Gravity waves passing by, and other nice time lapses!

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Gravity wave still
Mike Moss

In the course of checking out some new climatological data made available on the Iowa Environmental Mesonet web site, operated by Iowa State University, I ran across a page full of really nice time-lapse videos. Most of them relate to the behavior of convective cells in the form of cumulonimbus clouds crossing the field of view of the cameras.

One somewhat different movie really stands out, though, in terms of the ability of time-lapse video to illustrate physical processes that aren't always as apparent in real time.  In this case, a stable layer of air in the lower atmosphere within an otherwise unstable environment is disturbed by a nearby thunderstorm or squall line. The effect is not unlike tossing a pebble, or perhaps something elongated like a stick, into a pond and setting off a series of waves on the water's surface. In this case, though, the waves are called "internal gravity waves," and they propagate outward within the stable layer of air.

Like waves on a pond surface, the air is moving up going into the crests of the waves and is sinking as the troughs approach, but in the case of the air is also flowing along horizontally. In the absence of sufficient moisture, waves like these may go unnoticed, and in the presence of too much thick cloud cover they might be hard to see as well. Sometimes, though, there is a fairly shallow layer of moisture available that very nicely makes the waves visible, and that is the case in the linked YouTube video. Note that when the troughs of the undulating waves pass by, the clouds are thinner and allow more light through as some warming and drying occurs due to sinking, while the opposite occurs as the crests approach, causing the clouds to thicken and darken along those crests.

The first link I included is for a quick view of the gravity wave video on YouTube. The same video is available as a higher resolution/quality mpeg file at the second link, where you can also find a number of other interesting time-lapse movies, as I mentioned above. Enjoy!


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