Observer Comments

06:20 Sat May 25, 2019

Atmospheric Long Waves and Short Waves; not the hand kind

Often times in our forecasts there a couple terms we use that describe a wave pattern in the atmosphere. Normally, these terms are associated with low-pressure and high-pressure systems. We will say or write descriptions like; “the ridge of the high pressure will crest over the region early this morning” or “a shortwave trough will pass through the area and” bring some sort of weather phenomena to the region. What we are talking about are the different types of waves in the atmosphere. I grew up on the southern coast of Rhode Island and was fortunate enough to spend a lot of time at the beach and on the ocean, so I like to think about atmospheric waves as if they are ocean waves and waves that crash on to the beach. They are quite similar and quite different as well, they have different boundary conditions, forces acting on them, and are made of different substances, but both mediums behave the same and follow the same laws of physics.

There are two types of waves, generally speaking; long waves and short waves. Long waves are essentially the areas of high pressure and low pressure and short waves are waves that occur within a long wave feature. To relate to that, let think about the waves that crash onto the beach. Think of a long wave as a set of waves and the short waves to be the individual waves that make up the set (usually 3). Sometimes, there are not really ‘sets’ of waves per say, and they seem to be evenly spaced, but more often than not, they follow that general formation. I digress though; the shortwaves are riding the long wave in the same fashion that a shortwave in the atmosphere rides a longwave. Pictures always help so let us look at a map of the 500-mbar pressure level over the Continental US (see below).

This is a good example of some long wave features. Over the Mid-West, you can observe the ridge or crest of wave and the trough of said wave over New England. Each line you see on this map is called an Isobar, which is a line of constant pressure. The numbers along the isobars indicate the height of the 500-mbar pressure surface in decameters, so 588 decameters = 5880 meters. Another feature you may notice is that the wind barbs are mostly aligned with the isobars. The shortwaves are better seen closer to the surface (also seen below).

This map is the same as the previous one except it is centered over the Northeast and is at 850-mbars, about 4000 meters below the 500-mbar line. Here, the shortwave can be seen along the blue line, which was drawn by me and is oriented perpendicular to the shortwave along the isobars. The squiggle or kink in the isobars indicates the shortwave. It is ‘riding the larger, long wave and rotating around the center of the Low in a counter-clockwise fashion. These shortwaves usually are associated with areas of uplift and cause more intense weather within the system. 

I hope this helps anyone who was unfamiliar with the terminology, if it did not, and or you have more questions, or if it sparked some new questions, reach out to us on social media and ask us. A good place to ask is on our FB lives where we can verbally discuss it and maybe show some live maps!

Smiles,

 


Jay Broccolo, Weather Observer/Meteorologist
  
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