On July 3, 2014 a haboob occurred near Phoenix, Arizona. A haboob is defined by the American Meteorological Society Glossary as “A strong wind and sandstorm or duststorm in northern and central Sudan, especially around Khartoum, where the average number is about 24 a year.” The first recognized paper on this is from 1925 by L. J. Sutton in the Quarterly Journal of the Royal Meteorological Society. Despite the specific location in the definition above, haboobs occur in many parts of the world (mostly arid regions, and especially if these regions have a wet season with thunderstorms). A perfect example of this type of location is the American Southwest, where the North American Monsoon (NAM) brings increased precipitation and storm activity during the roughly July – August time frame.
To understand why this phenomenon arises, lets look at the diagram of winds below a thunderstorm once it reaches a mature state. The figure below shows the stages of a gust front, from a 1982 paper by Roger Wakimoto found here. Once precipitation particles reach a size where the force of gravity overcomes any upward motion from the storm, they begin to fall toward the ground. If there are a large number of drops, the surrounding air can be dragged downward as well. This effect may act in combination with the evaporation of precipitation, which cools the air (which is heavier and sinks) forming what are called downdrafts. The figure below shows that these downdrafts from the thunderstorm run into the ground at which point they spread laterally. As the thunderstorm-induced wind reaches the ground it can pick up loose dirt and dust which become suspended in the “precipitation roll”. This roll is the haboob that we see as above. Notice also that the gust front generally moves faster than and away from the parent storm and therefore is able to pick up the dry dirt not yet moistened by rain.
Haboobs are an active avenue of research, with increased interest in how the transported dust effects air quality and even incoming/outgoing radiation (sunlight/heat). As mentioned above, these phenomena occur during wet periods, such as the NAM. With the fast delivery of a large amount of moisture, flash flooding can occur which can have a substantial impact on an urban population. One such example of ongoing research in this area is the informally named PUFFS (Phoenix Urban Flash Flood Project), which has operated the last 3 years. And of course one side benefit of observing rainfall from the monsoon storms is seeing the occasional (and sometimes frequent) haboob from a mature thunderstorm.