The colors are the
different echo intensities (reflectivity) measured in dBZ (decibels
of Z) during each elevation scan. "Reflectivity" is the amount of
transmitted power returned to the radar receiver. Reflectivity
(designated by the letter Z) covers a wide range of signals (from
very weak to very strong). So, a more convenient number for
calculations and comparison, a decibel (or logarithmic) scale (dBZ),
is used.
The dBZ values increase as the
strength of the signal returned to the radar increases. Each
reflectivity image you see includes one of two color scales. One
scale (far left) represents dBZ values when the radar is in clear
air mode (dBZ values from -28 to +28). The other scale (near left)
represents dBZ values when the radar is in precipitation mode (dBZ
values from 5 to 75). Notice the color on each scale remains the
same in both operational modes, only the values change. The value of
the dBZ depends upon the mode the radar is in at the time the image
was created.
The scale of dBZ values is also
related to the intensity of rainfall. Typically, light rain is
occurring when the dBZ value reaches 20. The higher the dBZ, the
stronger the rainrate. Depending on the type of weather occurring
and the area of the U.S., forecasters use a set of rainrates which
are associated to the dBZ values.
These values are estimates of the rainfall per hour, updated each
volume scan, with rainfall accumulated over time. Hail is a good
reflector of energy and will return very high dBZ values. Since hail
can cause the rainfall estimates to be higher than what is actually
occurring, steps are taken to prevent these high dBZ values from
being converted to rainfall.
