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Radar just part of whole picture

Sat., Feb. 24, 2007

With all the rain and snow that we’ve been seeing lately, you’ve probably seen your fair share of radar images on television or the Internet. Just as weather satellites do not “see it all” from their vantage points in outer space, weather radars also have their limitations in what they can “see.” By learning some of the basics of how a weather radar works, you’ll be able to understand why sometimes the image you see on the radar display does not match up to what you’re experiencing at a given location.

A weather radar is not a mechanical “eye” that “sees” precipitation as it falls. The radar does two things: It sends out a pulse of energy, then listens for a return signal. The return signal occurs because the energy bounces off an object such as rain or snow, and some of the energy is reflected back to the radar. The distance, or range, of an object is determined by how long it takes for the pulse of energy to travel back to the radar. The reflectivity of the object is determined by how much energy is returned to the radar.

The pulse of energy sent out by the radar can bounce back from a number of things, not limited to precipitation. The radar spins a full 360 degrees and releases pulses of energy at a slight upward angle. At low radar beam elevations, close to the radar, the energy can bounce off trees and tall buildings, which produces a return signal that might be painted on the radar display. This is what we call ground clutter. Most of the time, software filters out this ground clutter so that you don’t ever see it. A return signal can also be produced from dust, a flock of birds, or even a large swarm of insects.

At high beam elevations, return signals can be produced by the water or ice in clouds themselves, which may or may not be precipitating. The radar beam elevation increases with distance from the radar because of the angle at which it is emitted in addition to the curvature of the Earth. This would explain the common display of rain or snow in central Washington that is moving eastward, but always seems to “disappear” before making it to the Idaho border. What you’re often really seeing is the result of the Spokane radar beam hitting the clouds in central Washington where it may or may not even be raining or snowing yet.

In mountainous areas such as the Northwest, another important caveat to consider when looking at a radar image is that the radar cannot “see” through a mountain. Seattle and Portland’s radars are effectively blocked by the Cascades from further eastward coverage. Likewise, Spokane’s and Missoula’s radars cannot see through the Rockies. The radar images you see on television and the internet are almost always composite images, combining the data from all of the area radars. In the Northwest we are covered by National Weather Service radars located in Portland, Seattle, Pendleton, Ore., Spokane and Missoula.

Finally, a weather radar cannot make a determination whether its energy is bouncing off rain or snow or anything in between. You may often see a display on television that is color-coded white for snow, green for rain and pink for all the stuff in between. These images are produced by additional software from various private weather graphics companies. The formulas and data used to determine precipitation type may differ from company to company but they are never perfect.

As indispensable of a tool weather radar is, the most complete weather picture is put together by combining information from satellite, radar and the human eye.


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