Again winds are calm and birds are aloft tonight. Radar beams for weather go out in a straight line which rotates, forming a disc or shallow cone. Because of the Earth's curvature, the beam samples higher and higher altitudes farther from the radar station. After sunset migrating birds fly up to their cruising altitude intercepting that disc or cone thickly enought to show as up as a fuzzy or even solid area, but far enough away from the station the radar is sampling higher than the birds and it fades out. The result is an unmoving circular fuzzy cloud centered on the radar station, bigger and thicker on heavy migration nights like this. If you watch as it forms in the evening it seems to blossom, while in the morning it shrinks and retracts as the birds descent. Using doppler radar show the birds are moving, and this is a circular sample of a vast cloud layer.
--Dave Nutter -- Cayugabirds-L List Info: http://www.NortheastBirding.com/CayugabirdsWELCOME http://www.NortheastBirding.com/CayugabirdsRULES http://www.NortheastBirding.com/CayugabirdsSubscribeConfigurationLeave.htm ARCHIVES: 1) http://www.mail-archive.com/[email protected]/maillist.html 2) http://www.surfbirds.com/birdingmail/Group/Cayugabirds 3) http://birdingonthe.net/mailinglists/CAYU.html Please submit your observations to eBird: http://ebird.org/content/ebird/ --
