San Francisco Weather
Radar Frequently Asked Questions (FAQ)
Precipitation intensity is measured by a ground-based radar that bounces radar waves off of precipitation. The Local Radar base reflectivity product is a display of echo intensity (reflectivity) measured in dBZ (decibels). "Reflectivity" is the amount of transmitted power returned to the radar receiver after hitting precipitation, compared to a reference power density at a distance of 1 meter from the radar antenna. Base reflectivity images are available at several different elevation angles (tilts) of the antenna; the base reflectivity image currently available on this website is from the lowest "tilt" angle (0.5°).
The maximum range of the base reflectivity product is 143 miles (230 km) from the radar location. This image will not show echoes that are more distant than 143 miles, even though precipitation may be occurring at these greater distances. To determine if precipitation is occurring at greater distances, link to an adjacent radar. In addition, the radar image will not show echos from precipitation that lies outside the radar's beam, either because the precipitation is too high above the radar, or because it is so close to the Earth's surface that it lies beneath the radar's beam.
In this mode, the radar is in its most sensitive operation. This mode has the slowest antenna rotation rate which permits the radar to sample a given volume of 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. Also, snow does not reflect energy sent from the radar very well. Therefore, clear air mode will occasionally be used for the detection of light snow. 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 in clear air mode as rain provides plenty of returning signals. In Precipitation Mode, the radar products update every 6 minutes.
The colors on the legend are the different echo intensities (reflectivity) measured in dBZ. "Reflectivity" is the amount of transmitted power returned to the radar receiver. Reflectivity covers a wide range of signals (from very weak to very strong). So, a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ), is used.
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. Depending on the type of weather occurring and the area of the U.S., forecasters use a set of rain rates which are associated to the dBZ values. These values are estimates of the rainfall per hour, updated each volume scan, with rainfall accumulated over time. Hail is a good reflector of energy and will return very high dBZ values. Since hail can cause the rainfall estimates to be higher than what is actually occurring, steps are taken to prevent these high dBZ values from being converted to rainfall.
NEXRAD (Next Generation Radar) can measure both precipitation and wind. The radar emits a short pulse of energy, and if the pulse strike an object (raindrop, snowflake, bug, bird, etc), the radar waves are scattered in all directions. A small portion 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 radar waves, time it took to travel to the object and back, and frequency shift of the pulse. The ability to detect the "shift in the frequency" of the pulse of energy makes NEXRAD a Doppler radar. The frequency of the returning signal typically changes based upon the motion of the raindrops (or bugs, dust, etc.). This Doppler effect was named after the Austrian physicist, Christian Doppler, who discovered it. You have most likely experienced the "Doppler effect" around trains.
As a train passes your location, you may have noticed the pitch in the train's whistle changing from high to low. As the train approaches, the sound waves that make up the whistle are compressed making the pitch higher than if the train was stationary. Likewise, as the train moves away from you, the sound waves are stretched, lowering the pitch of the whistle. The faster the train moves, the greater the change in the whistle's pitch as it passes your location.
The same effect takes place in the atmosphere as a pulse of energy from NEXRAD strikes an object and is reflected back toward the radar. The radar's computers measure the frequency change of the reflected pulse of energy and then convert that change to a velocity of the object, either toward or from the radar. Information on the movement of objects either toward or away from the radar can be used to estimate the speed of the wind. This ability to "see" the wind is what enables the National Weather Service to detect the formation of tornados which, in turn, allows us to issue tornado warnings with more advanced notice.The National Weather Service's 148 WSR-88D Doppler radars can detect most precipitation within approximately 90 mi of the radar, and intense rain or snow within approximately 155 mi. However, light rain, light snow, or drizzle from shallow cloud weather systems are not necessarily detected.
Echoes from objects like buildings and hills appear in almost all radar reflectivity images. This "ground clutter" generally appears within a radius of 25 miles of the radar as a roughly circular region with a random pattern. An mathematical algorithm can be applied to the radar data to remove echoes where the echo intensity changes rapidly in an unrealistic fashion. These "No Clutter" images are available on the web site. Use these images with caution; ground clutter removal techniques can remove some real echoes, too.
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.
Radar returns from birds, insects, and aircraft 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 miles of the radar and produce reflectivities of <30 dBZ.
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.
The Weather Underground maintains a NOAAPORT satellite dish which continuously ingests the Level III NEXRAD radar data directly from the National Weather Service Doppler radars. Included in the Level III NEXRAD data are the following products, all updated every 6 minutes if the radar is in Precipitation Mode or every 10 minutes if the radar is in Clear Air Mode:
- Base Reflectivity
- Composite Reflectivity
- Base Radial Velocity
- Storm Relative Mean Radial Velocity
- Vertically Integrated Liquid Water (VIL)
- Echo Tops
- Storm Total Precipitation
- 1 Hour Running Total Precipitation
- Velocity Azimuth Display (VAD) Wind Profile
This is a display of echo intensity (reflectivity) measured in dBZ. The base reflectivity images in Precipitation Mode are available at four radar "tilt" angles, 0.5°, 1.45°, 2.40°, and 3.35° (these tilt angles are slightly higher when the radar is operated in Clear Air Mode). A tilt angle of 0.5° means that the radar's antenna is tilted 0.5° above the horizon. Viewing multiple tilt angles can help one detect precipitation, evaluate storm structure, locate atmospheric boundaries, and determine hail potential.
