| Weather Radars: Measuring Rain & Detecting Storms Stanley Changnon |
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Initially the ISWS radar weather program focused on developing techniques to measure rainfall accurately using World War II surplus radars, but some scientific and technological limitations needed to be overcome. For example, lack of knowledge about raindrop size distributions, a key factor in the rain estimation equation, led ISWS scientists to pioneer the design and development of the first movie camera system to photograph raindrops in 1956-58. Field operations in Illinois provided information about raindrop distributions under various rainfall conditions such as: steady rainfall, showers, and storms. This effort led to accurate rain measurements, and to federal funding that facilitated ISWS operation of these cameras at many other sites around the world. This led to the development of a global climatology of raindrop distributions for use in radars to measure rainfall anywhere in the world by 1965.
Both radars had Doppler capability that made it possible to measure the direction and speed of airborne particles, a breakthrough for assessing in-cloud development of tornado funnels and strong straight-line winds. Use of these radars in studies at St. Louis and Chicago helped ISWS scientists define the effects of large cities on rain development, resulting in more rain and storms locally, another scientific breakthrough in the 1970s. Because they could be used to assess changes in water and ice particles in clouds, the radars were also invaluable during the Survey's cloud-seeding experiments designed to determine effective methods to increase rain in Illinois in the 1980s. Ultimately, the CHILL radar served as the prototype used in the 1980s by the National Weather Service in designing 145 radars built for current use across the nation. The HOT radar was taken to the southern Illinois hills near Anna in 1980 for use in a NSF-sponsored field study aimed at defining the local factors that led to 1 to 2 inches more summer rainfall in the hill area than in other surrounding flatland areas. The CHILL radar was located in lower Michigan during three winters during the 1980s as part of a NSF-funded field study aimed at defining the process whereby Lake Michigan created more precipitation over the lake and more snowfall over adjacent land areas. The CHILL was also used during the 1980s to detect the movement of airborne insect pests coming into Illinois, a study with scientists of the Natural History Survey. The Survey's weather-radar program was terminated in 1990. Answers to the weather questions that depended on radar measurements had been obtained.
The HOT and CHILL radars were given to other research institutions since they had fulfilled their duties at the Illinois State Water Survey.
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