In this three-part activity, you will begin by familiarizing yourself with the local geography. You will then learn to interpret the kind of local weather radar displays commonly accessible via cable television or the Internet. And lastly, you will examine regional and national composite maps of a national network of local radars. But first, let's introduce some basic understandings about weather radar.
RAdio Detection: Weather radars are both transmitters and receivers. Weather radars transmit a microwave beam and then "listen" for echoes that bounce back from precipitation-sized particles (or "targets") within or falling from clouds. (Doppler weather radars can also operate in a "clear air mode" in which cloud droplets can be detected.)
And Ranging: Since we know both the direction in which the radar transmitter is pointing and the speed at which microwaves travel (close to the speed of light), the direction and distance from the transmitter to the precipitation can be determined. (Distance to the precipitation echoes is calculated by dividing the travel time (outbound and inbound) in half and multiplying by the speed of light.) This permits mapping of precipitation over the region surrounding the radar site.
Precipitation intensity can be determined by measuring the strength of the echoes received by the radar antenna. The amount of energy reflected back to the radar is proportional to the precipitation intensity--the greater the energy reflected back to the radar, the greater the precipitation intensity. Echo strength is measured in units of DBZ (decibels). In general, DBZ values greater than 15 indicate areas where the precipitation is reaching the ground; DBZ values less than 15 indicates very light precipitation which may be evaporating before it reaches the ground (virga).
In addition, Doppler weather radar is capable of measuring whether precipitation echoes are moving toward or away from the radar antenna, and can therefore measure rotation within storms which may precede severe storms.
Using Doppler technology, the WSR-88D calculates both the direction and speed of motion of severe storms. By providing data on the wind patterns within developing storms, the new WSR-88D identifies the conditions leading to severe weather such as tornadoes. This means earlier detection of the precursors to tornadoes, as well as data on the direction and speed of tornadoes once they form.
The following WSR-88D products are available (most of these products are not widely accessible via the Internet):
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