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Timing of Bird Migration

   After sunset the surface of the earth cools, and columns of warm, rising air dissipate. As a result turbulence aloft decreases and the atmosphere becomes increasingly more stable. In terms of energy expenditure, birds can maximize their efficiency by flying in a less turbulent atmosphere. Many species of birds migrate at night to reap these energy-saving benefits.
   On a typical evening during spring and fall migration, birds often depart 30 to 45 minutes after sunset. This departure is known as exodus. Approximately 2 to 3 hours after exodus, numbers of birds peak in the atmosphere. The following base reflectivity images from GSP Greer, SC on the evening of 12 September, 1999 show an exodus occurring after sunset.
   In first frame notice the reddish-orange spike representing local sunset as well as extremely light reflectivity (-12 to -16 dBZ) and ground clutter close to the radar station. Very few targets are present in the atmosphere. However, in the second frame see that the number of reflectors detected in the radar scan has increased dramatically, reaching a moderate 12 dBZ in some areas or 81 birds per cubic km. These targets are mostly birds that have entered the radar beam in the time since sunset.

Initiation of exodus at Greer, SC
 
dBZBirds
/km3
ND
-28
-24
-20
-16
-12
-8
-4
0
458
865
1281
16123
20227
24489
281148
NEXRAD bird exodus
(C) 1999 weatherTAP.com
Radial Vel 124nm
Elev=0.5 deg
1 km²/pixel2
Greer SC
34.88N 82.22W
09/12/99 23:34
to
09/13/99 00:52
Clear Mode
VCP 32
Max: 47 dBz

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