This was the view to the northwest during a break in our April-May rainy season. It was shortly after sunset, and the valley just west of ours contained a thick layer of stratus clouds, settled about 150 meters (500 ft) above the humid ground below. As this low-lying humid air cooled below the dew point, its water vapor condensed into clouds.
But look closely at that distant mountain left of center (inset). To me, it looked like a volcano, with a plume of smoke blown eastward by the prevailing wind. I watched for 10-15 minutes as the plume gradually lengthened. After another 10 minutes it was gone. Was it a fire? A check of the Southwest Coordination Center (SWCC) wildfire map showed nothing.
Arizona did have volcanoes — several hundred thousand years ago. Their remnants can be seen all around the state. Check out this image, courtesy of NASA’s Terra land-imaging satellite. And Arizona still has many hot springs. But despite the appearance of the plume, this just couldn’t be a real volcano. There had to be a meteorological explanation.
That mountain is about 300 meters (1000 ft) higher than the surrounding terrain. When I thought about that, I realized what I was seeing. The “volcanic plume” was actually an orographic cloud, formed as warmer humid air, carried by the prevailing wind, flowed up and over the mountain. As the air rises in altitude, it expands and cools adiabatically. When its falling temperature reaches the dew point, it forms a cloud.
There are several types of orographic clouds. The most spectacular are lenticular clouds. If you’d like to see some beautiful examples, check out this collection of images.
The cloud I photographed, unfortunately, was a most mundane example of the orographic species. Technically known as a “banner cloud,” it simply forms as a long plume that eventually re-evaporates.
Careful observers of cloud patterns can tell a lot about what the air is doing. Pilots do it routinely, to help them anticipate updrafts, downdrafts, and turbulence. Some people think “clouds are clouds,” but the science of meteorology has a whole taxonomy of cloud types, understands how they form, and knows what kind of weather to expect from them.
Next Week in Sky Lights ⇒ What a Black Hole Looks Like v.2