What are the differences between the product type images shown here?
What makes APRSfl.net Unique
The National Weather Service Nexrad Radar Imagery seen here is created using the
software package Gempak.
TDWR Radar Imagery is created using the software package
GRLevel3. All images are created live from
the sites raw data via Unidata
LDM. Images are never screen scraped from another website.
Level II vs Level III
Level II images are shown with 128 colors instead
of 15 providing more detail in intensity. This allows the ability to delineate certain phenomena (e.g. gust fronts, hook
echoes) that are sometimes hard to see using Level III data. The Level II data is available in 0.5 dBZ increments,
while Level III data is only available in 5.0 dBZ increments. Today's Level II is commonly called
Super High Resolution Nexrad Doppler Radar.
NWS Nexrad vs FAA TDWR Radar Sites
Terminal Doppler Weather Radar sites are ran by the FAA at 45 airports around the country.
TDWR's show low altitude wind shear caused by microbursts and their associated
gust fronts, as well as precipitation intensities. Since these sites are designed
for high resolution detail near the airport, their range is limited to only 48 nautical miles and a .3 degree beam angle.
Learn more at NOAA Training.
Base Reflectivity is one of the basic quantities that Doppler radar measures. Color intensity corresponds to the amount of
radiation that is scattered or reflected back to the radar by whatever targets are located in the radar beam at a given
location. These targets can be hydrometeors (snow, rain drops, hail, cloud drops or ice particles) or other targets (dust,
smoke, birds, airplanes, insects).
Base and Radial Velocity
Base Velocity is the average radial velocity of the targets in the radar beam at a given location. Radial velocity is the
relationship between the target's motion and the direction of the radar beam. Positive values (warm colors) denote out-bound
velocities that are moving away from the radar. Negative values (cool colors) are in-bound velocities that are moving towards
Base Spectrum Width
Spectrum Width depicts a measure of velocity dispersion. It provides a measure of
the variability of the mean radial velocity estimates due to wind shear, turbulence, and/or the quality of the
velocity samples. It is used to estimate turbulence associated with boundaries,
Storm Total Rainfall Estimate
Storm Total Precipitation is an estimate of 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
1 Hour Rainfall Estimate
One Hour Total Precipitation is an estimate of one-hour precipitation accumulation on a 1.1x1.1 nm grid. This
product is useful for assessing rainfall intensities for flash flood warnings, urban flood statements and special weather
Composite Reflectivity displays the maximum reflectivities for each resolution grid box for all 16 of the elevation angles.
It is used to observe the highest reflectivities in a storm from any scanned elevation angle, determine intensity trends
and generate cross section through maximum
Vertically Integrated Liquid
Vertically Integrated Liquid displays reflectivity data converted into liquid water equivalent via an empirically-derived
relationship which assumes that all reflectivity returns are from liquid water. It is used to indicate presence and
approximate size of hail, locate the most significant thunderstorms or areas of possible heavy rainfall. Rapid decrease in
VIL values may signify the onset of wind damage.
Echo Tops displays the maximum height of precipitation echoes. The radar will not report echo tops below 5,000 feet or above
70,000 feet, and will only report those tops that are at a reflectivity of 18.5 dBZ or higher. In addition, the radar will
not be able to see the tops of some storms very close to the radar as the maximum
tilt angle of the radar (19.5 degrees) is not high enough to let the radar beam see
the top of the storm.
Clear Air versus Precipitiation Mode
In Clear Air mode the radar is in its most sensitive operation. This mode has the slowest antenna rotation rate
which permits the radar to sample 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. In clear air mode, the radar products update every 10 minutes.
When rain is occurring, the radar does not need to be as sensitive as rain provides plenty of returning signals. In
Precipitation Mode, the radar products update every 6
The dBZ Scale
The dBZ scale is shown by the color scale on the left of the image. 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 represents dBZ
values when the radar is in clear air mode (dBZ values from -28 to +28). The other scale represents dBZ values when the radar
is in precipitation mode (dBZ values from 5 to 75). 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.