In 1988, after twenty-four years of development, the U.S. National Weather Service began installing Doppler weather surveillance radar systems at 164 locations around the United States and its overseas territories. Capable of detecting wind movements and precipitation, Doppler radar significantly improved the monitoring and predictability of intermediate-size weather systems.
Centrally located on the North American continent between the Arctic and the Tropics and between two large oceanic basins, the United States, perhaps more than any other nation, routinely experiences severe weather. During the second half of the twentieth century, American scientists significantly improved electronic systems to monitor severe weather and thus enable the protection of life and property. In 1964, the U.S. National Oceanic and Atmospheric Administration (NOAA) commissioned the National Severe Storms Laboratory
Strategically sited in Norman, Oklahoma, a geographic epicenter for severe mesocyclonic or tornadic storms, the NSSL, which is affiliated with the University of Oklahoma, had ready access to the subject of its investigations. The NSSL’s efforts to improve the monitoring and forecasting of severe weather quickly focused on the development of improved radar technology for national deployment. Meteorologists had for some time used the existing Weather Surveillance Radar employed by the U.S. Weather Bureau and other operators since 1957. Little had changed in radar technology since its intensive development during World War II by the American, British, German, and Japanese governments. Radar owed its existence to the ingenuity of many scientists, including the German electronics specialist Hans Hollmann, who immigrated with his family to the United States after the war.
Hollmann’s industrial research and patents for microwave radar components were based on the scientific findings of James Clerk Maxwell and Heinrich Hertz during the later half of the nineteenth century. In 1871, Maxwell published his theory that energy occurs as transverse waves of electricity and magnetism moving at the speed of light. Subsequently, between 1885 and 1889, Hertz, working at the Polytechnical University of Karlsruhe in Germany, experimentally confirmed Maxwell’s theory of electromagnetism by generating, transmitting, and receiving electromagnetic waves between the frequencies of 50 and 500 megahertz.
Hertz elaborated the measurable electromagnetic properties of wavelength, frequency, amplitude, phase, polarization, refraction, reflection, diffraction, and interference, all necessary parameters for monitoring mesoscale (intermediate-size) weather systems using radar. Additionally, it was Hertz who first used reflectors to concentrate wavelengths into beams for transmitting and receiving, a feature essential to the success of electronic communication and imaging systems, including radar. In recognition of the scientist’s outstanding contributions to physics, the International System of Units named the unit of vibrational frequency for Hertz. One hertz is equivalent to one cycle or oscillation per second of any specified electromagnetic wavelength. Shorter wavelengths thus have higher vibrational frequencies, and longer electromagnetic wavelengths have lower frequencies. Shorter wavelengths, or microwaves, proved essential for the development of weather surveillance radar systems.
In Italy, Guglielmo Marconi built on the foundation of his predecessors, especially Hertz, by building the first wireless communications device. In 1895, Marconi’s invention wirelessly broadcast an encoded electric message a distance of 1.5 miles to an accompanying radio receiver. For his significant contributions to the improvement of the capabilities of radiotelegraphy during the first decade of the twentieth century, Marconi shared the 1909 Nobel Prize in Physics
Other researchers used radio and microwave technologies to develop the electronic components of radar (a term first used in 1940, from an American military acronym for “radio detection and ranging”). Understanding the potential applications of radio waves to the prevention of foul-weather shipping collisions, Christian Hülsmeyer patented his invention, the telemobiloscope, in 1904; this device was capable of detecting and crudely imaging distant metallic objects. Oddly, early electronics companies, including Telefunken of Germany, failed to appreciate the potential commercial applications of Hülsmeyer’s ingenious invention.
Subsequently, during the 1920’s and 1930’s, Hollmann and others investigated the commercial use of transmitted and reflected radio waves to detect large metallic objects, especially ships; the reflected waves were returned to an electronic receiver at the broadcast station for processing. With war looming, this time Telefunken paid attention. Hollmann developed many electronic tubes that continuously generated short electromagnetic waves, or microwaves, with ultrahigh frequencies; he secured more than three hundred patents, including seventy-six in the United States, that were useful for radar development.
