QUESTION: The other morning, I woke to find clouds rolling over the top of Ute Mountain, a nearby peak that stands just under 10,000 feet. Although the mass of them disappeared, there remained a solitary banner-shaped cloud hanging onto the top of the mountain and flying in the direction of the wind. It appeared to be in perpetual motion while refusing to let go of the peak. What was forming the stationary cloud?
Chris Moore, Cortez, Colo.
ANSWER: The cloud formed as a result of orographic lift – an air mass being forced from low to high elevation as it passes over the mountain. Orographic lift plays a critical role in weather patterns in mountainous regions around the world.
For example, it is responsible for the heavy precipitation characteristic of the Pacific Northwest. As the moist air from the Pacific Ocean is forced over the coastal mountains by the prevailing winds, it cools, causing the moisture in it to condense. On the other (lee) side of the mountain, air warms and dries as it descends, resulting in regions of very low precipitation.
When air masses pass over mountain ranges, water is not “squeezed” out of the air, as weather forecasters sometimes say. Air is not like a sponge. Instead, the water molecules, which are in a constant state of evaporation and condensation, slow down as the temperature decreases, and condensation starts to outpace evaporation.
In addition to influencing precipitation patterns, orographic lift leads to the formation of strange clouds because the change in speed and direction of the air blowing over mountains can produce standing waves in the atmosphere. The waves can remain nearly stationary for extended periods if the wind generating them has a fairly constant speed.
Banner clouds are produced when there is a sustained drop in pressure behind the mountain obstacle. Air expands due to the drop in pressure. Expansion of the air causes it to cool because heat energy is used to overcome the weak attractive forces between the molecules in the air.
Banner clouds are similar to the condensation trails that form as air passes over the wing of an airplane. Condensation forms continuously in the low-pressure region and evaporates beyond it.
Lenticulars (Altocumulus Lenticularis) are another interesting type of cloud caused by atmospheric standing waves over mountains. These striking, lens-shaped clouds are sometimes mistaken for UFOs.
Sherry Seethaler is a UCSD science writer and educator. Send scientific questions to her at Quest, San Diego Union-Tribune, P.O. Box 120191, San Diego, CA 92112-0191. Or e-mail sseethaler@ucsd.edu. Please include your name, city of residence and phone number.
