ATSC 113 Weather for Sailing, Flying & Snow Sports

Special Clouds > Billow Clouds

Learning Goal 1b. Recognize special clouds (e.g. castellanus, billow, lenticular, rotor, banner, fractus, etc.)

Wind shear (change of wind speed or direction with height) in the atmosphere can cause waves to form in a layer of air where cooler air underlies warmer air. These waves are called Kelvin-Helmholtz (KH) waves, and do not need mountains to form. These waves have relatively short wavelengths (crest to crest distance of hundreds of meters) and are relatively thin (hundreds of meters).

Such waves probably occur very often in the atmosphere, but are visible only rarely, when the rising cool air is relatively humid. Under the correct conditions, parallel bands of clouds form in the crest of each wave. To a viewer on the ground, these parallel bands of clouds look like billows of an accordion.

The waves often break, causing that cloud layer to become turbulent. While the waves are breaking, "cat's eye" swirls form, some of which are even visible on radar. Pilots prefer to avoid flying at these altitudes because of the turbulence induced by wind shear.

Billow clouds and KH waves

Because these waves happen in large horizontal regions, you can often see billow clouds as a high-cloud layer (cirrocumulus undulatus), or as mid-cloud layer (altocumulus undulatus). The photos below show some examples of billow clouds.

Here is a photo taken at sunset of high-altitude (cirrocumulus) billow clouds (in the top half of the photo):

cirrocumulus billows over Montreal

The next photos show mid-level (altocumulus) billow clouds:

billow cloud 1

billow cloud 2

billow clouds

The next photo shows lower billow clouds under an overcast mid-level cloud layer.

billow clouds 3

Next are two photos of breaking KH wave clouds, taken by Dr. May Wong (used with permission) at Boulder, Colorado in October 2016.  Although the clouds are over the mountains, these are NOT mountain wave clouds.  Instead these clouds are caused by wind shear at the altitude of the cloud.  If an aircraft were to fly at the altitude of this KH cloud, it would likely experience very strong turbulence.

KH waves over Boulder, CO, photo by Dr. May Wong

KH waves over Boulder, CO, photo by Dr. May Wong

And here is a photo showing breaking KH waves on the side of a cumulus cloud:

KH waves on side of Cu

Below is a photo of a thin top layer of clouds with breaking KH waves.
KH waves

You can also find many beautiful photos by web searching on images for "billow clouds" and for "kelvin helmholtz clouds".

Here are YouTube videos showing KH wave formation in a laboratory water tank, and in a computer simulation:

Here are YouTube videos of KH waves and billow clouds:

Key words: altocumulus undulatus, banner clouds, cirrocumulus undulatus, Kelvin-Helmholtz (KH) waves, wind shear

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, then search on "undulatus clouds", the official name for billow clouds.
  • USA.A.1. - American Meteorological Society (AMS): Glossary of Meteorology
  • USA.C.1. - Practical Meteorology (PrMet): Chapter 5 Stability, and Chapter 6 Clouds

Image credits. All figures and photos by Roland Stull, except as noted near the figures.   Two KH wave photos by Dr. May Wong, used with permission.