Each spring, Egypt is hit by sand storms, known as the khamsin, or the 50-day wind, that deposit a layer of fine sand on buildings and cars.
Khamsin can be triggered by depressions that move eastwards along the southern parts of the Mediterranean or along the North African coast from February to June.
In Egypt, khamsin usually arrives in April but occasionally occur in March and May, carrying great quantities of sand and dust from the deserts, with a speed up to 140 kilometers per hour, and a rise of temperatures as much as 20°C in two hours. It is believed to blow at intervals for about 50 days although it rarely occurs more than once a week and last for just a few hours at a time.
Sandstorms, also called dust storms, are caused by strong winds blowing over loose soil or sand, and picking up so much of that material that visibility is greatly reduced. The widespread abundance of loose sand in deserts makes them the most common locations for sandstorms to form.
In desert regions at certain times of the year, sandstorms become more frequent because the strong heating of the air over the desert causes the lower atmosphere to become unstable. This instability mixes strong winds in the middle troposphere downward to the surface, producing stronger winds at the surface.
The term sandstorm is used most often in the context of desert sandstorms, especially in the Sahara, or places where sand is a more prevalent soil type than dirt or rock, when, in addition to fine particles obscuring visibility, a considerable amount of larger sand particles are blown closer to the surface. The term dust storm is more likely to be used when finer particles are blown long distances, especially when the dust storm affects urban areas.
The Sahara and drylands around the Arabian peninsula are the main terrestrial sources of airborne dust and legend has it that in 524BC he 50,000 strong army of Cambyses II supposedly buried by a sandstorm en route to the Siwa Oasis.
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