Altocumulus undulatus clouds are a common cloud formation that can be observed in the late afternoon to early evening, when large masses of air are moving slowly enough to settle into convective layers according to temperature. Their wave shape resembles parallel waves on water or the ridges of sand dunes.

Even though they are a common cloud type, their exact formation mechanism is still poorly understood. AU clouds form during peaceful weather conditions that do not produce precipitation or high winds. Storm-generating clouds like cumulonimbus have been studied with satellite and radar measurements for weather prediction, since they produce tornadoes, hurricanes, or damaging precipitation and winds. However, fewer studies on AU clouds appear in the literature since they occur during uneventful weather conditions and have little predictive value.

Although AU clouds are commonly accepted as formed by wind shear, a qualitative relationship between wind speed, altitude, temperature or other predictors has not been established. In other words, a qualitative predictive model has not been found to accurately predict AU dimensions from environmental conditions.

Similar although distinct phenomena such as roll clouds, cloud streets, lee waves and gravity waves have been studied, and predictive models formulated for the convective layers as a function of wind velocity and temperature.

You can perform a simple experiment that produces similar band structures when preparing a cup of tea. Fill a mug with enough water to leave about 3 cm clearance between the water surface and lip. Add enough black tea bags (one is usually enough) so that foam bubbles appear on the surface. Remove the bag, and gently rock the mug back and forth, to set the tea rocking. Let go, and watch the bands appear.

Its critical that the tea be hot enough for the foam bubbles to move about easily, so don’t let the tea cool before rocking it. Also, the tea needs to be rocked vigorously enough to make the bands appear. Finding the natural resonant frequency of the tea takes a bit of practice, so don’t give up after your first try.

Similar bands appear in particulate matter that has settled to the bottom of a liquid. Continuing with caffeinated beverages, the same effect can be seen in fine coffee grounds at the bottom of the cup when sifted properly. Unlike the tea experiment, the temperature of the coffee appears irrelevant, since the coffee grounds are always free to move at the bottom of the mug. This experiment also works with any particulate matter that doesn’t adhere to itself or form clumps, such as cornmeal, cinnamon or cumin. Have fun with kitchen science!

You’ll notice that the bands always have a similar wavelength, which suggests that the band spacing is a function of the natural resonant frequency of the tea. Could a similar phenomenon be occurring in the atmosphere?

See also---

  • Bleeker, W. and Andre, M. J. Convective Phenomena in the Atmosphere. Journal of the Atmospheric Sciences, vol. 7, Issue 3, June 1950, p. 195-209.
  • Hanna, S. R. The Formation of Longitudinal Sand Dunes by Large Helical Eddies in the Atmosphere. Journal of
  • Applied Meteorology vol. 8, Dec 1969, p. 874-882.
  • Hill, G. E. On the Orientation of Cloud Bands. Tellus XX, 1968, p. 132-137.
  • Kuettner, J. The Band Structure of the Atmosphere. Tellus vol. 2, no. 3, Aug 1959. p. 267-294.
  • Pretor-Pinney, G. The Cloudspotter's Guide, G. Penguin Group, the Berkeley Publishing Group. 2006, P. 197-198.