Lenticular Clouds: Why Are These Aerosol Masses Shaped Like a UFO?

If you live near a hill, mountain, or volcano, you have a greater chance of seeing lenticular clouds. These cloud formations are famous for their strange appearance, resembling the traditional shape of flying saucers in science fiction.

What are Lenticular Clouds?

Lenticular clouds refer to lens-shaped orographic wave clouds that form downwind of an obstacle in the path of a strong air current. Their name comes from the Latin word 'altocumulus lenticularis' which means "like a lens".

Also known as lee waves, these clouds form most often in winter, although it is expected to see them at other times of the year. In general, the base of the lenticular clouds measures 6,500 - 16,500 feet (1,981 to 5,029 meters) high. Unlike ordinary clouds, lenticular clouds do not give off precipitation.

Lenticular clouds are a visible sign of mountain waves in the air. However, these waves can also be present beyond the clouds and may exist even when no clouds are formed.

On the ground, lenticular clouds can result in very gusty winds in one place, with stationary air only a few hundred meters away. Pilots of powered aircraft try to avoid them because the presence of lenticular clouds means there could be severe turbulence. On the other hand, skilled glider pilots like them because the shape of these clouds can help them tell where the air will be rising.

Lenticular clouds can be categorized into three main types. Based on their altitude above the ground, they can be identified as cirrocumulus standing lenticular (CCSL), stratocumulus standing lenticular (SCSL), and altocumulus standing lenticular (ACSL).

How Do Lenticular Clouds Form?

Lenticular clouds usually form when wind runs perpendicular to a mountain or a mountain chain from the same or similar direction at different heights through the troposphere. It could be a hill or even a tall building in some places.

During the process of expansional cooling, the air cools as it rises. As the air cools down to the dew point temperature, it can no longer hold the water vapor it once could when it was warmer. As a result, cloud droplets form and get suspended in the air.

Clouds form in specific altitudes, also known as lifted condensation levels. As the air rises, clouds continue to develop. At a level higher than the mountain peak, there is a stable layer or a level where winds run faster than below. This shears off the top of the cloud. The stable layer gets warmer than the air down. Both scenarios can occur, but both can give the cloud a sharp edge on top.

After passing the mountain ridge, the air sinks and then gets warm by a process known as compressional warming. The sinking and warming air evaporates the cloud droplets. This action gives the cloud a smooth edge on the left side of the mountain. The up-and-down movement of the air can last for up to 100 miles (161 kilometers) past the first wave of air.

While wind blows most clouds across the sky, lenticular clouds seem to stay in one place. Air moves up and over a mountain, where the lenticular cloud forms just past the mountaintop. Since the lenticular cloud evaporates on the downwind side, it appears stationary even though air moves through it.