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
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.