,600,000 square miles. It is the third biggest desert on earth right after Antarctica and the Arctic. The Sahara covers much of the northern parts of Africa excluding the Nile Valley of Sudan and Egypt, Atlas Mountain, and fertile region on the coast of the Mediterranean Sea. On the southern side, the desert is bounded Sudan, Sahel and a semi-arid tropical savanna belt located around the river Nile valley. The desert is divided into numerous regions including the Libyan Desert, Tenere Desert, Air Mountains, Tibesti mountains, central Ahaggar mountains, and Western Sahara. The desert stretches to various countries including Tunisia, Morocco, Sudan, Niger, Mauritania, Mali, Libya, Egypt, Chad, and Algeria.
The sky in the desert is usually clear plus the duration of sunshine is quite high throughout the desert. Most parts of the desert experiences over 3,600 hours of bright sun annually while the eastern parts have over 4,000 hours of sunlight. The highest value ever recorded is 4,300 hours of sunlight in Nubian desert and Upper Egypt. The average direct-solar irradiation experienced annually is over 3,800 kwh annually. During the hot months, the average temperature surpasses 40°C, and the highest recorded temperature is 47°C which was recorded in Bou Bernous. During the day the ground temperature can reach a maximum of 80°C. Due to low humidity and no cloud cover this region experiences high diurnal temperature variations between the night and day.