WEATHER FORECASTING

the prediction of the weather through application of the principles of physics, supplemented by a variety of statistical and empirical techniques. In addition to predictions of atmospheric phenomena themselves, weather forecasting includes predictions of changes on the Earth's surface caused by atmospheric conditions-e.g., snow and ice cover, storm tides, and floods. the prognostication of the weather through application of the principles of physics, supplemented by a variety of statistical and empirical techniques. In addition to predictions of atmospheric phenomena themselves, weather forecasting includes predictions of changes on the Earth's surface caused by atmospheric conditions-e.g., snow and ice cover, storm tides, and floods. Scientific weather forecasting developed only after instruments for measuring atmospheric temperature, pressure, and humidity became available. The invention in the 17th century of the thermometer and the barometer, along with improvements of the hygrometer, permitted the measurement of these three basic elements of the atmosphere. Correlations were made between such measurements and those of other aspects of local weather, such as wind speed and precipitation. Initially, however, the number of observations were too small and the variety in observational techniques too great to allow accurate patterns of atmospheric conditions to be developed. Furthermore, the study of such patterns depends on a rapid exchange of data between many observing stations over a large area, and this was impossible before the development of the electric telegraph in the late 1830s. From this time on, networks of weather stations on land increased rapidly, although the development of oceanic stations lagged behind for many decades. National and international organizations were established by the late 19th century to standardize the recording of weather and to oversee the first attempts at forecasting, initially largely for ocean shipping. In recent years, advances in observational apparatus and techniques-most notably the use of radiosondes carried by weather balloons, radar, and Earth-orbiting satellites and high-altitude aircraft equipped with special sensors-have helped to revolutionize weather forecasting. Another major breakthrough has been the generation of numerical weather prediction (NWP) models, made possible by the development of electronic digital computers. These NWP models consist of mathematical representations of the physical conservation laws of motion, mass, heat, and moisture in the form of nonlinear, partial differential equations, which enable weather forecasters to approximate relations for solution on a three-dimensional grid mesh and integrate them forward in time. This grid mesh corresponds to the area for which future weather is to be extrapolated; it can represent a specific region, such as western North America, or the entire Earth. The integration of such atmospheric models with high-speed supercomputers has made it possible to predict temperature anomalies and pressure fields and, to a lesser degree, precipitation about five to seven days in advance. Public weather forecasts beyond 12 hours are virtually all based on these highly complex mathematical models. For prognostications of a shorter period, a method known as "nowcasting" is frequently employed. In this technique, satellite and radar observations of local atmospheric conditions are processed by computers in order to project the details of future weather within a limited area. Additional reading Richard A. Anthes et al., The Atmosphere, 3rd ed. (1981), contains general discussions. The history of weather forecasting is recounted in Gisela Kutzbach, The Thermal Theory of Cyclones: A History of Meteorological Thought in the Nineteenth Century (1979); A. Kh. Khrgian, Meteorology: A Historical Survey, 2nd ed. rev. (1970; originally published in Russian, 2nd ed. rev., 1959); Patrick Hughes, "American Weather Services," Weatherwise, 33(3):100-111 (June 1980); and Frederick G. Shuman, "Numerical Weather Prediction," Bulletin of the American Meteorological Society, 59(1):5-17 (January 1978). Instruments used in weather analysis are examined in the classic texts by W.E. Knowles Middleton and Athelstan F. Spilhaus, Meteorological Instruments, 3rd ed. rev. (1953, reprinted 1960); and W.E. Knowles Middleton, History of the Barometer (1964), A History of the Thermometer and Its Uses in Meteorology (1966), and Invention of the Meteorological Instruments (1969); as well as in Leo J. Fritschen and Lloyd W. Gay, Environmental Instrumentation (1979), with coverage limited to measurements at the Earth's surface; Stuart G. Bigler, "Radar: A Short History," Weatherwise, 34(4):158-163 (August 1981); Louis J. Battan, Radar Observation of the Atmosphere, rev. ed. (1973); R.S. Scorer, Cloud Investigation by Satellite (1986); Vincent J. Oliver, "A Primer: Using Satellites to Study the Weather," Weatherwise, 34(4):164-170 (August 1981); Eric C. Barrett and David W. Martin, The Use of Satellite Data in Rainfall Monitoring (1981); and Eric C. Barrett, Climatology from Satellites (1974). A good overview of forecasting is found in Lance F. Bosart, "Weather Forecasting," ch. 4 in David D. Houghton (ed.), Handbook of Applied Meteorology (1985), pp. 205-279. Methods and problems of forecasting are presented in Jaromir Nemec, Hydrological Forecasting: Design and Operation of Hydrological Forecasting Systems (1986); D.M. Burridge and E. Klln (eds.), Problems and Prospects in Long and Medium Range Weather Forecasting (1984); K.A. Browning (ed.), Nowcasting (1982); and three articles from Bulletin of the American Meteorological Society: Donald L. Gilman, "Long-Range Forecasting: The Present and the Future," 66(2):159-164 (February 1985); Jerome Namias, "Remarks on the Potential for Long-Range Forecasting," 66(2):165-173 (February 