A description of what Satellite Images and Radar Images show and not show,
including their advantages, limitations, and accuracy of the images they display.
A Description of Satellite And Radar Images

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Due to the destruction of our Post Home building by Hurricane Katrina we are now sharing
a building owned by the Elks Lodge 12010 Klein Road off of Three Rivers Road in Gulfport.
The location is about 4 miles North of I-10 Exit 38, Cowan-Lorraine Road.
American Legion Post 119 Is Requesting
Donations To Rebuild Our Post Home.
See below for Contact Information.

All communications with the Post, regarding donations to rebuild
our Post Home destroyed by Hurricane Katrina, should be made to
Post Commander Jim Tolar, 11220 Hughes Road Lot 90  Gulfport, MS.

Jim Tolar's Home Phone 1-228-206-1936  Cell Phone 228-332-0350 or by
 

EMAIL

UNITED STATES SATELLITE IMAGES
UNITED STATES RADAR IMAGES
WORLDWIDE SATELLITE IMAGES

What do Satellite Images show and not show?
Extracted from descriptions provided by GHCC.

Visible Images

Albedo: The fraction of radiation reflected by a surface.
The satellite measures sunlight reflected by the clouds and surface of the earth.
Water absorbs a lot of sunlight (it reflects just a little) so it appears dark. The
percent of sunlight reflected by the land is called the surface albedo. The albedo
of land ranges from about 10-30%, except for snow covered surfaces where the
albedo is much higher. A cloud's albedo is generally high, but can vary with its
thickness and composition. Thick clouds have high albedos and show up bright
in the satellite image. Thin cirrus clouds have low albedos and are usually
semi-transparent to sunlight. The structure of clouds in the satellite image
can tell the meteorologist a lot about the weather and animations
tell him/ her about the movement of weather systems.

Infrared Images

The satellite also measures the temperature of the clouds and the surface of the
Earth with an infrared sensor. This allows for the detection of changes in the
temperature of clouds and that of the surface during the day and at night. Clouds
are usually colder than the surface (land or water). The temperature of the clouds
also indicates how tall they are since temperature is inversely proportional to
height in the atmosphere. When the satellite meteorologist processes the infrared
data, he/she makes the warm clouds gray, the cool clouds whiter, and the very cold
clouds bright white. Meteorologists may also "enhance" (color code)
imagery in order to more easily interpret the data.

Water Vapor Images

This imagery represents a special kind of infrared measurement which measures
the temperature of clouds and water vapor in a layer of the atmosphere about
6 to 10 kilometers above the surface. At this altitude, steering currents such as jet
streams control the movement of weather systems around the Earth. The water
vapor imagery therefore captures these jet streams (elongated dark regions
with adjacent clouds and bright regions), "dry" blocking high pressure systems
(dark regions), and other weather systems (gray and bright white cloud patterns).
By studying these features and tracking them over time, meteorologists
can produce more accurate weather forecasts.

Enhanced or Colorized Images

The colder the cloud the more likely it is to produce rain. The temperature structure
of clouds also tells the meteorologist how hard it may be raining and whether the
storm may be producing severe weather. In the absence of clouds, the satellite
measures the temperature of the surface, which could be land or ocean. In the
infrared image, warm temperatures are dark and cold temperatures are lighter. In
the image, arid regions are hot and therefore dark, while regions at higher latitudes
are usually cooler and brighter. The infrared image can also be used to monitor
sea-surface temperature (SST). Since about 70% of the Earth is ocean, this
allows the scientist to study how changes in SST (such as El Nino and La Nina)
are related to global weather events (such as droughts, hurricanes, and floods).

Limitations on Satellite Images

Infrared (IR) and Water Vapor (WV) satellite images will display during
daylight or nighttime hours. Visible (VIS) images display only during daylight
hours. Sunlight is necessary for Visible (VIS) images to be displayed.
Visible images may appear delayed in time when the sun angle is too low
to generate sufficient reflected light...especially near sunrise and sunset.
High cloud tops will be displayed before low cloud tops near sunrise,
and high cloud tops will remain displayed longer after sunset.

White or black streaks and/or banding indicates temporary loss of satellite
data at a moment in time. The satellite "loops" cover a longer period
of time, and the offending image/images can be excluded.

Geostationary: Position in orbit is fixed with respect to a point on the earth.
This is due to the GOES-8 Satellite geostationary position above the Earth,
some States will not appear at or near the center of the satellite images.

Due to the GOES-8 Satellite geostationary position above the Earth,
western states appear canted (tilted) eastward. The GOES-10 Satellite
could have been selected, but close-up images would not be available.

Regarding Visible Satellite Images

Due to changes in the angle of sunlight throughout the year, visible
satellite images will appear somewhat different from month to month.

During summer in the northern hemisphere, reduced sunlight is
available in the southern hemisphere. During winter in the northern
hemisphere, reduced sunlight is available in the northern hemisphere.
The opposite is true for those living in countries the southern hemisphere.
This reduction of sunlight is severe for those living near the polar regions.

What do Radar images show and not show?
Extracted from descriptions provided by the National Weather Service (NWS).

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 Electromagnetic wave energy.

If the energy strikes an object (building, aircraft, 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 1,300 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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