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• 1 media/Big Header Photo Blowing wind.jpg 2016-06-01T12:01:28+00:00 Blowing Wind 49 image_header 2020-01-09T20:12:05+00:00 Introduction
  
  Blowing wind is an important mechanism for changing the surface of bodies in our Solar System, particularly where there is little or no water.  On Earth, we see the outcomes of blowing wind particularly well in deserts, which are regions that have potential to lose more water due to evaporation and transpiration (water lost to the atmosphere from plants), together both of these processes are called "evapotranspiration", than the region receives as precipitation (be it rain, snow, fog, or mist).  So, we can define a desert as any region where potential evapotranspiration exceeds precipitation.  On Earth, any region that receives less than 250 mm of precipitation per year is generally considered to be desert.
  
  In deserts wind is the main agent of both erosion of bedrock to form sedimentary particles (gravel, sand, silt, etc.) and transportation of sedimentary particles within the desert environment.  In deserts wind is a particularly effective geological agent because of the absence of plant life (due to the extreme dryness) which tends to protect the substrate from erosion and transport by wind.
  
  On Earth there are a variety of features that are produced by erosion, transport and deposition by wind that have potential to be seen from space on other solar system bodies.  In this section we describe several of these features.
  
  Erosional features
  
  The major erosional features of a desert are those produced by the erosion or removal of rock materials by blowing wind. We often think of deserts as being seas of sand (a "sand sea" is formally called an "erg", even though only about 20% of an erg is covered by sand).  But about 50% of the surface of modern deserts on Earth is covered by what what we call a "desert pavement,' which is an erosional surface that has had most of the sand-size material blown away by wind, leaving only the coarser material, that is too heavy to move, behind to cover the surface.  Desert pavements are extensive stony surfaces that act to "armour" the substrate from further erosion by wind.  The figure below shows a desert pavement with the surface mantled by pebbles left behind by blowing wind.
  
  
  
  While desert pavements are the most extensive erosional features of modern deserts on Earth another important erosional feature is one called a "yardang".  Yardangs are the erosional remnant of bedrock that crops up above the mobile sediment cover of sand and/or gravel.  They are abraded by blowing wind and dust and excavated as the substrate is removed by wind.  Many yardangs are aerodynamically sculpted by abrasion so that they have a high blunt end facing into the wind and become lower and narrower in the downwind direction (see photo of a yardang, below, formed by predominant winds blowing from left to right over the bedrock).  Some archeologists have suggested that some sphinges (that's plural for sphinx) of Egypt are built from outcrops that were originally yardangs. 
  
  
  
  
  
  Transport features
  
  In desert areas blowing wind is the major means of moving sediment, particularly sand to dust size material.  Dust storms in Earth's deserts can have global repercussions because fine material can be carried upwards into the upper levels of the atmosphere where winds can distribute it around the world.  It has been noted by medical researchers that shortly following a major dust storm in the Sahara desert of north Africa there is an increase in asthma attacks in people living in Florida (which is downwind of upper atmospheric winds blowing over the Sahara).
  
  Large scale dust storms in desert areas are spectacular to see at ground level (see image below).  In northern Africa the term for a dust storm with a rolling front like the one shown are called a "haboob". 
  
  
  Dust storms are even more spectacular when seen from space.  In the next photograph we can see the extent of atmospheric haze across the Red Sea and adjacent lands of North Africa.
  
  
  
  Dust devils are produced by rotating vortices of wind at the ground level that picks up fine sand and dust from a desert surface and carries them along like a small tornado.  They not only transport fine sediment, they also erode the substrate to produce shallow linear troughs that preserve the path or track that the dust devil followed while in contact with the ground surface.
  
  
  
  The following video shows an energetic dust devil close up at ground level.
  
  
  
  
  Deposition
  
  We typically think of active sand dunes as being depositional features but they are actually an important element of sediment transport by blowing wind.  Dunes are actually only temporary deposits because sediment moves with them as the dune itself migrates over the desert surface.  Dunes migrate in the wind direction and they do so as sediment is eroded from their upwind side and deposited on their leeward (downwind) side; sand is temporarily stored within the dune between the brief periods of erosion, transport and deposition that causes dune migration in the downwind direction.  The duration of the period of storage within the dune increases with the size of the dunes.  
  
  On Earth "eolian dunes" make up the most common depositional structures in deserts.  They are formed where there is adequate sand cover over the ground surface so that it can be transported and sculpted by the wind into various shapes that are determined in larger part by the characteristics of the wind.  In modern deserts dunes can vary is scale from meters to hundreds of meters in height and meters to many kilometers in length.
  
  In this section we'll look at some of the common dune types that are found in deserts on Earth. 
  
  Barchan dunes
  
  Barchan dunes are common in many desert areas, particularly where the wind direction does not vary significantly over time. These are asymmetrical structures that have a gentle slope that dips into the wind direction and a steeper slope of about 35 degrees facing down wind.  The most distinctive feature of this dune type is that the crest of a barchan dune (that extends across the wind direction) is strongly curved so that the dune appears to have "horns" that point in the predominant wind direction.  The dunes migrate in the downwind direction as sand is eroded from their upwind facing slopes and avalanches or slides down the downwind facing slope where it is deposited; with each period of erosion and deposition the dune move a little further along in the direction that the wind is blowing.  Note that the downwind side of the dune is often called the "slip face" because of the way sand avalanches down its surface.  The local wind direction can easily be inferred from the orientation of the dunes sloping surfaces.
  
  
  The following is a photograph of well-developed barchan dunes on Earth.
  
  
  
  
  Transverse (Linear) dunes
  
  These are also asymmetrical dunes that have a gently sloping upwind side and a steeply (about 35 degrees) downwind side.  Like barchan dunes you can infer the predominant wind direction from the orientation of the dune (as shown in the figure, below).  The crests of transverse dunes are much longer (across the wind direction) and lack the prominent horns that characterize barchan dunes.  Despite the absence of horns it is easy to determine the local wind direction based on the overall form: wind blows along a line that is at 90 degrees to the crest and up the gently sloping surface.  Thus, transverse dunes migrate downwind in much the same manner as barchan dunes.
  
  
  
  Longitudinal (linear) dunes
  
  Longitudinal (sometimes called linear) dunes have relatively long, straight crests but their crests are oriented parallel to the average wind direction.  In cross section (across their crests) they may be symmetrical (i.e., the slopes on either side are similar) and a well-developed slip face is notably absent.  This form of dune develops when the wind direction varies with time over several tens of degrees as shown by the arrows indicating wind direction in the figure below.  Superimposed on the flanks of longitudinal dunes are often smaller dunes that are asymmetrical (forms of  barchan and transverse dunes), have well-developed slip faces and are oriented in the average wind direction (slip faces sloping downwind).  The crest of the longitudinal dune is "along" the wind direction but the crest alone cannot be used to infer the absolute direction in which the wind is blowing; direction of the wind can be inferred from the orientation of smaller dunes with slip faces.
  
  
  Star dunes
  
  Star dunes are more complex dunes than those described above and they form where the wind varies significantly in direction over time.  These dunes have multiple crests that often radiate from a central point (or a number of points) and do not have well developed slip faces.