How does the radar work?

NEXRAD (Next Generation Radar) obtains weather information
(precipitation and wind) based upon returned energy.

The radar emits a burst of energy.

If the energy strikes an object (rain drop, bug, bird, etc,), the energy is
scattered in all directions (blue). A small fraction of that scattered
energy is directed back toward the radar.

This reflected signal is then received by the radar during its listening period.
Computers analyze the strength of the returned pulse, time it took to travel to
the object and back, and phase shift of the pulse. This process of emitting a
signal, listening for any returned signal, then emitting the next signal,
takes place very fast, up to around 1300 times each second.

What are the different types of radar images?

Base Reflectivity

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), select an
adjacent radar, or link to the National Reflectivity Mosaic.

Composite Reflectivity

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 of storms.

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. To determine if precipitation is occurring at greater distances
link to an adjacent radar or link to the National Reflectivity Mosaic.

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.

How often are the images updated?

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.

Is everything I see on the images
an accurate picture of my weather?

Weather surveillance radars such as the WSR-88D can detect most precipitation
within approximately 80 nautical miles (nm) of the radar, and intense rain or snow
within approximately 140 nm. However, light rain, light snow, or drizzle from
shallow cloud weather systems are not necessarily detected.

Echoes from surface targets appear in almost all radar reflectivity images. In the
immediate area of the radar, "ground clutter" generally appears within a radius
of 20 NM. This appears as a roughly circular region with echoes that show little
spatial continuity. It results from radio energy reflected back to the radar from
outside the central radar beam, from the earth's surface or buildings. Under highly
stable atmospheric conditions (typically on calm, clear nights), the radar beam can
be refracted almost directly into the ground at some distance from the radar,
resulting in an area of intense-looking echoes.

This "Anomalous Propagation" phenomenon (commonly known as AP) is much
less common than ground clutter. Certain sites situated at low elevations on
coastlines regularly detect "sea return", a phenomenon similar to ground
clutter except that the echoes come from ocean waves.

Returns from aerial targets are also rather common. Echoes from migrating birds
regularly appear during nighttime hours between late February and late May, and
again from August through early November. Return from insects is sometimes
apparent during July and August. The apparent intensity and a real coverage of
these features is partly dependent on radio propagation conditions, but they
usually appear within 30 NM of the radar and produce reflectivities of less
than 30 dBZ (decibels of Z). However, during the peaks of the bird migration
seasons, in April and early September, extensive areas of the south-central
United States may be covered by such echoes.

Finally, aircraft often appear as "point targets" far from the radar, particularly in
composite reflectivity images. The radar is also limited close in by its inability to
scan directly overhead. Therefore, close the radar, data are not available
due to the radar's maximum tilt elevation of 19.5°. This area is
commonly referred to as the radar's "Cone of Silence".

Though surface echoes appear in the base and composite reflectivity images,
special automated error checking generally removes their effects from
precipitation accumulation products. The national reflectivity mosaic product is
also automatically edited to detect and remove most non-precipitation features.

Even with limited experience, users of unedited products can differentiate
precipitation from other echoes, if they are aware of
the general meteorological situation.

Note from American Legion Post 119 Gulfport, Mississippi

The paragraphs, following this note, describe Clutter.
"Ground Clutter"..."Aerial Clutter"..."Sea Clutter"

Clutter produces Radar returns which cause the resulting radar images to
display spots, streaks, blotches, and lines that actually have nothing to do
with the weather you are trying to view. Understanding "clutter" is essential
to understand, or interpret Radar images. For awhile, viewers, new to viewing
Radar images, will have to compare the actual weather near and around
their locations to the Radar images displayed. After a few weather systems
approach and/or leave your location, "clutter" will become easier to ignore.

Now back to the National Weather Service's discussion of Radar...

Is everything I see on the images
an accurate picture of my weather?

Echoes from surface targets appear in almost all radar reflectivity images. In the
immediate area of the radar, "ground clutter" generally appears within a radius
of 20 NM. This appears as a roughly circular region with echoes that show little
spatial continuity. It results from radio energy reflected back to the radar from
outside the central radar beam, from the earth's surface or buildings. Under highly
stable atmospheric conditions (typically on calm, clear nights), the radar beam can
be refracted almost directly into the ground at some distance from the radar,
resulting in an area of intense-looking echoes.

This "Anomalous Propagation" phenomenon (commonly known as AP) is much
less common than ground clutter. Certain sites situated at low elevations on
coastlines regularly detect "sea return", a phenomenon similar to ground
clutter except that the echoes come from ocean waves.

Returns from aerial targets are also rather common. Echoes from migrating birds
regularly appear during nighttime hours between late February and late May, and
again from August through early November. Return from insects is sometimes
apparent during July and August. The apparent intensity and a real coverage of
these features is partly dependent on radio propagation conditions, but they
usually appear within 30 NM of the radar and produce reflectivities of less
than 30 dBZ (decibels of Z). However, during the peaks of the bird migration
seasons, in April and early September, extensive areas of the south-central
United States may be covered by such echoes.

Finally, aircraft often appear as "point targets" far from the radar, particularly in
composite reflectivity images. The radar is also limited close in by its inability to
scan directly overhead. Therefore, close the radar, data are not available
due to the radar's maximum tilt elevation of 19.5°. This area is
commonly referred to as the radar's "Cone of Silence".

Though surface echoes appear in the base and composite reflectivity images,
special automated error checking generally removes their effects from
precipitation accumulation products. The national reflectivity mosaic product is
also automatically edited to detect and remove most non-precipitation features.

Even with limited experience, users of unedited products can differentiate
precipitation from other echoes, if they are aware of
the general meteorological situation.

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