[TN-Bird] Re: Passerine migration underway on radar

  • From: Bill Pulliam <bb551@xxxxxxxxxxxxx>
  • To: Tennessee Birds <tn-bird@xxxxxxxxxxxxx>
  • Date: Mon, 8 Mar 2010 23:48:30 -0600

Additional info:

Go to the Nashville weather radar page at:


That large circular blue haze surrounding the radar site out to about  
80 miles or so is composed primarily of night-migrating birds (there  
may be a few bats in the mix as well).  Farther out than the blue  
haze extends the radar beam is shooting too high up in the air to see  
the birds.  At sundown you can see the haze grow; in some places you  
can actually use the loop animation to watch the birds taking off  
from areas of particularly high concentrations.  If you click on the  
link in the left sidebar for "Velocity: Base" you can see what  
direction they are flying.  Red is moving away from the radar, green  
is moving towards the radar, gray is moving at right angles to the  
radar beam.

You can only see the birds well on rain-free nights when the radar is  
operating in the high-sensitivity "clear air" mode.  When there are  
storms around, it is often operating in a less sensitive mode and the  
birds are not visible.  At peak times when the number of birds is  
largest, their density will often be high enough to register as green  
or even yellow in parts of the image.

For a really interesting view, navigate down to the radars in the  
Florida Keys.  At sundown you can use the animation loop to watch the  
birds departing Cuba headed for the U.S. mainland.  The radars along  
the central gulf coast will show the arrival of the trans-gulf  
migrants in late afternoon as well, as a blue haze that advances  
gradually northward.  It is also fascinating to watch the New Orleans  
radar at sundown as the birds get up from the coast and head inland;  
you can often see the front advancing across lake Ponchartrain.   
Closer to home, in West Tennessee at sundown if you look at the  
Memphis radar you will see the outlines of the larger rivers as the  
birds first get up in the evening; these riparian corridors are major  
stopping points for migrants crossing the open coastal plain.

Most commercial outlets for weather radar images (e.g. Weather  
Channel, Weather Underground, etc.) automatically filter out the  
birds as "ground clutter."  You need to go directly to the NOAA pages  
to see the unfiltered images if you want to see the birds.

Bill Pulliam
Hohenwald TN

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