1985); and L. Bengtsson, "Medium-Range Forecasting-The Experience at ECMWF," 66(9):1133-46 (September 1985). John J. Cahir History of weather forecasting Early measurements and ideas The Greek philosophers had much to say about meteorology, and many who subsequently engaged in weather forecasting no doubt made use of their ideas. Unfortunately, they probably made many bad forecasts, because Aristotle, who was the most influential, did not believe that wind is air in motion. He did believe, however, that west winds are cold because they blow from the sunset. The scientific study of meteorology did not develop until measuring instruments became available. Its beginning is commonly associated with the invention of the mercury barometer by Evangelista Torricelli, an Italian physicist-mathematician, in the mid-17th century and the nearly concurrent development of a reliable thermometer. (Galileo had constructed an elementary form of gas thermometer in 1607, but it was defective; the efforts of many others finally resulted in a reasonably accurate liquid-in-glass device.) A succession of notable achievements by chemists and physicists of the 17th and 18th centuries contributed significantly to meteorological research. The formulation of the laws of gas pressure, temperature, and density by Robert Boyle and Jacques-Alexandre-Csar Charles, the development of calculus by Isaac Newton and Gottfried Wilhelm Leibniz, the development of the law of partial pressures of mixed gases by John Dalton, and the formulation of the doctrine of latent heat (i.e., heat release by condensation or freezing) by Joseph Black are just a few of the major scientific breakthroughs of the period that made it possible to measure and better understand theretofore unknown aspects of the atmosphere and its behaviour. During the 19th century, all of these brilliant ideas began to produce results in terms of useful weather forecasts. The emergence of synoptic forecasting methods Analysis of synoptic weather reports An observant person who has learned nature's signs can interpret the appearance of the sky, the wind, and other local effects and "foretell the weather." A scientist can use instruments at one location to do so even more effectively. The modern approach to weather forecasting, however, can only be realized when many such observations are exchanged quickly by experts at various weather stations and entered on a synoptic weather map to depict the patterns of pressure, wind, temperature, clouds, and precipitation at a specific time. Such a rapid exchange of weather data became feasible with the development of the electric telegraph in 1837 by Samuel F.B. Morse of the United States. By 1849 Joseph Henry of the Smithsonian Institution in Washington, D.C., was plotting daily weather maps based on telegraphic reports, and in 1869 Cleveland Abbe at the Cincinnati Observatory began to provide regular weather forecasts using data received telegraphically. Synoptic weather maps resolved one of the great controversies of meteorology-namely, the rotary storm dispute. By the early decades of the 19th century, it was known that storms were associated with low barometric readings, but the relation of the winds to low-pressure systems, called cyclones, remained unrecognized. William Redfield, a self-taught meteorologist from Middletown, Conn., noticed the pattern of fallen trees after a New England hurricane and suggested in 1831 that the wind flow was a rotary counterclockwise circulation around the centre of lowest pressure. The American meteorologist James P. Espy subsequently proposed in his Philosophy of Storms (1841) that air would flow toward the regions of lowest pressure and then would be forced upward, causing clouds and precipitation. Both Redfield and Espy proved to be right. The air does spin around the cyclone, as Redfield believed, while the layers close to the ground flow inward and upward as well. The net result is a rotational wind circulation that is slightly modified at the Earth's surface to produce inflow toward the storm centre, just as Espy had proposed. Further, the inflow is associated with clouds and precipitation in regions of low pressure, though that is not the only cause of clouds there. In Europe the writings of Heinrich Dove, a Polish scientist who directed the Prussian Meteorological Institute, greatly influenced views concerning wind behaviour in storms. Unlike the Americans, Dove did not focus on the pattern of the winds around the storm but rather on how the wind should change at one place as a storm passed. It was many years before his followers understood the complexity of the possible changes. Principles and methodology of weather forecasting Short-range forecasting Objective predictions When people wait under a shelter for a downpour to end, they are making a very-short-range weather forecast. They are assuming, based on past experience, that such hard rain usually does not last very long. In short-term predictions the challenge for the forecaster is to improve on what the layperson can do. For years the type of situation represented in the above example proved particularly vexing for forecasters, but since the mid-1980s they have been developing a method called nowcasting to meet precisely this sort of challenge. In this method, radar and satellite observations of local atmospheric conditions are processed and displayed rapidly by computers to project weather several hours in advance. The U.S. National Oceanic and Atmospheric Administration operates a facility known as PROFS (Program for Regional Observing and Forecasting Services) in Boulder, Colo., specially equipped for nowcasting. Meteorologists can make somewhat longer-term forecasts (those for six, 12, 24, or even 48 hours) with considerable skill because they are able to measure and predict atmospheric conditions for large areas by computer. Using models