Thursday, January 30, 2014

Snow Dunes

Many of the schools in the area are closed again today due to blowing and drifting snow.  As I was driving to work earlier this week I noticed how the snow was acting like sand and forming the same types of dunes that would be found in sand.  Under the right conditions, snow can act very much like sand - cold (well below freezing) temperatures that do not allow the snow to melt , fine "dry" snow crystals that do not easily stick together, and of course wind.

I went out into the open field behind the Conservation District office and photographed these small snow "dunes".  None of these features was more than a few inches in height, but they display the classic features of certain types of dunes.

The first feature is a barchan dune.  Barchan dunes form in open areas where there are no obstructions.  A barchan dune is shaped like an arch with the curve of the arch pointing into the wind and arms extending downwind.  In this first picture, there are two barchan dunes on the right half of the image.
Snow "dune" features - Barchan dunes to the right of the image
Here is the same image with the wind direction indicated with an arrow and the dunes highlighted with curved lines.  The dunes slowly move downwind an particles are picked up from the face of the dune and deposited on the arms or in the lee space created behind the peak of the dune.

Snow "dune" features - note wind direction (arrow) and barchan dunes (arcs)
Over time, the arms of barchan dunes will intersect with those of adjacent dunes.  The resulting undulating line of dunes in now known as a barchanoid dune

Snow "dune" features - barchanoid dune in center of image
Here is the same image with wind direction and dune indicated.  The barchan dunes from image 1 and 2 are to the right of this picture.

Snow "dune" features - note wind direction (arrow) and barchanoid dune (curving line)
Eventually, barchoid dunes will begin to lose their undulating curves and become straight or nearly straight lines.  This type of dune is known as a transverse dune.  Often many transverse dunes will be found together in rows of ridges and valleys.

This image shows the transition between barchanoid and transverse dune forms.  The closest formation still shows the undulating character of a barchanoid dune, but the dunes doing off into the distance have straightened out forming a series of ridges perpendicular to the wind.

Snow "dune" features - transverse and barchanoid dunes
Another copy of the same image with wind direction and dunes indicated.  the nearest line is a barchoid dune and the further lines have evolved into straighter transverse dunes.

Snow "dune" features - note the wind direction (arrow), barchanoid dune (closest line), and transverse dunes (furthest three lines)
For more information on these and other dune types visit the Great Sand Dunes National Park website.

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