In the mid-1930s, with the situation in Europe deteriorating rapidly, the director of Britain’s Air Ministry asked Robert Watson-Watt, superintendent of a radio department at England’s National Physical Laboratory, if there was some way to develop a “death ray” that could shoot down aircraft from a distance. The request resulted not in a death beam but in Watson-Watt’s report “Detection and Location of Aircraft by Radio Methods,” which detailed how certain radio waves might be reflected off aircraft and back to the origin point, revealing the planes’ positions.
Watson-Watt’s invention came to be called Radio Detection and Ranging, or radar, and by the beginning of World War II, the coast of England bristled with radar installations. On those early radar screens, radar echoes from large storms would often obscure the images of approaching planes, and large areas of rain would show up as a green fog. By the end of the war, both the Axis and the Allies would depend on radar just as military forces do today.
There must have been an “aha!” moment when meteorologists first saw those radar echoes. After all the guesswork and ground observations used in the past to track weather systems, here was a system that could actually see the weather systems in motion. Radar was a forecaster’s dream come true. After the war, surplus radar systems were pressed into service by the US Weather Bureau to track weather systems. Further research led to more powerful radars, which the bureau began to install along the coastline in 1954 as part of a hurricane early warning system.
The surplus radar units served their purpose, but as the years went by and the systems aged, spare parts became scarce and breakdowns were more frequent. Additionally, the old radar units were unable to detect developing tornadoes or accurately measure rainfall amounts. It became obvious that something new was needed.
In the 1960s, the US Weather Service began experimenting with Doppler radar, which was a big improvement over the older types. During the late 1970s and early 1980s, Doppler radar began to appear at a few television stations, and around that time NOAA and the Department of Commerce joined forces to produce a next-generation radar system—NEXRAD—that would greatly improve severe weather forecasting. NEXRAD used the Doppler effect to spot rotating weather systems that often indicate a tornado is forming.
Named after the nineteenth-century Austrian mathematician and physicist Christian Andreas Doppler, the Doppler effect describes the change in wavelengths (of sound or light) between two objects as a result of motion. For example, the change in sound as a motorcycle approaches, then passes, a stationary observer demonstrates the Doppler effect.
Light waves were much too fast to experiment with in the nineteenth century, so in 1845 Christoph Hendrik Diederik Buys Ballot, a recent graduate of the Netherlands’ Utrecht University, set out to debunk Doppler’s theory with a real-world test using sound waves. Ballot put a group of trumpeters on a train that would pass by a group of listeners. As the train passed with the trumpeters blasting away, the listeners heard the din rising in frequency as the train approached and then dropping as it moved away. On the train, however, the trumpets’ pitch stayed the same.
Instead of refuting Doppler’s theory, Ballot’s experiment proved that the frequency of light or sound depends on the speed of an object’s movement in relation to the viewer. The word “frequency” refers to how fast the peaks and valleys of a sound or light wave are moving past an observer. Let’s say you’re standing at a station watching an approaching train. When the engineer sounds the horn, the pitch will seem to rise because the speed of the moving train as it comes toward you is added to the speed of those sound waves, meaning the sound waves are pressed closer and closer together as they arrive at your ear. Once the train passes, the distance between the wave peaks is farther apart because the speed of the train is subtracted from the speed of the sound waves, and so the horn seems to shift to a lower pitch.
In Doppler radar, pulses of microwave radiation are used instead of sound waves, but the effect is the same. When a Doppler beam is aimed at a storm, the echoes that return are coded by color: areas of precipitation moving toward the radar are shown in one color, while areas moving away from the radar are displayed in another. The National Weather Service’s Weather Surveillance Radar 1988 Doppler (WSR-88D) uses green to indicate rain that’s approaching the radar, and paints receding showers in red. When the radar sees green and red in close proximity, it’s a sign of rotation within the storm that can indicate a developing tornado.
Doppler radar can identify gust fronts and microbursts as well, something conventional weather radar can’t do. Peering deep within storms, the Doppler beam can identify mesocyclones (rotating air masses inside a thunderstorm) swirling inside. This allows forecasters to discover a region that may spawn a tornado and give them much more time to alert those in its path. Because about 30 percent of mesocyclones generate tornadoes and 95 percent produce severe weather, Doppler radar has become a welcome addition to a forecaster’s arsenal.