Imagine an accident in which a nuclear power plant releases radioactive gas. The cloud starts moving with the wind. Clearly, the authorities will want to evacuate anyone in its path, but what is that path? Local wind information is meaningless without information about terrain, a mountain range or series of valleys can divert both wind and gas in unpredictable directions. To make 'downwind' a useful term, scientists at Lawrence Livermore National Laboratory have put the United States on a computer, the entire United States-every hill, every valley, every mile of seashore. Within minutes of a disaster, they can give meteorologists a context for weather data, and thus the ability to predict how toxic gases might spread. The database for this computer map is a series of altitude measurements, made over many years by the Defense Department and the U.S. Geological Survey. They represent the height above sea level of over a billion separate points--a grid of points 200 feet apart, spanning the country. Armed with these data, plus a program that manipulates them, a Cray-1 computer can produce an image of any piece of terrain, seen from any angle, illuminated by an imaginary sun at any time of day placing the 'observer' at any altitude from zero to 40,000 feet. 'We use a technique called ray tracing,' says Patrick Weidhaas, one of the Livermore computer scientists who wrote the program. The computer is told where the observer is. The program traces an imaginary ray from there outward until it 'intersects' with one of the points' of altitude recorded in the machine' s memory. The computer then puts a dot of color at the proper place on the screen, and the program traces another ray. At its highest resolution of 2,000 horizontal and 1,700 vertical dots per picture, the computer has to trace several million rays. Even on the Cray, the most powerful computer in the world, this takes about a minute. Reducing the resolution to 400～300 (a TV screen has 800 x 700) speeds it up to about eight seconds. 'We can't produce a movie simulating flight on the screen in real time,' says Weidhaas. There is a way around the problem: Two movies have been made using still pictures generated by the computer as individual frames. 'The results were impressive,' he says, 'but it was cumbersome to do. At twenty-four frames per second, it takes fourteen hundred separate computer images to make a one-minute film.' Another limitation. The computer can access only enough memory to cover a 15-mile-square area. An 'observer' high up will see blank spaces beyond those limits. Weidhaas wants to add information about what overlies the terrain-cities, vegetation, roads, and so on. 'Making the image as realistic as possible will make our advice more effective,' he says, 'and might lead to uses we haven't thought of yet.' As used in the first paragraph, terrain most clearly means ______.