The Great Sand Dunes based on the name is composed of sand. Much of this sand is created from the weathering of the San Juan Mountain Range, which was explained in the last post. The process of erosion (movement of weathered particles) is what carries the sand to the eastern boarder of the San Luis Valley where the sand dunes lie. The erosion process regularly occurs through wind currents, which are seen regularly in the valley.
Picture Showing Wind Direction Within the San Luis Valley
Labeled in the above picture is the Alamosa Rose Wind. The Alamosa Rose is a Southwesterly wind which heads North up through the San Luis Valley until it finally reaches the base of the Sangre De Christo Mountains. The wind force (air pressure) is created through thermal variance in the valley. Since the San Luis Valley is relatively flat and low compared to the mountains surrounding it, the air in the valley is considerably warmer. Professor Allen's Third Law of Geography states wind always blows from high to low pressure, which can be seen in the San Luis Valley. Warm air (high pressure) wind blows the sand towards the mountains where the elevation is higher, resulting in cooler air masses. This process is commonly known as a cyclone because warm high pressure air from the valley flows to cool low pressure air in the mountains. As the winds rise up the Sangre De Christo Mountain Range, the heavier sand falls and is deposited in the base of the mountain. This occurs because the wind currents exit the valley through passes (Music, Medano, and Mosca) in the mountain range. This process is more noticeable during months where temperature varies drastically between the San Luis Valley and Sangre De Chriso Mountains.
Picture Showing Condensation Level of Clouds
As the warm air from the San Luis Valley reaches and climbs the Sangre De Christo Mountains it cools and reaches dew point which is when clouds are formed. This process is known as orographic lifting (when an air mass is forced from low to high elevation). As the particles and moisture in the air cool they condense in the atmosphere. This process does not occur until the air reaches the Lifting Condensation Level, which is the dew point temperature.
Picture Showing the Rain Shadow Effect
The San Luis Valley is located on the windward side of the Sangre De Christo Mountains and the rainshadow side of the San Juan Mountains. This allows for the valley to receive between 8 and 10 inches of rain a year. This effect allows us to see clouds form close to the mountain peaks of the Sangre De Christo Mountains. Because the clouds contain so much moisture, which makes them heavy, they are unable to move until they release enough water for the wind to move them.
