My house is at 9,200 feet elevation, and it looks west over the Tularosa Basin, which is at 4,300 feet. Occasionally a layer of clouds forms well below my elevation and fills the basin, as on December 2, 2009. This time, I noticed a ridge in the cloud layer that I strongly suspected lay over the four-lane divided highway (US 70) that runs NE to SW across the basin. The cloud layer was quite low, but I don't know if it was touching the ground. There had been about a half inch of precipitation in the basin in the three days before December 2. In the attached photo, taken at 7:30 a.m., the mountains in the background are about 40 miles away; the ridge in the clouds is about 20 miles away and about six miles across.
I am curious to know why a ridge in the clouds appeared over the highway. Was it caused by a difference in temperature between the pavement and the sandy soil on either side? By heat from the highway traffic? By vehicles moving the air along the highway? By something else?
Caption: A bright ridge in the fog covering the Tularosa Basin of New Mexico at 7:30 a.m. on December 2, 2009, looking west.
Sunspot, New Mexico
I corresponded with Kit seeking more information. By comparing similar photos from the same vantage point, one with basin fog and one without, he clearly established that the ridge in the cloud layer was over the highway. He supplied a diagram, superposing his camera view over a DigitalGlobe image of the Tularosa Basin, taken with clear skies. The red line marks the highway. He also sent surface observations from Holloman Air Force Base and the Alamogordo Airport, both located on the image. Both sites reported fog with visibility of ¼ mile or less, sky obscured, and temperatures near 30°F at the time of the photo.
Caption: A DigitalGlobe image with camera view angle, the highway, and two surface observing sites marked. The White Sands National Monument lies at upper right.
Why did a ridge appear in the fog above the highway? The air temperature had been in the upper 20s before the photo was taken. I wondered if exhaust from traffic might contain ice nuclei on which ice crystals could form. The release of latent heat associated with crystal growth might have been enough to elevate the fog over the highway. This line of reasoning is a dead end because very few ice nuclei are active at such high temperatures, and such nuclei aren't present in the exhaust from automobiles and trucks. Differences in solar heating between the road surface and the surrounding desert can be ruled out because the sun had just risen, and the fog was obscuring the sky, as indicated by the nearby surface observations. That leaves heat in the exhaust from cars and trucks, and turbulence generated by moving traffic as the most likely causes for the ridge-like elevation of the fog layer over the highway.
The photo poses another interesting question: why is the ridge noticeably brighter than the fog itself?
For decades, ship tracks in the ocean have been clearly visible in satellite images when the ships pass through fog or low stratus clouds, which occur frequently off the U.S. West Coast. The tracks appear as bright lines, superposed on a somewhat darker field of low clouds and fog. This effect bears the name of Sean Twomey, who first explained it in 1974.
Cloud condensation nuclei (CCN) are minute particles upon which water vapor readily condenses when the relative humidity is close to 100%. When a source of CCN from anthropogenic pollution is introduced into an otherwise clean environment where low clouds or fog already exist, the available vapor can condense on many more particles per unit volume than before, resulting in a greater number of smaller droplets compared with a lower number of larger droplets. Twomey was the first to point out that clouds containing large numbers of small droplets are more highly reflective (brighter) than those containing fewer, larger droplets but the same amount of liquid water per unit volume.
Ships passing through fog or low clouds off the West Coast, where the clean air has a relative dearth of CCN, increase their concentration by orders of magnitude. As the CCN from the ship's stacks mix with the cloudy air, the number concentration of smaller drops greatly increases, brightening the clouds along the track and making them plainly visible.
It's quite likely that the same process occurred in the Tularosa Basin. The desert air is relatively clean as the fog forms overnight. The exhaust from the highway traffic contains high concentrations of CCN, which mix with the surrounding foggy air and prompt a shift in the size spectrum of cloud droplets toward smaller sizes. The air over the highway doesn't move because there's no wind, so a bright ribbon appears in the fog above the highway when the sun comes up.
Weatherwise Contributing Editor THOMAS W. SCHLATTER is a retired meteorologist and volunteer at NOAA's Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author at firstname.lastname@example.org, or by mail in care of Weatherwise, Taylor & Francis, 325 Chestnut St. Suite 800, Philadelphia, PA 19106.