National Weather Service
February 09, 2009
The radar scan above from the Staunton, Virginia radar site shows several interesting peculiarities, none of which are related to precipitation. First, note the blue spike at right center. This has nothing to do with the band of precipitation it seems to be pointing toward, rather it's caused by the Sun. This scan was made just as the Sun was rising on this midsummer morn (about 6:00 a.m. Eastern Daylight Time). On occasion, the rising or setting sun will appear as a spike on National Weather Service radar screens -- in the direction of the Sun. The spike featured here is pointing toward the northeast and not the east because this is the direction of sunrise about a month after the summer solstice, at this latitude (39 degrees north). Whenever the Sun is very close to the horizon, its electromagnetic energy can confound the Doppler radar receiver. Radar algorithms designed to detect rainfall sometimes display this energy as a "spike."
Secondly, note the blue/green blotch around the Nation's Capital. Again, this isn't a rainfall signal. The radar is picking up the clutter of densely packed buildings and concrete in and about Washington, D.C.
Thirdly, note the blue ring at bottom center. An odd precipitation artifact indeed! Actually, this ring is a result of birds -- birds leaving their roost at the crack of dawn. Radar can misinterpret tightly packed flocks of birds, as well as groupings of insects and bats, as rainfall rings. Click on image for a larger view. Radar scan from July 24, 2008.