National Weather Service
JetStream - Online School for Weather
Doppler Radar
- Introduction
- How Radar Works
- Web Views
- Radar Images
- RIDGE
- Downloading
- The GIS(t) of it All
- Radar Faq's
- Review Questions
- Additional Info:
- Lesson Plan Overview
- Topic Matrix
- JetStream News
Frequently Asked Questions
About the RIDGE version- What the...!?! Why fix something that wasn't broken?
- Why does it take so long to load?
- 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?
- Why is the image squashed? What projection is this?
- Why doesn't the loop fully load?
- Where is the old version?
- How can I set the way I want the overlays to display?
- How can I zoom and pan on a storm or feature?
- How often does the radar display refresh with AutoUpdate turned on?
- The radar stopped working - what have you done?
- Are there any other radar images available besides the current six?
- What are the different types of radar images?
- What is Range Folding (RF)? What does the purple color mean?
- What do the colors mean in the reflectivity images?
- What do the colors mean in the velocity images?
- How does the radar work?
- Is everything I see on the images an accurate picture of my weather?
- What is UTC Time?
- Why can't I save a radar image on my computer?
- Where is the sweeping line showing where the radar is pointed?
- How often are the images updated?
- When I attempt to load a loop using Internet Explorer under the Vista operating system all I get is an "X". Why will the looping function not work? New
What the...!?! Why fix something that wasn't broken?
The old version of generating and rendering the radar image was becoming very unreliable. We experienced many network and hardware problems with the old displays that caused outages on the website even when the radar was functioning. Additionally, there were requests for the radar image to be made available for GIS users and the old version was incapable to producing the output.
The new radar displays are much more real time (within 1-2 minutes from when the radar transmits the data) are more reliable. Also, if the radar displays go blank, its likely due to the radar physically not functioning. Finally, new displays also provide the ability to easily update geographic reference information.
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?
The looping images in the "Standard Version" radar display are animated gifs. Some third-party browser utilities include "pop-up blockers" which include a feature to disable animated ads. Set your preference to allow animated ads and the radar loop should load properly.
Why can't I loop radar images in the standard version with my dial-up connection?
The looping images in the "Standard Version" radar display are animated gifs. Web accelerator services that are designed to decrease your download times do so by many methods, one of which is to compress graphics.
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 webpage 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 doesn't the loop fully load?
First, check to see if Javascript and/or Java is turned off in your browser settings. If so, enable Javascript and/or Java.
But more likely the loop problems are attributed to a version of Java older than 1.4.2 that is installed on your machine. There are computer locks and/or computer performance issues, especially when older versions of java are trying to run the applet for the radar loop. Please visit www.java.com/en for more information on Java or download the most recent version. Check to see if you have the most recent version.
Where is the old "black background" version?
In the "Standard Version" of the radar display, the background color was changed from black to white for accessibility reasons. The black background was less friendly to those with colorblindness. In fact the black background with green and red data (highways and county lines) is impossible for the 1 in 20 males to use. The white background complies with Section 508 accessibility requirements - which cannot be said for the black background.
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?
There may several reasons why a radar is not in operation but primarily will be due to either a part that has failed or it is just routine maintain. Check the Status reports page of the Doppler radar. These are messages describing the anticipated time the radar is to be out of service.
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. Information on the other products and the type of computer equipment you need to receive the products can be found at the Radar Product Central Collection/Distribution Service webpage.
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 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 is from the lowest "tilt" angle (0.5°). This means the radar's antenna is tilted 0.5° above the horizon. Learn more about Base Reflectivity.
- 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. Learn more about composite reflectivity.
- 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. Learn more about Base Velocity images.
- 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. Learn more about Storm Relative Motion images.
- 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. Learn more about the one-hour precipitation image.
- 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). 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 and 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). 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 has no numerical value, but is 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 (mm^6/m^3). Mathematically:
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 mm^6/m^3 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. Learn more about these reflectivity color scales.| |
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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.
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).
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.
Learn more about Base Velocity and Storm Relative Motion images.
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.
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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".
