What do the colors mean in the reflectivity products?

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.


dBZ	Rainrate (inches per hour)	Description
65	16+	Extreme thunderstorms, possible hail
60	8.00	Very heavy thunderstorms, possible hail
55	4.00	Heavy thunderstorms
52	2.50	Thunderstorms
47	1.25	Very heavy rain or sleet
41	0.50	Heavy rain or sleet
36	0.25	Moderate rain or sleet
30	0.10	Light rain or moderate snow
20 or less	Trace	Very light rain or snow

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.

What do the colors mean in the velocity images?

The colors are the different radial velocities measured by the radar. In velocity images, red colors indicated wind moving away from the radar with green colors indicating motion toward the radar. The transition zone between incoming and outgoing winds are indicated the gray-ish colors between the two.

Each velocity image includes one of two velocity scales regardless of the radar's operation mode. One scale (far left) represents radial velocities in the base velocity image. The other scale (near left) represents the "storm relative motion" radial velocities. Note: As in the case of reflectivity images, the color on each scale remains the same in both velocity images, only the values change. The velocity of the wind is measured in knots (1 knot = 1.15 mph).

Since these colors represent values relative to the radar, to interpret these images correctly, it is most important to know where the radar is located each velocity image. For example, a region with outbound wind in one radar will be represented by red colors. That same region's wind could be inbound on an adjacent radar image and represented by green colors.

FORCE 0	LESS THAN 1 KNOT	SEA LIKE A MIRROR
FORCE 1	1 - 3 KNOTS	WAVE HEIGHT .1 M(.25 FT.); RIPPLES WITH APPEARANCE OF SCALES, NO FOAM CRESTS.
FORCE 2	4 - 6 KNOTS	WAVE HEIGHT .2 - .3 (.5 - 1 FT.); SMALL WAVELETS, CRESTS OF GLASSY APPEARANCE, NOT BREAKING.
FORCE 3	7 - 10 KNOTS	WAVE HEIGHT .6 - 1 M (2 - 3 FT.); LARGE WAVELETS, CRESTS BEGIN TO BREAK, SCATTERED WHITECAPS.
FORCE 4	11 - 16 KNOTS	WAVE HEIGHT 1 - 1.5 M (3.5 - 5 FT.); SMALL WAVES BECOMING LONGER, NUMEROUS WHITECAPS.
FORCE 5	17 - 21 KNOTS	WAVE HEIGHT 2 - 2.5 M (6 - 8 FT.); MODERATE WAVES, TAKING LONGER FORM, MANY WHITECAPS, SOME SPRAY.
FORCE 6	22 - 27 KNOTS	WAVE HEIGHT 3 - 4 M (9.5 - 13 FT.); LARGER WAVES FORMING, WHITECAPS EVERYWHERE, MORE SPRAY.
FORCE 7	28 - 33 KNOTS	WAVE HEIGHT 4 - 5.5 M (13.5 - 19 FT.); SEA HEAPS UP, WHITE FORM FROM BREAKING WAVES BEGINS TO BE BLOWN IN STREAKS ALONG DIRECTION OF WIND.
FORCE 8	34 - 40 KNOTS	WAVE HEIGHT 5.5 - 7.5 M (18 - 25 FT.); MODERATELY HIGH WAVES OF GREATER LENGTH, EDGES OF CRESTS BEGIN TO BREAK INTO SPINDRIFT, FOAM IS BLOWN IN WELL MARKED STREAKS.
FORCE 9	41 - 47 KNOTS	WAVE HEIGHT 7 - 10 M (23 - 32 FT.); HIGH WAVES, SEA BEGINS TO ROLL, DENSE STREAKS OF FOAM ALONG WIND DIRECTION, SPRAY MAY REDUCE VISIBILITY.
FORCE 10	48 - 55 KNOTS	WAVE HEIGHT 9 - 12.5 M (29 - 41 FT.); VERY HIGH WAVES WITH OVERHANGING CRESTS, SEA TAKES WHITE APPEARANCE AS FOAM IS BLOWN IN VERY DENSE STREAKS, ROLLING IS HEAVY AND SHOCKLIKE, VISIBILITY IS REDUCED.
FORCE 11	56 - 63 KNOTS	WAVE HEIGHT 11.5 - 16 M (37 - 52 FT.); EXCEPTIONALLY HIGH WAVES, SEA COVERED WITH WHITE FOAM PATCHES, VISIBILITY IS STILL MORE REDUCED.

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