From March through mid- November one can view nocturnal bird migration on Nexrad
weather radar in real time or through a 5 day archive.
Birds do not take off (termed “exodus”) until at least 30 minutes after sunset and peak
concentrations do not develop until after 10 p.m. CDT.
You can consult
http://www.wunderground.com/ for the sunrise/sunset times for your area.
Wisconsin has 4 Nexrad stations providing coverage. Begin at
http://www.rap.ucar.edu/weather/radar/
Nexrad images are labeled and archived using Universal Time
-e.g. 00:00 UTC= 7:00 p.m. CDT
From the main page of RAP you can click on “Contiguous US” for a view of the
Nexrad map for the entire 48 states. Click on the individual station symbols for local
coverage. Click again on the reflectivity image (see below, indicates bird densities) and
you will get a velocity detection image which tells you the direction of the radar targets
(see explanation image below). Velocity is generally not usable for birds when the
radar is in precipitation mode- mode is indicated on the reflectivity image label. From
RAP you can get regional composites, a five day archive and you can view loops of
data and watch exodus or dawn fallout develop over several hours.
Look for nightly peak concentrations after 10 or 11 p.m. (0300 or 0400 UTC). Do not
assume that a good night will always result in good dawn fallout of birds in your area.
Check radar about 1 hour before dawn to see the densities still being detected as birds
land.
These graphics illustrate Nexrad concepts and detections of birds.
The detection image as seen on radar is often called a donut shape due to the nature of
the radar beam’s sweep through the bird swarm. In WWII in England where radar
was first used the radar images of migrating birds at night were termed “angels”
because they slowly developed only at night and appeared wispy and ethereal.
Note the hard edged images of ‘weather” vs. the soft donut shape of a bird detection.
The birds over the Lake in this image are ascended –they increase their height at dawn
or pre-dawn when they detect water below as they make decisions to get to land.
Radar modes- reflectivity and bird density. The highest bird reflectivities we see occur
in May and September and approach 35 dbz. This could indicate a density of over
2500 birds per cubic kilometer.
Reflectivity from Nexrad detections available from Weather Underground are in
shades of green- these are not as easy to use as the contrasting colors of RAP. The
small image is the velocity image from the larger reflectivity image.
A typical bird detection from RAP. Note the highest reflectivity of 20-25 dbz (green
detection) This value can be extrapolated throughout the entire region and detection
map as the current density of birds migrating- it is not just at 20-25 dbz within that
small area.
Here’s the velocity for the same image.
Extrapolation of bird densities and traffic rate- one can assume that the entire region
on this map including tan areas between radar detections are covered by a bird swarm
of 15-20 dbz or around 200 birds per cubic kilometer; this can be converted into a
traffic rate (see graphic) -
For those who wish to learn a lot more about what one is seeing the CUROL site
is essentially a mini-course about Nexrad basics and bird detections.
http://virtual.clemson.edu/groups/birdrad/
Another site under construction concentrates on the Western Great Lakes and
explores concepts and some Wisconsin examples more deeply
http://my.execpc.com/CE/5F/idzikoj/nexrad/nexweb/nex.htm
John Idzikowski
idzikoj@uwm.edu
