A Dust Storm by Any Other Name
Dust storms are sometimes referred to by the winds that generate them:
- Haboob: The word means "strong wind" in Arabic, but haboobs are also called "black blizzards."
- Shamal: The persistent northwest winds in Iraq and the Persian Gulf that pick up dust into the atmosphere. Shamal winds cause the most hazardous weather in the region.
- Sirocco: Very strong pre-frontal winds that originate in the Sahara and blow up dust over North Africa and Southern Europe. In Libya it's called ghibli. Sirocco winds rival hurricane wind speeds and can blow for up to 50 consecutive days.
- Dust Devils: Common worldwide, these dust-filled vortexes look like tornadoes, but smaller and less intense. They typically last only a few minutes.
Weather and Dust Storm Formation
Meteorological conditions play a big role in determining the size, duration, and intensity of dust storms. Weather patterns can be broadly categorized into two categories: large-scale -- or synoptic -- weather systems, and smaller patterns known as mesoscale systems.
Synoptic systems are frontal systems that cover great distances; they range anywhere from 620 to 1,500 miles (998 to 2,414 kilometers) across [source: National Oceanic and Atmospheric Administration]. Dust storms are created when these fronts sweep onto hot, arid surfaces. When cold fronts undercut warm air, the pressure gradient increases, and winds shift and become quite gusty -- as high as 50 to 100 miles per hour (80 to 160 kilometers per hour) [source: University Corporation and Atmospheric Administration].
Additionally, the shifting winds cause turbulence and the high surface temperature creates convective currents at the leading edge of the storm. These forces essentially act to lift particles higher and keep them aloft for longer. Synoptically induced storms can carry dust for very long distances. For instance, each year millions of tons of desert dust ride the trade winds from the North African coast to the Caribbean basin [source: U.S. Geological Survey]. The summer shamals in the Persian gulf and African sirocco winds are synoptic systems that routinely kick up dust storms that can last as long as five days [source: University Corporation and Atmospheric Administration].
Mesoscale systems are smaller weather systems that range from 50 to 100 miles (80 to 160 kilometers) across [source: National Oceanic and Atmospheric Administration]. Although smaller than synoptic systems, mesoscale systems don't always generate smaller storms. Wind currents generated on this scale fall into three groups: down-slope winds, gap winds and convection. Down-slope winds are very strong, gusty winds that blow down the slope of a mountain and reach their peak strength at the foot. Gap winds are low-level winds that pass through relatively small channels or gaps in mountain barriers -- these are the type that blow across the Bodele Depression and end up crossing the Atlantic Ocean. Convection is the vertical movement of air. Although convective winds are generated at all cold fronts, convection is most commonly associated with thunderstorms -- these systems produce haboobs.
Haboobs are what most people think of when they hear the words dust storm. Let's take a closer look at these impressive walls of dust in the next section.