RIDGE Radar Frequently Asked Questions

About the RIDGE version
  1. Why does it take so long to load?
  2. Why can't I loop radar images in the standard version with my broadband connection?
  3. Why can't I loop radar images in the standard version with my dial-up connection?
  4. Why is the image squashed? What projection is this?
  5. Why doesn't the loop fully load?
  6. How can I set the way I want the overlays to display?
  7. How can I zoom and pan on a storm or feature?
  8. How often does the radar display refresh with AutoUpdate turned on?
  9. The radar stopped working - what have you done?
Radar images
  1. Are there any other radar images available besides the current six?
  2. What are the different types of radar images?
  3. What is Range Folding (RF)? What does the purple color mean?
  4. What do the colors mean in the reflectivity images?
  5. What do the colors mean in the velocity images?
General questions
  1. How does the radar work?
  2. Is everything I see on the images an accurate picture of my weather?
  3. What is UTC Time?
  4. Why can't I save a radar image on my computer?
  5. Where is the sweeping line showing where the radar is pointed?
  6. How often are the images updated?

About the RIDGE version

Why does it take so long to load?

The enhanced version initially takes longer to load due to added functionality such as the toggle on/off ability of graphics, calculating the distance from a storm to your approximate location and lat/lon info for hurricane tracking.

However, many of the overlays are static in that they do not require to be retransmitted to update the page. Since these images are "cached" on your computer, subsequent visits to your favorite radar site do not require retransmission of most graphics leading to a much reduced download file size.

For most folks, the topography image is the largest file the needs to be transferred. The file size of the topo overlay is smallest for radar located near the ocean or Great Lakes. The largest topo files are for radars in the Rocky Mountain regions due to large differences in terrain.

Why can't I loop radar images in the standard version with my broadband connection?

Why can't I loop radar images in the standard version with my dial-up connection?

When an accelerator service compresses the radar loop, it essentially keeps the oldest image (first in the loop) and deletes the remaining images. To view the loop in the "Standard Version" you need to disable the accelerator service to prevent this compression of the radar image.

Yes, the files are large and it will be slow on dial-up but the overall file size, including the web page and all graphics is typically much smaller than the old original looping version (about 25% smaller).

Why is the image squashed? What projection is this?

The images appear elongated because of the Geographic Coordinate System (GCS) way of displaying the information. In order for GIS-based programs to ingest the Doppler radar data, the information is displayed in an UN-projected format.

This means the display of information, intended for a spherical earth, becomes distorted when the image is viewed on a flat surface. Find out more about GIS.

Why does the loop not fully load?

How can I set the way I want the overlays to display?

See "Preserving Your Toggle Settings"

How can I zoom and pan on a storm or feature?

The zoom feature is based on a Java applet and that is only seen on the looping mode of the radar. Select the looping image you desire then select the "zoom-pan" button (located just below the main radar image). Your pointer should change to a finger.

To ZOOM: Click on the image where you want to zoom. The more you click the image the more you zoom.
To PAN: If you "click and hold" the mouse button down you can then drag the image around inside of the applet window.

How often does the radar display refresh with AutoUpdate turned on?

Every five minutes. Read more about the AutoUpdate feature.

The radar stopped working - what have you done?


Radar Images

Are there any other radar images available besides the current six?
The National Weather Service has a central collection of WSR-88D radar products in process. While we currently only display six of those products (Base Reflectivity, Composite Reflectivity, Base Velocity, Storm Relative Motion, One-Hour Precipitation, and Storm Total Precipitation) through these local radar pages, you can receive all products through a "multicast" flow or via standard anonymous FTP from the Gateway file servers.

What are the different types of radar images?
There are six different types of images currently available for the RIDGE radar display: Base Reflectivity, Composite Reflectivity, Base Velocity, Storm Relative Motion, One-hour Precipitation and Storm Total Precipitation.

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 reflected off an object and returned to the radar receiver. Base Reflectivity images are used to detect precipitation, evaluate storm structure, locate atmospheric boundaries and determine hail potential.
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.
Base Velocity
This display of radial velocity represents the overall wind field. Green colors indicate wind moving toward the radar with red colors indicating wind moving away from the radar. The maximum range of this product is 124 nm (about 143 miles) from the radar location.
Storm Relative Motion

This display is of radial velocity of the wind relative to the storm's motion. The result is a picture of the wind as if the storms were stationary.

