Page created and maintained by Roger Edwards, Storm Prediction Center
Radar receivers in the national network of Doppler radars (WSR-88D) can detect sunsets. Here, in a composite of reflectivity images across the east-central U.S., seven sites did so. A radar antenna was located at each location numbered in yellow, with the sun at a low enough angle to the ground to be intercepted head-on by the radar beam.
Since it was winter -- only a few days after solstice -- the sun sets in the southwest in these areas; so the sunset echoes appear as blue line segments southwest of each radar site. The tilt of the earth causes the sun to set later at more southerly latitudes in winter, which is why the sunsets show up farther east as you look from Minnesota to Tennessee. By contrast, in the summer, the sunset echoes would be facing northwest; and they would appear farther west as you go southward. At the start of spring and fall, each sunset echo would face east-west, and the whole series of them would be arranged in a north-south band. In nationwide radar reflectivity loops, the sunset can often be tracked east-west across the country by following the appearance of the sun echoes over several hours.
[The large area of blues and greens over Iowa was a belt of snow. Some radars failed to show sunsets because of precipitation and dense cloud cover obscuring the sun.]
These radars can pick up the sun as an echo because the sun transmits radiation at all wavelengths. This includes the 10-cm long microwaves sent and received by the radars. When the sun is low in the sky, and a radar beam moves across it, the sun's 10-cm microwave emissions are strong enough to register in the radar receiver. Unfiltered, these echoes pass through the computer software processing and show up on radar displays. They are easily distinguished from precipitation echoes by their character: occurring only around sunrise or sunset, shaped straight, nearly uniform in intensity, elongated with the radar beam and facing the direction of the sun.