The thing with Sahara dust is that it catches the wind easily and can be blown a long way. The reason for this is that it is made up of flat platelets that have little mass and large surface area, relatively. When these plates land on a flat surface like a conny roof they sit on top of one another like tiles on a roof.
If a dust storm is followed by a little rain the plates form up flat against each other and gain cohesion through friction. A little soap helps to get it off before a final rinse.
I don't get out much