This often unmasks storms that rotate (supercells) which can be a precursor to the formation of tornadoes. Green colors indicate wind moving toward the radar with red colors indicating wind moving away from the radar. The maximum range of this product is 124 nm (about 143 miles) from the radar location.

One-hour Precipitation

This is an image of estimated one-hour precipitation accumulation. 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 image will not display accumulated precipitation more distant than 124 nm, even though precipitation may be occurring at greater distances. To determine accumulated precipitation at greater distances you should link to an adjacent radar.

Storm Total Precipitation

This image is of estimated accumulated rainfall, continuously updated, since the last one-hour break in precipitation. This image 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. To determine accumulated precipitation at greater distances link to an adjacent radar.

What is Range Folding (RF)? What does the purple color mean?
Range Folding is basically when the radar is unable to determine the wind's velocity. This is due to the speed at which the radar transmits signals, called the pulse repetition frequency (PRF).

Sample radar image with range folded regions.
The faster the pulses are sent by the radar the less time it has to listen for any returned signals. It occurs when the return from a prior pulse is detected during the listening period for the current pulse. Both reflectivity and velocity data are affected by this.

The occurrence of range folding can usually be detected by radar software and reflectivity data can be "unfolded" using special programs. However, velocity data cannot be accurately unfolded.

Therefore the effective range with which Doppler radars can detect velocity data is limited by the frequency of the radar pulses; the higher the pulse rate, the shorter the range within which the velocity field can be determined.

When the radar is unable to "unfold" the information, we paint the region purple as an indicator of the problem. There are some ways to minimize range folding and we have recently implemented a program which will sample the atmosphere with different PRF to do just that.

What do the colors mean in the reflectivity images?
The colors are the different values of energy that are reflected back toward the radar. Called echoes, the reflected intensities are measured in dBZ (decibels of z).

Reflectivity color scale.
As the strength of the signal returned to the radar increases the dBZ values increases. The Doppler radar does not determine where rain is located, only areas of returned energy.

The "dB" in the dBz scale is logarithmic and unitless used only to express a ratio. The "z" is the ratio of the density of water drops (measured in millimeters, raised to the 6th power) in each cubic meter (mm6/m3). Mathematically:

dBz= 10 * log (z/z0), where z = reflectivity factor and z0 is defined to be 1 mm6/m3

When the "z" is large (many drops in a cubic meter), the reflected power is large. A small "z" means little returned energy.

In fact, "z" can be less than 1 mm6/m3 and since it is logarithmic, dBz values will become negative, as often in the case when the radar is in clear air mode and indicated by earthtone colors.

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.

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.

  • Storm Relative Motion scale
  • Base Velocity scale
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 = 1.85 km/h).

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.


General questions

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 (green). 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. Learn more about the Doppler radar.

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 areal 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 <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 U.S. 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 to 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".

Cone of Silence - The area above 19.5 degrees where the radar cannot see.

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.

What is UTC Time?

Weather observations around the world (including radar observations) are always taken with respect to a standard time. By convention, the world's weather communities use a twenty four hour clock, similar to "military" time based on the 0° longitude meridian, also known as the Greenwich meridian.

To obtain your local time here in the United States, you need to subtract a certain number of hours from UTC depending on how many time zones you are away from Greenwich (England). The table (right) shows the standard difference from UTC time to local time.

The switch to daylight saving time does not affect UTC. It refers to time on the zero or Greenwich meridian, which is not adjusted to reflect changes either to or from Daylight Saving Time.

However, you need to know what happens during daylight saving time in the United States. In short, the local time is advanced one hour during daylight saving time.

As an example, the Eastern Time zone difference from UTC is a -4 hours during daylight saving time rather than -5 hours as it is during standard time.

