Sunset at Arches National Park Campground

Photo copyright by Craig Johnson, WeatherBriefing.com

This photo was taken at sunset from the campground at Arches National Park near Moab, Utah. Temperatures had cooled considerably from mid-afternoon highs. The warmth of the day and a passing upper air disturbance had set off scattered thundershowers with gusty winds and a chilly rain. This photo shows the remnants of the thundershowers that by then had dissipated.

The setting Sun created a faint hint of yellow on the underside of altostratus clouds. On the left side of the photo fall streaks of rain not reaching the ground are seen hanging below the cloud base. Fall streaks are common in desert areas because dry low level air causes the rain to evaporate before it reaches the ground.

Fall Streaks reaching the ground at Arches National Park. Photo Copyright 2012 by Craig Johnson, Weather Briefing, LC

Deserts are dry for many reasons. In this case, Arches National Park is located in the interior of the North American Continent to the the east of several mountain ranges. As air flows over the continent from the Pacific Ocean its moisture is dropped on the windward side of the mountains. Rain shadows are found east of each mountain range. A rain shadow area is located downwind of mountain ranges on the lee side of the mountain ridges. In this case the west to east flow causes more precipitation to fall on the west side of the mountains than to the east. Successive north-south arranged mountain ridges force the air to rise and drop precipitation leaving little moisture for areas to the east.

Photo at Canyonlands National Park, Utah, copyright 2012 by Craig Johnson, Weather Briefing, LC

Notice the hint of green on top of the mesa in the lower left of the photo. This photo was taken in late September 2012 from Canyonlands National Park, Utah. There had been late summer thundershowers that encouraged new growth on top of the mesa. Vegetation in desert areas must take advantage of any rain it receives. The thick white clouds in the distance are late afternoon thundershowers. It often rains at the top of the mesa while the bottom of the canyon receives little rain because of evaporation of the raindrops. The clouds are moving right to left. Notice the shadows of clouds on the ground covering parts of the mesa and the walls of the canyon in the distance. As the shadows move there are small changes in temperature near the ground.