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.
Clear Air Mode - (dBZ values from -28
In this mode, the radar is in its most sensitive operation. This mode has the
slowest antenna rotation rate which permits the radar to sample a given volume
of the atmosphere longer. This increased sampling increases the radar's
sensitivity and ability to detect smaller objects in the atmosphere than in
precipitation mode. A lot of what you will see in clear air mode will be
airborne dust and particulate matter. Also, snow does not reflect energy sent
from the radar very well. Therefore, clear air mode will occasionally be used
for the detection of light snow. Note that radar can also detect insects or
Precipitation Mode - (dBZ values from 5
When rain is occurring, the radar does not need to be as sensitive as in clear
air mode since rain provides plenty of returning signals.
Base Reflectivity (short
This is a display of echo intensity (reflectivity) measured in dBZ (decibels of
Z, where Z represents the energy reflected back to the radar). "Reflectivity" is
the amount of transmitted power returned to the radar receiver. Base
Reflectivity images are available at several different elevation angles (tilts)
of the antenna and are used to detect precipitation, evaluate storm structure,
locate atmospheric boundaries and determine hail potential.
The base reflectivity image
currently available on this website is from the lowest "tilt" angle (0.5°). This
means the radar's antenna is tilted 0.5° above the horizon.
The maximum range of the "short
range" (S Rng) base reflectivity product is 124 nm (about 143 miles) from the
radar location. This view will not display echoes that are more distant than 124
nm, even though precipitation may be occurring at greater distances. To
determine if precipitation is occurring at greater distances, select the "long
range" (L Rng) view (out to 248 nm/286 mi).
Composite Reflectivity (long
This display is of maximum echo intensity (reflectivity) from any elevation
angle at every range from the radar. This product is used to reveal the highest
reflectivity in all echoes. When compared with Base Reflectivity, the Composite
Reflectivity can reveal important storm structure features and intensity trends
The maximum range of the "long
range" (L Rng) composite reflectivity product is 248 nm (about 286 miles) from
the radar location. The "blocky" appearance of this product is due to its lower
spatial resolution on a 2.2 * 2.2 nm grid. It has one-fourth the resolution of
the Base Reflectivity and one-half the resolution of the Precipitation products.
Although the Composite
Reflectivity product is able to display maximum echo intensities 248 nm from the
radar, the beam of the radar at this distance is at a very high altitude in the
atmosphere. Thus, only the most intense convective storms and tropical systems
will be detected at the longer distances.
Because of this fact, special
care must be taken interpreting this product. While the radar image may not
indicate precipitation it's quite possible that the radar beam is overshooting
precipitation at lower levels, especially at greater distances.
For a higher resolution (1.1 *
1.1 nm grid) composite reflectivity image, select the short range (S Rng) view.
The image is less "blocky" as compared to the long range image. However, the
maximum range is reduced to 124 nm (about 143 miles) from the radar location.
This is an image of estimated one-hour precipitation accumulation on a 1.1 nm by
1 degree grid. This product is used to assess rainfall intensities for flash
flood warnings, urban flood statements and special weather statements. The
maximum range of this product is 124 nm (about 143 miles) from the radar
location. This product will not display accumulated precipitation more distant
than 124 nm, even though precipitation may be occurring at greater distances.
Storm Total Precipitation
This image is of estimated accumulated rainfall, continuously updated, since the
last one-hour break in precipitation. This product is used to locate flood
potential over urban or rural areas, estimate total basin runoff and provide
rainfall accumulations for the duration of the event.
The maximum range of this product
is 124 nm (about 143 miles) from the radar location. This product will not
display accumulated precipitation more distant than 124 nm, even though
precipitation may be occurring at greater distances.
How often are the images
Image updates are based upon the operation mode of the radar at the time the
image is generated. The WSR-88D Doppler radar is operated in one of two modes --
clear air mode or precipitation mode. In clear air mode, images are updated
every 10 minutes. In precipitation mode, images are updated every five or six
minutes. The collection of radar data, repeated at regular time intervals, is
referred to as a volume scan.
correspond to levels
of precipitation intensity in terms of inches of rainfall per hour. The higher
the dBZ level, the more intense the precipitation; they are directly
10 DBZ - Very light rain
20 DBZ - Drizzle or light rain or snow
30 DBZ - Light to moderate rain (about 0.2 cm/hr or 1/10 inch/hour) or
40 DBZ - Moderate to heavy rain (about 1 cm/hr or 1/2 inch/hour) or snow
45 DBZ - Heavy rain (about 2.5 cm/hr or one inch/hour), or snow
50 DBZ - Very heavy rain or thunderstorms (about 6.0 cm/hr or two to
three inches/hour), or snow; hail possible
55 DBZ - Very heavy rain and hail, some large - severe
Radar information from the
National Weather Service