Interpreting Radar Images
Weather radars send out pulses of microwave energy and listen between the transmitted pulses for part of that the energy to be reflected back to the radar. Raindrops and snow produce reflections that become stronger as the size of the drop or flake increases.
The unit of radar reflectivity is dbz. High values of dbz (color scale to the right of the image) indicate large drops and heavy precipitation. Our loop shows the signals recorded by several radars in the northwest over the last several hours.
The coverage of the Pacific Northwest by weather radar is by no means uniform. The beam can be blocked by mountains, and some areas are simply too far away from any radar. So, rain will occur in some places (such as the western side of the Olympic Mountains) without it showing up on our loop.
Weather radars can also pick up returns from nearby objects on the ground (ground clutter) and flying insects. See http://www.wikipedia.org/wiki/Weather_radar for a more extensive discussion.