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NAME: Dust storm magnitude, duration, and frequency
BRIEF DESCRIPTION: The frequency, duration and magnitude (intensity) of dust storms are gauges of the transport of dust and other fine sediments from arid and semi-arid regions [see wind erosion]. Desert winds carry more fine sediment than any other geological agent: the Sahara probably moves 60-200 million tonnes/yr of dust. An increased flux of dust has been correlated with periods of drier and/or windier climates in arid regions, historically and from proxy records in ocean and ice cores. Material picked up in the Sahara Desert is known to be transported in the atmosphere across the Atlantic Ocean.
SIGNIFICANCE: Local, regional and global weather patterns can be strongly influenced by accumulations of dust in the atmosphere. Dust storms remove large quantities of surface sediments and topsoil with nutrients and seeds: in the 1930s, drought and dust storms created the 'Dust Bowl', greatly reducing agricultural production on the North American prairies at that time. Wind-borne dust, especially where the grain size is less than 10µ m, and salts are known hazards to human health. Dust storms are also an important source of nutrients for soils in desert margin areas.
HUMAN OR NATURAL CAUSE: Dust storms are natural events, but the amount of sediment available for transport may be related to surface disturbances such as overgrazing, ploughing, or removal of vegetation.
ENVIRONMENT WHERE APPLICABLE: Arid and semi- arid regions, temperate, tropical and sub- tropical latitudes.
TYPES OF MONITORING SITES: Downwind of source areas, near urban and/or agricultural areas, and away from local wind barriers.
SPATIAL SCALE: landscape to mesoscale / regional to continental
METHOD OF MEASUREMENT: Determine frequency, length of storm season, volume of transported material, with visibility observations at first-order meteorological stations. Reduction of visibility to WMO specified limits gives an index of event intensity: the duration gives an approximation of magnitude. Satellite measurements of dust storms are being developed for regional monitoring and tracing pathways.
FREQUENCY OF MEASUREMENT: Each event should be recorded. The best statistic is the annual frequency of occurrence. Estimate long- range sediment transport at least every 10 years.
LIMITATIONS OF DATA AND MONITORING: restricted geographical distribution of monitoring sites.
APPLICATIONS TO PAST AND FUTURE: Good index of aridity and/or wind speeds. Paleo-records may be developed from ancient dust storm deposits found in ice cores, ocean sediments and loess.
POSSIBLE THRESHOLDS: Wind speeds of more than 5-10 m/sec are required for entrainment. Thresholds are strongly affected by the character of the ground surface and the vegetation cover.
Goudie, A.S. & N.J.Middleton 1992. The changing frequency of dust storms throughout time. Climatic Change, 20: 97- 225.
Lancaster, N. 1996. Geoindicators from desert landforms. In Berger, A.R. & W.J.Iams (eds). Geoindicators: Assessing rapid environmental changes in earth systems:251-268. Rotterdam: A.A. Balkema.
Pye, K. 1987. Aeolian dust and dust deposits. London: Academic Press.
OTHER SOURCES OF INFORMATION: Geological surveys, environment agencies, desert research institutes. Environmental and meteorological agencies may have some useful data from air quality measurements. WMO
RELATED ENVIRONMENTAL AND GEOLOGICAL ISSUES: Wind erosion, changes in hydrological systems
OVERALL ASSESSMENT: Dust storms can be a major contributor to reduced air quality, and can cause hazards to human health. Their magnitude, duration and frequency are valuable indicators.
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