Earth Notes
4:38 pm
Wed June 30, 2010

Earth Notes: Southwestern Cloudscapes

Flagstaff, AZ – Earth Notes: Southwestern Cloudscapes


The climate and varied topography of the Colorado Plateau shape many aspects of the natural world, including some on spectacular display high overhead.

During dry times of year, lenticular clouds often form over southwestern mountains. Formed where high ground pushes up large standing waves of air, they have very clearly defined shapes and appear to stay in place even as the wind blows.

In reality, a lenticular cloud is constantly being formed anew on a peak's upwind side even as it evaporates away on the downwind side. The result is a distinctive lens shape, often seen stacked one beside another like a fleet of giant flying saucers.

The moisture of summer monsoon storms produces huge cumulonimbus clouds that can tower as much as 70,000 feet. That's above the freezing level, and the ice crystals that form as a result give the cloud top a striped appearance.

But perhaps more characteristic is the flat-topped anvil shape that forms when upward-moving air is diverted laterally high in the atmosphere. Such clouds are often accompanied by theatrical displays of thunder, lightning, heavy showers and hail. But rain does not always reach the ground; a thunderhead's moisture can evaporate in mid-air, forming the hanging veil known as "virga."

The most spectacular cloudscapes of all might be the classic southwestern sunsets formed whenever the setting sun shines from cloud-free skies to the west. Any clouds that happen to be overhead are lit from underneath, producing a visual symphony in flaming hues.

-Diane Hope