that apply their accumulated expert knowledge quickly, accurately, and in a statistically valid form, meteorologists are now capable of making forecasts objectively (see above). As a consequence, the same results are produced time after time from the same data inputs, with all analysis accomplished mathematically. Unlike the prognostications of the past made with subjective methods, objective forecasts are consistent and can be studied, reevaluated, and improved. Another technique for objective short-range forecasting is called MOS (for Model Output Statistics). Conceived by Harry R. Glahn and D.A. Lowry of the U.S. National Weather Service, this method involves the use of data relating to past weather phenomena and developments to extrapolate the values of certain weather elements, usually for a specific location and time period. It overcomes the weaknesses of numerical models by developing statistical relations between model forecasts and observed weather. These relations are then used to translate the model forecasts directly to specific weather forecasts. For example, a numerical model might not predict the occurrence of surface winds at all, and whatever winds it did predict might always be too strong. MOS relations can automatically correct for errors in wind speed and produce quite accurate forecasts of wind occurrence at a specific point, such as Heathrow Airport near London. As long as numerical weather prediction models are imperfect, there may be many uses for the MOS technique. Predictive skills and procedures Short-range weather forecasts generally tend to lose accuracy as forecasters attempt to look farther ahead in time. Predictive skill is greatest for periods of about 12 hours and is still quite substantial for 48-hour predictions. An increasingly important group of short-range forecasts are economically motivated. Their reliability is determined in the marketplace by the economic gains they produce (or the losses they avert). Weather warnings are a special kind of short-range forecast; the protection of human life is the forecaster's greatest challenge and source of pride. The first national weather forecasting service in the United States (the predecessor of the Weather Bureau) was in fact formed, in 1870, in response to the need for storm warnings on the Great Lakes. Increase Lapham of Milwaukee urged Congress to take action to reduce the loss of hundreds of lives incurred each year by Great Lakes shipping during the 1860s. The effectiveness of the warnings and other forecasts assured the future of the American public weather service. Weather warnings are issued by government and military organizations throughout the world for all kinds of threatening weather events: tropical storms variously called hurricanes, typhoons, or tropical cyclones, depending on location (see above Major forms of weather disturbances); great oceanic gales outside the tropics spanning hundreds of kilometres and at times packing winds comparable to those of tropical storms; and, on land, flash floods, high winds, fog, blizzards, ice, and snowstorms. A particular effort is made to warn of hail, lightning, and wind gusts associated with severe thunderstorms, sometimes called severe local storms (SELS) or simply severe weather. Forecasts and warnings also are made for tornadoes, those intense, rotating windstorms that represent the most violent end of the weather scale (see above). Destruction of property and the risk of injury and death are extremely high in the path of a tornado, especially in the case of the largest systems (sometimes called maxi-tornadoes). Because tornadoes are so uniquely life-threatening and because they are so common in various regions of the United States, the National Weather Service operates a National Severe Storms Forecasting Center (NSSFC) in Kansas City, Mo., where SELS forecasters survey the atmosphere for the conditions that can spawn tornadoes or severe thunderstorms. This group of SELS forecasters, assembled in 1952, monitors temperature and water vapour in an effort to identify the warm, moist regions where thunderstorms may form and studies maps of pressure and winds to find regions where the storms may organize into mesoscale structures. The group also monitors jet streams and dry air aloft that can combine to distort ordinary thunderstorms into rare rotating ones with tilted chimneys of upward rushing air that, because of the tilt, are unimpeded by heavy falling rain. These high-speed updrafts can quickly transport vast quantities of moisture to the cold upper regions of the storms, thereby promoting the formation of large hailstones. The hail and rain drag down air from aloft to complete a circuit of violent, cooperating updrafts and downdrafts. By correctly anticipating such conditions, SELS forecasters are able to provide time for the mobilization of special observing networks and personnel. If the storms actually develop, specific warnings are issued based on direct observations. This two-step process consists of the tornado or severe thunderstorm watch, which is the forecast prepared by the SELS forecaster, and the warning, which is usually released by a local observing facility. The watch may be issued when the skies are clear, and it usually covers a number of counties. It alerts the affected area to the threat but does not attempt to pinpoint which communities will be affected. By contrast, the warning is very specific to a locality and calls for immediate action. Radar of various types can be used to detect the large hailstones, the heavy load of raindrops, the relatively clear region of rapid updraft, and even the rotation in a tornado. These indicators, or an actual sighting, often trigger the tornado warning. In effect, a warning is a specific statement that danger is imminent, whereas a watch is a forecast that warnings may be necessary later in a given region.