Duststorm

(Or dust storm;

also called duster, black blizzard.) An unusual, frequently severe weather condition characterized by strong winds and dust-filled air over an extensive area.
Prerequisite to a duststorm is a period of drought over an area of normally arable land, thus providing the very fine particles of dust that distinguish it from the more common sandstorm of desert regions. A duststorm usually arises suddenly in the form of an advancing dust wall that may be many kilometers long and and a kilometer or so deep, ahead of which there may be some dust whirls, either detached or merging with the main mass. Ahead of the dust wall the air is very hot and the wind is light. In U.S. weather observing practice, if blowing dust reduces visibility to between 5/8 and 5/16 statute mile, a "duststorm" is reported; if the visibility is reduced to below 5/16 statute mile, it is reported as a "severe duststorm." Duststorm winds can also be associated with thunderstorm outflows and gust fronts. While these are often shorter-lived than synoptically forced duststorms, they can be quite intense, with an impressive leading edge of the dusty gust front, sometimes called a dust wall.
Compare haboob.

Any function of time f(t) is periodic with period t if f(t) = f(t + τ) for all times t, where τ is the smallest number for which this equality holds.

(Or dust storm;