Special Clouds > Rotor Clouds
Learning Goal 1b. Recognize special clouds (e.g. castellanus,
billow, lenticular, rotor, banner, fractus, etc.)
Rotor clouds are turbulent,
ragged-looking clouds that form at low altitude under the crests of
mountain waves. They rotate about a horizontal axis parallel to the
mountain range. They are associated with violent turbulence that can
break wings off of aircraft, and their strong up- and down-drafts can
cause pilots of small aircraft to crash while attempting to land and
take off.
Diagram 1. The average wind is blowing from left to right,
across the mountain rainge. The wavy black lines represent the movement
of air as it blows downwind as a mountain wave.
But notice that on the bottom side of the rotor,
the air is blowing in the opposite direction to the average wind
direction at higher altitude. In these examples, the middle and
high-altitude winds are from the west (from the left side of the
diagram), while the bottom of the rotor has winds from the east. This
change in wind direction over relatively short horizontal and vertical
distances makes flying extremely difficult and dangerous.
Diagram 2. If the air is humid enough, then clouds
can form in the updraft portions of the wave and rotor
circulations. Lenticular clouds can form in the top part of the
mountain waves, and rotor clouds
can form in the updraft portion of the rotor circulation. If the
air is flowing over a long mountain range instead of an isolated peak,
then the rotor cloud can be a long horizontal rough-looking turbulent
cylinder parallel to the ground (the diagram above shows only one end
of the cylinder). Similarly, the lenticular clouds can be long
bands above and parallel to the mountain range (the diagram above shows
only the end of these lenticular cloud bands).
Here is a video simulation, showing a rotor circulations forming on the lee (downwind) side of a mountain range: https://www.youtube.com/watch?v=xOjrIbcKGpc
The next two photos show the ragged, turbulent rotor clouds under
smoother-looking lenticular clouds.
Same photos with labels:
If the air is relatively dry, then neither lenticular clouds nor
rotor clouds will form, even though dangerous mountain waves and rotors
can still be present. In other words, if there are strong winds blowing
across the mountains, then there can be dangerous mountain waves
and rotors even if you cannot see any rotor or lenticular clouds.
Unfortunately, the few photos that I have with rotor clouds also
have so many other clouds that you cannot distinguish the rotor clouds.
Also, a word of caution: a Google image search on "rotor clouds" shows
many images that are NOT really rotor clouds.
Here is a time-lapse video of rotor clouds in Boulder, Colorado: https://www.youtube.com/watch?v=Cy0_PZiagBw
The rotor cloud is visible rotating in the background.
The video camera is located at the "x" in Diagram 2 above, and the wind is blowing toward the video camera.
Keywords: mountain waves, rotor, rotor clouds,
lenticular clouds
Extra info for experts;
Not Needed for this
Course.
Use this expert_resources link to find all the items
listed below:
- Item
- Topic
- World.A.1. - World Meteorological Organization (WMO)
"International Cloud Atlas"
- USA.B.1. - Wikipedia list
of cloud types
- World.A.2. - WMO: Manual on Cloud Observations (an old,
outdated document)
- USA.A.1. - American Meteorological Society (AMS): Glossary of
Meteorology
- USA.C.1. - Practical Meteorology (PrMet): Chapter 17, Regional Winds
Image credits. All figures by Roland Stull.