Standard Time (UTC offset)
Z-time Guam
(+10)
Hawaii
(-10)
Alaska
(-9)
Pacific
(-8)
Mountain
(-7)
Central
(-6)
Eastern
(-5)
Atlantic
(-4)
00z 10 a.m. 2 p.m.* 3 p.m.* 4 p.m.* 5 p.m.* 6 p.m.* 7 p.m.* 8 p.m.*
01z 11 a.m. 3 p.m.* 4 p.m.* 5 p.m.* 6 p.m.* 7 p.m.* 8 p.m.* 9 p.m.*
02z 12 noon 4 p.m.* 5 p.m.* 6 p.m.* 7 p.m.* 8 p.m.* 9 p.m.* 10 p.m.*
03z 1 p.m. 5 p.m.* 6 p.m.* 7 p.m.* 8 p.m.* 9 p.m.* 10 p.m.* 11 p.m.*
04z 2 p.m. 6 p.m.* 7 p.m.* 8 p.m.* 9 p.m.* 10 p.m.* 11 p.m.* 12 mid
05z 3 p.m. 7 p.m.* 8 p.m.* 9 p.m.* 10 p.m.* 11 p.m.* 12 mid 1 a.m.
06z 4 p.m. 8 p.m.* 9 p.m.* 10 p.m.* 11 p.m.* 12 mid 1 a.m. 2 a.m.
07z 5 p.m. 9 p.m.* 10 p.m.* 11 p.m.* 12 mid 1 a.m. 2 a.m. 3 a.m.
08z 6 p.m. 10 p.m.* 11 p.m.* 12 mid 1 a.m. 2 a.m. 3 a.m. 4 a.m.
09z 7 p.m. 11 p.m.* 12 mid 1 a.m. 2 a.m. 3 a.m. 4 a.m. 5 a.m.
10z 8 p.m. 12 mid 1 a.m. 2 a.m. 3 a.m. 4 a.m. 5 a.m. 6 a.m.
11z 9 p.m. 1 a.m. 2 a.m. 3 a.m. 4 a.m. 5 a.m. 6 a.m. 7 a.m.
12z 10 p.m. 2 a.m. 3 a.m. 4 a.m. 5 a.m. 6 a.m. 7 a.m. 8 a.m.
13z 11 p.m. 3 a.m. 4 a.m. 5 a.m. 6 a.m. 7 a.m. 8 a.m. 9 a.m.
14z 12 mid 4 a.m. 5 a.m. 6 a.m. 7 a.m. 8 a.m. 9 a.m. 10 a.m.
15z 1 a.m.# 5 a.m. 6 a.m. 7 a.m. 8 a.m. 9 a.m. 10 a.m. 11 a.m.
16z 2 a.m.# 6 a.m. 7 a.m. 8 a.m. 9 a.m. 10 a.m. 11 a.m. 12 noon
17z 3 a.m.# 7 a.m. 8 a.m. 9 a.m. 10 a.m. 11 a.m. 12 noon 1 p.m.
18z 4 a.m.# 8 a.m. 9 a.m. 10 a.m. 11 a.m. 12 noon 1 p.m. 2 p.m.
19z 5 a.m.# 9 a.m. 10 a.m. 11 a.m. 12 noon 1 p.m. 2 p.m. 3 p.m.
20z 6 a.m.# 10 a.m. 11 a.m. 12 noon 1 p.m. 2 p.m. 3 p.m. 4 p.m.
21z 7 a.m.# 11 a.m. 12 noon 1 p.m. 2 p.m. 3 p.m. 4 p.m. 5 p.m.
22z 8 a.m.# 12 noon 1 p.m. 2 p.m. 3 p.m. 4 p.m. 5 p.m. 6 p.m.
23z 9 a.m.# 1 p.m. 2 p.m. 3 p.m. 4 p.m. 5 p.m. 6 p.m. 7 p.m.
Note: *The previous day   %The next day
Daylight Saving Time (UTC offset)
Z-time Guam
(+10)
Hawaii
(-10)
Alaska
(-8)
Pacific
(-7)
Mountain
(-6)
Central
(-5)
Eastern
(-4)
Atlantic
(-3)
00z 10 a.m. 2 p.m.* 4 p.m.* 5 p.m.* 6 p.m.* 7 p.m.* 8 p.m.* 9 p.m.*
01z 11 a.m. 3 p.m.* 5 p.m.* 6 p.m.* 7 p.m.* 8 p.m.* 9 p.m.* 10 p.m.*
02z 12 noon 4 p.m.* 6 p.m.* 7 p.m.* 8 p.m.* 9 p.m.* 10 p.m.* 11 p.m.*
03z 1 p.m. 5 p.m.* 7 p.m.* 8 p.m.* 9 p.m.* 10 p.m.* 11 p.m.* 12 mid
04z 2 p.m. 6 p.m.* 8 p.m.* 9 p.m.* 10 p.m.* 11 p.m.* 12 mid 1 a.m.
05z 3 p.m. 7 p.m.* 9 p.m.* 10 p.m.* 11 p.m.* 12 mid 1 a.m. 2 a.m.
06z 4 p.m. 8 p.m.* 10 p.m.* 11 p.m.* 12 mid 1 a.m. 2 a.m. 3 a.m.
