The average annual rainfall ranges from very low in the northern and southern fringes of the desert to nearly non-existent over the central and the eastern part. The thin northern fringe of the desert receives more winter cloudiness and rainfall due to the arrival of low pressure systems over the Mediterranean Sea along the polar front, although very attenuated by the rain shadow effects of the mountains and the annual average rainfall ranges from 100 mm (3,93 in) to 250 mm (9,84 in). For example, Biskra, Algeria and Ouarzazate, Morocco are found in this zone. The southern fringe of the desert along the border with the Sahel receives summer cloudiness and rainfall due to the arrival of the Intertropical Convergence Zone from the south and the annual average rainfall ranges from 100 mm (3,93 in) to 250 mm (9,84 in). For example, Timbuktu, Mali and Agadez, Niger are found in this zone. The vast central hyper-arid core of the desert is virtually never affected by northerly or southerly atmospheric disturbances and permanently remains under the influence of the strongest anticyclonic weather regime and the annual average rainfall can drop to less than 1 mm (0.04 in). In fact, most of the Sahara receives less than 20 mm (0.79 in). Of the 9,000,000 km2 of desert land in the Sahara, an area of about 2,800,000 km2 (about 31% of the total area) receives an annual average rainfall amount of 10 mm (0.39 in) or less, while some 1,500,000 km2 (about 17% of the total area) receive an average of 5 mm or less. The annual average rainfall is virtually zero over a wide area of some 1,000,000 km2 in the eastern Sahara comprising deserts of Libya, Egypt and Sudan (Tazirbu, Kufra, Dakhla, Kharga, Farafra, Siwa, Asyut, Sohag, Luxor, Aswan, Abu Simbel, Wadi Halfa) where the long-term mean approximates 0.5 mm per year. The rainfall is very unreliable and erratic in the Sahara as it may vary considerably year by year. In full contrast to the negligible annual rainfall amounts, the annual rates of potential evaporation are extraordinarily high, roughly ranging from 2,500 mm/year to more than 6,000 mm/year in the whole desert. Nowhere else on Earth has air been found as dry and evaporative as in the Sahara region. With such an evaporative power, the Sahara can only be desiccated and dried out further more and the moisture deficit is tremendous.

What is the reason the northern fringe receives more   cloudiness and rainfall?