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Clouds Near Australia

Clouds Near Australia

On October 17, 2017, the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite captured this image of an unusual cloud pattern off the coast of southern Australia.

Clouds appear to be streaming out from a cold front, indicated by the dark blue line on a weather map published that day by Australia’s Bureau of Meteorology (BOM). But according to Paul Lainio, BOM meteorologist, that’s not actually what’s happening.

Instead, the pattern is caused by a phenomenon in the atmosphere called “gravity waves.” Similar to a boat’s wake, which forms as water is pushed upward by the boat and pulled downward again by gravity, these clouds are formed by the rise and fall of air columns. As the wave moves along the cloud band, the wave peaks appear cloudy and the troughs appear cloud-free. In this case, the gravity waves developed as a result of instability on the flank of a strong jet stream moving ahead of the cold front.

“This type of effect is relatively unusual since it requires a strong anticyclonic-curved jet that develops gravity waves of sufficient magnitude,” Lainio said. “The gravity waves are the atmosphere’s way of restoring balance, and they usually don’t last for lengthy periods.”

Others suggested that the clouds might be transverse cirrus bands. Lainio agrees that is also possible, but he added that the regular spacing between bands leads him back to gravity waves. “I'm fairly confident,” Lainio said, “but as with much meteorology, I cannot be 100 percent sure.”

NASA Earth Observatory image by Joshua Stevens, using VIIRS data from LANCE/EOSDIS Rapid Response. Story by Kathryn Hansen.

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