07z 5 p.m. 9 p.m.* 11 p.m.* 12 mid 1 a.m. 2 a.m. 3 a.m. 4 a.m.
08z 6 p.m. 10 p.m.* 12 mid 1 a.m. 2 a.m. 3 a.m. 4 a.m. 5 a.m.
09z 7 p.m. 11 p.m.* 1 a.m. 2 a.m. 3 a.m. 4 a.m. 5 a.m. 6 a.m.
10z 8 p.m. 12 mid 2 a.m. 3 a.m. 4 a.m. 5 a.m. 6 a.m. 7 a.m.
11z 9 p.m. 2 a.m. 3 a.m. 4 a.m. 5 a.m. 6 a.m. 7 a.m. 8 a.m.
12z 10 p.m. 2 a.m. 4 a.m. 5 a.m. 6 a.m. 7 a.m. 8 a.m. 9 a.m.
13z 11 p.m. 3 a.m. 5 a.m. 6 a.m. 7 a.m. 8 a.m. 9 a.m. 10 a.m.
14z 12 mid 4 a.m. 6 a.m. 7 a.m. 8 a.m. 9 a.m. 10 a.m. 11 a.m.
15z 1 a.m.# 5 a.m. 7 a.m. 8 a.m. 9 a.m. 10 a.m. 11 a.m. 12 noon
16z 2 a.m.# 6 a.m. 8 a.m. 9 a.m. 10 a.m. 11 a.m. 12 noon 1 p.m.
17z 3 a.m.# 7 a.m. 9 a.m. 10 a.m. 11 a.m. 12 noon 1 p.m. 2 p.m.
18z 4 a.m.# 8 a.m. 10 a.m. 11 a.m. 12 noon 1 p.m. 2 p.m. 3 p.m.
19z 5 a.m.# 9 a.m. 11 a.m. 12 noon 1 p.m. 2 p.m. 3 p.m. 4 p.m.
20z 6 a.m.# 10 a.m. 12 noon 1 p.m. 2 p.m. 3 p.m. 4 p.m. 5 p.m.
21z 7 a.m.# 11 a.m. 12 noon 1 p.m. 2 p.m. 3 p.m. 4 p.m. 5 p.m.
22z 8 a.m.# 12 noon 2 p.m. 3 p.m. 4 p.m. 5 p.m. 6 p.m. 7 p.m.
23z 9 a.m.# 1 p.m. 3 p.m. 4 p.m. 5 p.m. 6 p.m. 7 p.m. 8 p.m.
Note: *The previous day   %The next day
Where is the sweeping line showing where the radar is pointing?

Prior to Doppler radars, the old analog type of radar continually transmitted a signal and its return would be projected on a screen as a line sweeping around the scope. As the radar made its sweep, the updated information would overwrite the older info.

Doppler radars store information like all digitized systems; in 1's and 0's. Because of this, all Doppler radars wait until the radar makes one complete 360° rotation before any information is transmitted to the user. Gone is the old sweeping line. It no longer applies to Doppler's digital data.

Old habits are hard to die however. You may occasionally notice a sweeping line (with data being updated as it makes its sweep) during the weather report of some television stations. This is fake.

Special software had to be written to make it appear the information is being updated as the beam moves. The National Weather Service does not fool you this way, which is why you do not see any sweeping lines.

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 four to six minutes. The collection of radar data, repeated at regular time intervals, is referred to as a volume scan. Learn more about the two radar operating modes.