The Chergui or Sharqi, from the Arabic sharq = east, is an east or southeast desert wind in Morocco (North Africa), especially in the north, most frequent in July and August. It is persistent, very dry and dusty, hot in summer, cold in winter. In summer, the Chergui might bring life to a standstill. Desiccating hot it makes life acutely uncomfortable. Dust is blowing everywhere and haze in building up quickly. East of the Atlas Mountains, the vigorous gusts can turn the Sahara into a choppy sea filled with whitecaps of sand, and temperatures rise well above 40°C for days.
The easterly Chergui in Morocco is often explained as just another term and variant of the Scirocco in the Mediterranean and Northern Africa. Especially as both regi
onal wind systems are strongest at around the same time of the year, the 40 to 50 days following summer solstice, a period known as the Smam (or Simoom in other parts of Africa).
However, from a synoptic point of view this is only half of the truth, as the Scirocco, together with its brothers the Chili, Ghibli, Khamsin, Samum, etc., is a more or less southerly wind. While the Scirocco family is always related to a low over the Mediterranean Sea, the Chergui is triggered by the strengthening north-easterly trades in early July interacting with the Atlas mountain ridge.
It all starts with high pressure over the Mediterranean and a shallow Saharan heat low south of it with the isobars almost parallel to the coasts. The resulting strong northeasterly winds eventually hit into a barrier formed by the Atlas Mountains, stretching northeast to southwest. As the air is crossing the 10,000 to 13,000ft high ( 3,000 to 4,000m) ridge it will eventually loose its last tiny bit of humidity and the main wind direction will become somewhat deflected to the right into a more easterly to south-easterly wind. Temperatures over western Morocco will jump up by ten or more degreesC.
Downwind of the Atlas mountains, over the Atlantic Sea west of Morocco and Portugal, the accelerated air flow will give birth to a shallow lee trough or week low travelling a relatively short distance east towards the Iberian Peninsula, where it fills and eventually dissolves. However, its associated cold front will bring a little relief to north-western Morocco.