An ensemble of particles of dust or sand energetically lifted to great heights by a strong and turbulent wind. For details, dust storms or sandstorms generally occur in areas where the ground is covered with loose dust or sand; sometimes, after having travelled over more or less great distances, they may be observed over areas where no dust or sand covers the ground. The forward portion of a dust storm or sandstorm may have the appearance of a wide and high wall which advances more or less rapidly. Walls of dust or sand often accompany a cumulonimbus which may be hidden by the dust or the sand particles; they may also occur without any clouds along the forward edge of an advancing cold air mass.
WMO: International Cloud Atlas Volume I, Manual on the Observation of Clouds and other Meteors, 120-121, 1975.