Many of the most damaging and life threatening types of weather, such as torrential rains and severe thunderstorms, begin quickly, strike suddenly, and disappear rapidly, destroying small regions while leaving neighboring areas untouched. Conventional (普通) computer models of the atmosphere have limited value in predicting short-lived local storms because the available weather data are generally not detailed enough to allow computers to see clearly the small atmospheric changes that come before these storms. In most nations, for example, weather-balloon observations are taken just once every twelve hours at location typically separated by hundreds of miles. With such limited data, conventional forecasting models do a much better job predicting general weather conditions over large regions than they do forecasting specific local s.
Until recently, the observation-intensive approach needed for accurate, very short range forecasts, or "nowcasts", was impracticable. The cost of equipping and operating many thousands of conventional weather stations was extremely high, and the difficulties involved in rapidly collecting and processing the raw weather data from such a network were beyond overcoming. Fortunately, scientific and technological advance have overcome most of these problems. Radar systems, automated weather instruments, and satellites are all capable of detailed, nearly continuous observations over large regions at a relatively low cost. Communication satellites can transmit data around the world cheaply and immediately, and modem computers can quickly compile and yze this large volume of weather information. Meteorologists (气象学家) and computer scientists now work together to design computer programs and video equipment capable of transforming raw weather data into words, symbols, and vivid graphic displays that forecasters can interpret easily and quickly. As meteorologists have begun using these new technologies in weather forecasting offices, nowcasting is becoming a reality. Things have been changed by the following EXCEPT ______