Measuring weather

remote observation - radar

Radar is an acronym for radio detection and ranging. Radar is an object detection system that uses electromagnetic radiation to measure the distance to an object. It sends out signal pulses to measure distant samples and so it is an active remote sensing instrument. In the weather field, the returned signal can be analyzed to determine information about precipitation type and intensity, as well as wind speed and direction, taking into account the Doppler effect.

The radar instrument consists of an antenna mounted on a rotating parabolic dish. As the antenna rotates 360 degrees and tilts at certain angles to point higher or lower in the sky, a hollow-cone-like volume scan of the atmosphere is done. Weather radar can effectively see about 140 miles. The farther out it sees, the higher off the ground it is looking since it is tipped upwards to avoid seeing trees, buildings and mountains and because of the curvature of the earth. This lowest level reflectivity, called base reflectivity, is the most reliable of all levels with respect to an estimate of precipitation that will reach the ground. Another common product called composite reflectivity, shows the strongest radar return in each vertical column and is important for very short-term prediction of precipitation intensity. Overlapping coverage of multiple radar instruments helps to fill in the gaps. In the United States, radar coverage is nearly complete with difficulties in the mountain areas of the west.

Next page   -> remote observations, radar

Links and resources