Weather Radar
Radio Detection And Ranging systems, known as radar, were developed in Britain in the 1930s as a defense against German bombing raids. While their military use flourished during World War II, radar was not used commercially until the 1950s. Today, radar has become commonplace. Flight crews routinely use radar tracking features to navigate aircraft to their destinations safely. Radar is also commonly used by meteorologists to track weather patterns. For most television viewers of the weather forecast, the image of a green, circular radar screen--complete with a sweeping arm of light--is a familiar one. Using a high-intensity microwave transmission, meteorologists can detect and follow large masses of precipitation, whether they be rain, snow, or cloud. A beam of pulsed microwaves travels until it hits an obstacle (for meteorological purposes, a cloud or band of precipitation). It is then reflected back to the source, where it is received by a radar antenna. By measuring the time taken for the signal to reach the obstacle and return, its distance can be easily calculated. With thousands of pulses emitting and returning, a two-dimensional image of the weather formation is displayed on a cathode-ray tube, showing its precise position.
The first use of radar to study the atmosphere occurred in 1945, when Major Harry Wexler of the U.S. Air Weather Service published the first radar image of a hurricane.
After World War II, the U.S. Navy gave the civilian Weather Bureau 25 surplus radars, and the Bureau began to establish a radar network for tracking tornado movements in the Midwest. Later in the 1950s, radar networks were extended to the hurricane prone eastern seaboard of the United States. In the mid fifties, the Weather Bureau began to install a few Doppler radars. A more elaborate version of radar tracking, Doppler radar uses a continuous signal rather than a pulsed wave. Doppler radar can determine both the direction and velocity of wind patterns, as well as areas of precipitation. Doppler radar measures the shift in frequency caused by a moving particle. If the returning frequency is higher than when transmitted, the particle is moving toward the source; if it is lower, the particle is moving away. However, the system only works when a particle is approaching or receding from the transmitter; Doppler radar cannot detect the velocity of a particle moving perpendicular to the radar signal. For this reason, signals from more than one radar source must be combined to produce an image free of gaps. Although standard radar can often detect tornadoes by a characteristic hook shaped echo, a Doppler system can detect the presence of funnel clouds and tornadoes much more reliably, and is now used quite commonly by weather forecasters, as well as radio and television stations, to monitor thunderstorms for the presence of strong winds and tornadoes. Doppler radar forms an important part of the Next Generation Radar (NEXRAD) network, an extensive upgrade to the radar facilities of the National Weather Service put in place in the 1980s and 90s. With NEXRAD, forecasters are now able to continuously track severe storms in any region of the country, providing potentially life-saving weather information.
This is the complete article, containing 521 words
(approx. 2 pages at 300 words per page).
