Additionally, the beam may intercept ground targets, creating confusing clutter on radar images. This is commonly called "anomalous propagation." In this example, the radar beams superrefracted enough that they intercepted some local topography that was about 50 km (30 mi) from the radar.Superrefraction occurs mainly under the following conditions:Nocturnal radiation causing a temperature inversion near the ground and a sharp decrease in moisture with heightA flow of warm, moist air over cooler surfaces, especially waterA downdraft cooling the area underneath a thunderstorm, resulting in a temperature inversion in the lower troposphere. This is infrequent but can be very important because of its proximity to the storm.
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