Hans Hollmann was a committed pacifist, but ironically his patented inventions were exploited by military scientists. In 1935, Robert Alexander Watson-Watt, a meteorologist in the United Kingdom who was originally interested in using radio waves to detect thunderstorms, patented an electronic device similar to that previously developed by Hülsmeyer. Relying on technology invented by Hollmann, Braun, and others, Watson-Watt created a device that could be used to detect aircraft. His patented invention became the basis for England’s radar defenses against German aircraft during the Battle of Britain (1940).
Not unlike weather radar, military radar was effectively used for defensive purposes and the protection of lives and property during World War II. During the war, many radar specialists perceived weather systems to be a troublesome interference to the detection of enemy aircraft. After the war, many enthusiasts realized the value of radar for other uses, especially weather detection. In the United States, those benefits were championed by the U.S. Weather Bureau and its successor, the National Weather Service. In 1964, scientists at the NSSL immediately initiated improvements to the Weather Bureau’s existing weather surveillance radar. They added a black-and-white video display that depicted the detected locations of mesoscale weather systems such as thunderstorms.
Mesoscale weather features, which are from 1 to 200 kilometers (about a half mile to 124 miles) in size, have life spans of from three minutes to six hours as measured by radar. Although satellite imaging proved useful for detecting and monitoring large-scale meteorological features in excess of 2,000 kilometers (more than 1,200 miles) with life spans of many days, radar proved to be a superior technology for allowing meteorologists to monitor smaller, short-lived features such as thunderstorms, tornadoes, and microbursts within its effective 230-kilometer (roughly 140-mile) range.
In addition to making ground observations and measurements, NSSL meteorologists investigated the dynamic movements within scanned thunderstorms, which are detectable as shifts in frequency between transmitted and returned microwave pulses. Consequently, the emerging weather surveillance radar was named after Christian Doppler, the mathematician who first described perceived shifts in color and sound as a consequence of changes in distance between an object of interest and its observer. The radar improvements made by scientists at the NSSL first detected a mesocyclonic tornadic vortex in a severe Oklahoma thunderstorm during May, 1973.
Using horizontally transmitted pulses of microwave frequencies, the researchers coupled the qualities of return echoes to computer programs that transformed measured electromagnetic parameters into useful data regarding the location, movement, and intensity of wind-driven water within the rotating radar beam during its average revolution of five to six minutes. In addition, the NSSL investigators learned that alternated, polarimetric pulses of horizontal and vertical microwaves gave them even greater resolution for measuring and imaging the dynamic structure of a severe storm, including precipitation types, drop sizes, volumes, and rates.
The National Weather Service became convinced of Doppler radar’s ability to revolutionize mesoscale weather system monitoring and forecasting, especially for flash floods, hail, high winds, and tornadoes. In 1988 and extending through the early 1990’s, the National Weather Service, the Federal Aviation Administration, and the Department of Defense collaborated in installing around the United States and in overseas locations the system of Doppler radars known formally as Weather Surveillance Radar, 1988, Doppler, and informally as NEXRAD (for Next-Generation Radar).
NEXRAD technology and its products were made available to private enterprises, including television stations, to enable the forecasting and public broadcasting of information regarding weather systems, especially severe storms. The widespread availability of this information allowed individuals and organizations throughout the United States to make preparations to minimize the damage caused by storms and to take precautions against weather-related emergencies. With NEXRAD system’s success, many other governments—in Europe, Asia, South America, and Australia—deployed similar systems by the end of the twentieth century.
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Bringi, V. N., and V. Chandrasekar. Polarimetric Doppler Weather Radar: Principles and Applications. New York: Cambridge University Press, 2001. Discusses the theory behind and practical applications for polarimetric radar for weather monitoring and forecasting. Detailed, technical work is aimed at readers with backgrounds in science. -
Doviak, Richard J., and Dušan S. Zrnić. Doppler Radar and Weather Observations. 2d ed. San Diego, Calif.: Academic Press, 1993. Scientists with the National Severe Storms Laboratory provide a comprehensive examination of Doppler radar and its advantages for weather surveillance and forecasting. -
Thumm, Manfred. Historical German Contributions to Physics and Applications of Electromagnetic Oscillations and Waves. Karlsruhe, Germany: University of Karlsruhe, 2003. Detailed discussion and illustrated chronology of the development of electronic science, including microwave radar, with a focus on German scientists. The University of Karlsruhe has an extensive history in electromagnetic research, including that of electronics pioneer Heinrich Hertz.
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