Although hot and extremely dry today, it is believed that the Sahara Desert has undergone various climatic shifts for the last few hundred thousand years. For example, during the last glaciation, it was bigger than it is today because precipitation in the area was low. But from 8000 BCE to 6000 BCE, precipitation in the desert increased because of the development of low pressure over ice sheets to its north. Once these ice sheets melted however, the low pressure shifted and the northern Sahara dried out but the south continued to receive moisture due to the presence of a monsoon.
Around 3400 BCE, the monsoon moved south to where it is today and the desert again dried out to the state it is in today. In addition, the presence of the Intertropical Convergence Zone, ITCZ, in the southern Sahara Desert prevents moisture from reaching the area, while storms north of the desert stop before reaching it as well. As a result, the annual rainfall in the Sahara is below 2.5 cm (25 mm) per year.
In addition to being extremely dry, the Sahara is also one of the hottest regions in the world. The average annual temperature for the desert is 86°F (30°C) but during the hottest months temperatures can exceed 122°F (50°C), with the highest temperature ever recorded at 136°F (58°C) in Aziziyah, Libya.