National Weather Service Doppler radar has indicated a severe thunderstorm …” When you hear or read this statement, it is advantageous to be forewarned of a storm's location, direction, and speed. But the most effective reports for severe weather preparation include information about damage or ground conditions—details that are furnished by volunteer weather spotters in the NWS's SKYWARN® system.
Doppler radar, based on the Doppler effect, is a curious creature. After an antenna transmits radio waves toward rainfall at consistent speeds, the waves bounce off raindrops and return to the antenna. The radar system measures how those bounces change the speeds of those transmitted signals, and so they calculate the distance and direction of the rain. The challenge is that, due to the upward tilt of these radar beams in relation to the curvature of the earth's surface (a tilt that begins at 0.5 degrees), Doppler radar can only analyze data at a minimum of 1,000 feet above ground level. So while it is helpful to have colors on a weather map, indicating predicted or actual severity of a severe weather system, those colors cannot tell you what is actually happening on the ground.
GREG WAXBERG is a writer and magazine editor for The Pingry School and an award-winning freelance writer.