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.
| UTC Time |
off- set |
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| AST | -4 | 8p* | 9p* | 10p* | 11p* | 12M | 1a | 2a | 3a | 4a | 5a | 6a | 7a | 8a | 9a | 10a | 11a | 12N | 1p | 2p | 3p | 4p | 5p | 6p | 7p |
| EST | -5 | 7p* | 8p* | 9p* | 10p* | 11p* | 12M | 1a | 2a | 3a | 4a | 5a | 6a | 7a | 8a | 9a | 10a | 11a | 12N | 1p | 2p | 3p | 4p | 5p | 6p |
| CST | -6 | 6p* | 7p* | 8p* | 9p* | 10p* | 11p* | 12M | 1a | 2a | 3a | 4a | 5a | 6a | 7a | 8a | 9a | 10a | 11a | 12N | 1p | 2p | 3p | 4p | 5p |
| MST | -7 | 5p* | 6p* | 7p* | 8p* | 9p* | 10p* | 11p* | 12M | 1a | 2a | 3a | 4a | 5a | 6a | 7a | 8a | 9a | 10a | 11a | 12N | 1p | 2p | 3p | 4p |
| PST | -8 | 4p* | 5p* | 6p* | 7p* | 8p* | 9p* | 10p* | 11p* | 12M | 1a | 2a | 3a | 4a | 5a | 6a | 7a | 8a | 9a | 10a | 11a | 12N | 1p | 2p | 3p |
| AK | -9 | 3p* | 4p* | 5p* | 6p* | 7p* | 8p* | 9p* | 10p* | 11p* | 12M | 1a | 2a | 3a | 4a | 5a | 6a | 7a | 8a | 9a | 10a | 11a | 12N | 1p | 2p |
| Hi | -10 | 2p | 3p* | 4p* | 5p* | 6p* | 7p* | 8p* | 9p* | 10p* | 11p* | 12M | 1a | 2a | 3a | 4a | 5a | 6a | 7a | 8a | 9a | 10a | 11a | 12N | 1p |
| Guam | +10 | 10a | 11a | 12N | 1p | 2p | 3p | 4p | 5p | 6p | 7p | 8p | 9p | 10p | 11p | 12M% | 1a% | 2a% | 3a% | 4a% | 5a% | 6a% | 7a% | 8a% | 9a% |
| Note: AST - Atlantic AK - Alaska time HI - Hawaii time *The previous day %The next day | |||||||||||||||||||||||||
| UTC Time |
off- set |
0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| EDT | -4 | 8p* | 9p* | 10p* | 11p* | 12M | 1a | 2a | 3a | 4a | 5a | 6a | 7a | 8a | 9a | 10a | 11a | 12N | 1p | 2p | 3p | 4p | 5p | 6p | 7p |
| CDT | -5 | 7p* | 8p* | 9p* | 10p* | 11p* | 12M | 1a | 2a | 3a | 4a | 5a | 6a | 7a | 8a | 9a | 10a | 11a | 12N | 1p | 2p | 3p | 4p | 5p | 6p |
| MDT | -6 | 6p* | 7p* | 8p* | 9p* | 10p* | 11p* | 12M | 1a | 2a | 3a | 4a | 5a | 6a | 7a | 8a | 9a | 10a | 11a | 12N | 1p | 2p | 3p | 4p | 5p |
| PDT | -7 | 5p* | 6p* | 7p* | 8p* | 9p* | 10p* | 11p* | 12M | 1a | 2a | 3a | 4a | 5a | 6a | 7a | 8a | 9a | 10a | 11a | 12N | 1p | 2p | 3p | 4p |
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. Learn more about Z-Time.
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. Also these older weather radars required an operator to manually tilt the antenna up and down whenever a vertical cross section of a storm was desired, or when the upper parts of a storm needed to be investigated.
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 special effect and is not real. 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 use this effect, 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.
Why will the looping function not work in Vista?
The Java applet that controls the looping function is "unsigned". By default, Vista blocks unsigned applets. The work around is this: Add radar.weather.gov to your IE "trusted zone".
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