Clouds, Wind Pressure, and Fronts of the San Luis Valley

     Based on the sand composition of the Great Sand Dunes, it can be concluded that the sand originated mainly in the San Juan Mountains.  As explained in the last post, this is due to the process of weathering the San Juan Mountains.  The process of erosion (movement of weathered sediments) is what is responsible for the carrying of sand to the eastern boarder of the San Luis Valley were the active dune-filed lies.  Most of the time, this erosion process is fueled by wind currents which occur in the valley.

Picture showing the direction of wind in the San Luis Valley.

     One such wind current is the Alamosa Rose Wind.  This is a Southwesterly wind which North through the San Luis Valley until it reaches the base of the Sangre De Christo Mountains.  This predominant wind force (air pressure) is created through thermal variance in air cells in the San Luis Valley.  Since the valley floor is relatively flat and low in elevation (when compared to the mountains surrounding the valley) it can be concluded that the air in the valley is warmer than the air found nearer the mountains.  As the third law of geography states, wind always blows from high to low pressure.  Since warm air usually has higher pressure than cool air, the wind in the valley blows the sand towards the mountains where the elevation is higher thus resulting in cooler air masses.  This process can be thought of as a cyclone (when warm high pressure air flows to cooler low pressure air).  As the winds rise up the Sangre De Christo range, the sand falls out and becomes deposited at the base of the mountains.  The sand is deposited in this particular location in the valley because the wind exits the valley over the Medano, Mosca, and Music Passes (low spots at the peak of) the Sangre De Christo Mountain Range.  It is here where the winds funnel over the mountains.  If a map of wind currents was to be done at these passes, the isobars would be very close together.  This process is more noticeable during the spring months since the variance in temperature between the mountains and the valley is greater during this time.

Picture showing the lifting condensation level of clouds.

     As the warm air climbs the Sangre De Christo Mountains it becomes cooler and reaches its dew point this is when clouds form.  This is known as orographic lifting (when topographic features force air to rise).  As the moisture in the air cools it condenses on particulate matter in the atmosphere.  This does not occur until the air reaches the Lifting Condesation Level (at which the temperature is correct for dew (clouds) to form). 

Diagram showing the rain-shadow effect.

 It is interesting that the San Luis Valley is located in the rain-shadow (leeward side) of the San Juan Mountains and receives 8 inches of precipitation a year.  However, the valley is also located on the windward side of the Sangre De Christo Mountains.  As the air rises up the mountains clouds can be seen forming at the peaks of the Sangre De Christo Mountains.  These clouds cannot proceed over the mountains until they release their water weight.

Interesting facts:  The sand dunes absorb lots of heat during the summer, and results in a surface temperature of about 140 degrees Fahrenheit

Picture of a fulgurite.

When lightning strikes the dunes, it produces Fulgurites.  These are columns of sand that have been melted together from the extreme heat of a lightning strike.