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A radar observes precipitation by transmitting a pulse of energy into the atmosphere and the "listening" for how much energy is reflected back to the radar dish. Because of buildings, trees, etc. at the surface, the radar beam cannot follow the ground. Instead the radar beam is transmitted at a slight angle to the earth's surface to the radar beam rises above obstructions. In addition, as the beam moves far from the radar, the curvature of the earth also results in the beam elevation above the surface increasing. The result is that 100 miles from the radar the beam is 10000 ft above the surface. This means a portion of the storm will not be observed by the radar.Most particles in the atmosphere all reflect energy. However, how well energy is reflected depends on several factors including particle size and its composition. So even dust and bugs near the ground can reflect energy back to the radar and appear as precipitation on a radar scope. Some clouds without precipitation will also reflect energy back to the radar depending on how big the cloud droplets are. The strongest returns come from rain and hail. Finally to best observe storms, the National Weather Service employs a volume scan which examines several layers of the atmosphere in a few minutes. This provides a picture of the atmosphere (and thunderstorm) in three dimensions. |
