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2024 Hennepin County All -Jurisdiction Hazard Mitigation Plan <br />Volume 2 — Hazard Inventory <br />4.3.9.5. Geographic Scope of Hazard Blc <br />The winds involved with dust storms can be as small as "dust devils" or as large as fast moving regional air <br />masses. Dust storms occur most frequently over deserts and regions of dry soil, where particles are loosely <br />bound to the surface. Dust storms don't only happen in the middle of the desert, however. They happen <br />in any dry area where loose dirt can easily be picked up by wind. Grains of sand, lofted into the air by the <br />wind, fall back to the ground within a few hours, but smaller particles remain suspended in the air for a <br />week or more and can be swept thousands of miles downwind. Dusts storms can reach as high as 10,000 <br />feet with an aerial coverage on the leading edge that can stretch for hundreds of miles. However, on <br />average, they only travel around 25 to 50 miles. <br />4.3.9.6. Chronologic Patterns <br />Dust storms are not common around Minnesota, but they can happen any time of year, and have occurred <br />in the past. They are most common in desert regions, including the US Southwest and often are triggered <br />by downdraft winds from monsoon thunderstorms. They are slightly more common during the afternoons <br />and evenings than at cooler times of day, but only because of the importance of thunderstorms, which <br />tend to be most numerous and most intense during afternoons or evenings. Otherwise, diurnal cycles of <br />heating and cooling have no effect on dust storm behavior or probability. <br />In Minnesota, dust storms are most likely during persistently dry conditions, and/or when dry and loose <br />soil is also unprotected by mature vegetation. Because the growing season features higher rates of <br />moisture conduction between plants and soils, and because the same plants will shield underlying soils <br />from wind erosion, dust storms will tend to favor the pre -green -up periods of Late March into May, or <br />late September into early November. <br />GRAPH 4.3.9A shows the critical wind erosion period in Minnesota. It shows that March, April, and May <br />are the periods of the year where agricultural fields are particularly vulnerable to wind erosion, and to <br />extension dust storms, due to higher wind speeds with direction of prevailing wind than normal and low <br />vegetative cover on fields. <br />GRAPH 4.3.9A Critical wind erosion <br />30 <br />25 <br />20 <br />1.5 <br />1.0 <br />5 <br />U <br />lan <br />Percent of All EirOsive Will <br />it ire ire e 1po II it , MN <br />Feb (March Apr <br />May .1 un <br />Dull <br />Aug <br />Sep <br />OCL NOV <br />IDec <br />155 <br />