TEXT C Many of the most damaging
and life-threatening types of weather—torrential rains, severe thunderstorms,
and tornadoes—begin quickly, strike suddenly, and dissipate rapidly, devastating
small regions while leaving neighboring areas untouched. One such event, a
tornado, stuck the northeastern section of Edmonton, Alterta, in July 1987.
Total damages from the tornado exceeded $ 250 million, the highest ever for any
Canadian storm. Conventional computer models of the atmosphere have limited
value in predicting short-lived local storms like the Edmonton tornado, because
the available weather data are generally not detailed enough to allow computers
to discern the subtle atmospheric changes that precede these storms. In most
national weather reports, for example, weather balloon observations are taken
just once every twelve hours at locations 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 events. Until recently, the
observation-intensive approach needed for accurate, very short-range forecasts,
or "Nowcasts," was not available. The cost of equipping and operating many
thousands of conventional weather stations was prohibitively high, and the
difficulties involved in rapidly collecting and processing the raw weather data
from such a network were insurmountable. Fortunately, scientific and
technological advances have overcome most of these problems. Radar systems,
automated weather instruments, and satellites are all capable of making
detailed, nearly continuous observations over large regions at a relatively low
cost. Communications satellites can transmit data around the world cheaply and
instantaneously, and modern computers can quickly compile and analyze this large
volume of weather information. Meteorologists 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, Now casting is becoming a
reality. The word "they" in line 11 refers to ______.