[CALBIRDS] RE: Bird Migration Radar Images
- In a message dated 5/5/2004 12:48:18 AM Mountain Standard Time,
These could be
caused by one of three things, clouds, birds or bats.
Or bugs. NEXRAD is very sensitive and can even show reflectivity from smoke
(range / forest fires). It's great at picking up evening flights of bugs
(moths, etc.), especially in the Southeast at night. You'll see a nice "bloom"
around the radar sites, usually going in the direction of the prevailing winds.
Check the winds -- if the direction of movement shown on radar (radial
velocity) is against the prevailing winds, then it's probably something mobile
enough to proceed against the breeze (birds) and not most bugs (which generally
flow with the prevaling winds, depending on strength).
During the daytime, flights of birds can be seen on radar as a "donut" shape
around the radar shape -- an orangish circle or crescent, depending on whether
or not the bird "frontal migration" is just approaching the radar or whether
it is all around the site. This is because the birds generally fly at a given
altitude range, and since the radar shoots at an angle from the ground, it
will only start picking up reflectivity from birds a certain distance away from
the radar site (corresponding to the elevation of the birds). At least this
is the case over the Gulf of Mexico and slightly inland. I'm not sure how
birds migrate once far inland...
Evening flights of moths, (or the evening departure of birds from the ground,
gaining altitude) show up as a multi-colored "bloom" instead of a donut,
because of the reflectivity at altitudes from ground-level to much higher. Again,
prevailing breezes can provide a clue as to whether or not these are birds or
It's fun stuff to watch, but can be tricky to interpret correctly. It's
easier to note arrivals off the Gulf of Mexico in spring than it is to observe and
interpret departures in fall!
Stacy Jon Peterson
4442 Sijan St. Apt. A
Mtn Home AFB, ID 83648
USDA zone 6a; Sunset zone 3, AHS Heat zone 7
Race for the Rain Forest!
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