Planetary waves: The biggest coherent pressure structures in the Earth’s atmosphere

Water vapour measurements by satellite uncover a planetary wave structure.  Credit: NASA

Water vapour measurements by satellite uncover a planetary wave structure. Credit: NASA

Sometimes, when water vapor content in the Earth’s atmosphere is measured from above strange structures appear at around 30 or 60 degrees latitude North or South.

Looking at the picture, you will find the structures to resemble two-dimensional projections of ocean waves. In one respect, they actually are similar. They are waves, which can break and form vortices as they are called technically. With an ocean wave, a surfer can ride through these vortices.

The waves depicted here appear because a jet stream is subject to the Coriolis force and begins to meander. Very often high-altitude winds drain the jet stream of the energy required for the meandering. Sometimes they do not. Then the meandering will cause a succession of alternating high-pressure and low-pressure volumes around the jet stream.

Alternating high-pressure and low-pressure volumes which are coupled are called pressure waves, of which these so-called planetary waves are the ones with the largest wavelength of sometimes more than a 1000 km (or around 600 miles).

When I write about coupled volumes, I mean that they influence each other. This being the Earth’s atmosphere, the interaction between adjacent low- and high-pressure volumes may not be described easily, and mathematical equations have to be applied to the real-life situation. Click here, for a simplified homework calculation from a course at the University of Oregon.

Of course, these calculations can be made arbitrarily complex, and physicists may crank up the level difficulty quite a bit. We will come back to this, but let me mention already now, that mathematics are a wonderful tool, if applied on the background of proper expectations.


2015-03-14 18.46.20Alexander is a physicist, teacher and science communicator who is currently working at the Norwegian Centre for Space-related Education at Andøya Space Center in Norway. Even though, in his case, work and play do overlap, the content on this webpage is entirely private. You can follow Alexander on Twitter, Facebook and Google +.

 

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