The maximum range of the "short range" base reflectivity product is 124 nautical miles (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.
This display is of maximum echo intensity (reflectivity) measured in dBZ from all four radar "tilt" angles, 0.5°, 1.45°, 2.40°, and 3.35°. 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 "short range" composite 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.
This is the velocity of the precipitation either toward or away from the radar (in a radial direction). No information about the strength of the precipitation is given. This product is available for just two radar "tilt" angles, 0.5° and 1.45°. Precipitation moving toward the radar has negative velocity (blues and greens). Precipitation moving away from the radar has positive velocity (yellows and oranges). Precipitation moving perpendicular to the radar beam (in a circle around the radar) will have a radial velocity of zero, and will be colored grey. The velocity is given in knots (10 knots = 11.5 mph).
Where the display is colored pink (coded as "RF" on the color legend on the left side), the radar detected an echo but was unable to determine the wind velocity, due to inherent limitations in the Doppler radar technology. RF stands for "Range Folding".
The true wind direction can be determined on a radial velocity plot only where the radial velocity is zero (grey colors). Where you see a grey area, draw an arrow from negative velocities (greens and blues) to positive velocities (yellows and oranges) so that the arrow is perpendicular to the radar beam. The radar beam can be envisioned as a line connecting the grey point with the center of the radar. To think of it another way, draw the wind direction line so that the wind will be blowing in a circle around the radar (no radial velocity, only tangential velocity).
In order to determine the wind direction everywhere on the plot, a second Doppler radar positioned in a different location would be required. Research programs frequently use such "dual Doppler" techniques to generate a full 3-D picture of the winds over a large area.
If you see a small area of strong positive velocities (yellows and oranges) right next to a small area of strong negative velocities (greens and blues), this may be the signature of a mesocyclone--a rotating thunderstorm. Approximately 40% of all mesocyclones produce tornadoes. 90% of the time, the mesocyclone (and tornado) will be spinning counter-clockwise.
If the thunderstorm is moving rapidly toward or away from you, the mesocyclone may be harder to detect. In these cases, it is better to subtract off the mean velocity of the storm center, and look at the Storm Relative Mean Radial Velocity.
This is the same as the Base Radial Velocity, but with the mean motion of the storm subtracted out. This product is available for four radar "tilt" angles, 0.5°, 1.45°, 2.40°, and 3.35°.
VIL is the amount of liquid water that the radar detects in a vertical column of the atmosphere for an area of precipitation. High values are associated with heavy rain or hail. VIL values are computed for each 2.2x2.2 nm grid box for each elevation angle within 124 nm radius of the radar, then vertically integrated. VIL units are in kilograms per square meter--the total mass of water above a given area of the surface. VIL is useful for:
1) Finding the presence and approximate size of hail (used in conjunction with spotter reports). VIL is computed assuming that all the echoes are due to liquid water. Since hail has a much higher reflectivity than a rain drop, abnormally high VIL levels are typically indicative of hail. The following example shows high VIL levels associated with a thunderstorm that produced 1.5 inch hail and an F1 tornado near Lansing, Michigan in August 2003.
2) Locating the most significant thunderstorms or areas of possible heavy rainfall.
3) Predicting the onset of wind damage. Rapid decreases in VIL values frequently indicate wind damage may be occurring.
A handy VIL interpretation guide is available from the Oklahoma Climatological Survey.
The Echo Tops image shows 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. For very tall storms close to the radar, the maximum tilt angle of the radar (19.5 degrees) is not high enough to let the radar beam reach the top of the storm. For example, the radar beam at a distance 30 miles from the radar can only see echo tops up to 58,000 feet. The following example taken from Hurricane Claudette in 2003 shows this limitation.
Echo top information is useful for identifying areas of strong thunderstorm updrafts. In addition, a sudden decrease in the echo tops inside a thunderstorm can signal the onset of a downburst--a severe weather event where the thunderstorm downdraft rushes down to the ground at high velocities and causes tornado-intensity wind damage.
The Storm Total Precipitation image is of estimated 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 event.
The 1 Hour Running Total Precipitation image 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 statements.
The VAD Wind Profile image presents snapshots of the horizontal winds blowing at different altitudes above the radar. These wind profiles will be spaced 6 to 10 minutes apart in time, with the most recent snapshot at the far right. If there is no precipitation above the radar to bounce off, a "ND" (Non-Detection) value will be plotted. Wind are plotted in knots using the standard station model.
Altitudes are given in thousands of feet (KFT), and the time is GMT (5 hours ahead of EST). The colors of the wind barbs are coded by how confident the radar was that it measured a correct value. High values of the RMS (Root Mean Square) error (in knots) mean that the radar was not very confident that the wind it is displaying is accurate--there was a lot of change in the wind during the measurement.
The Storm Attributes Table is a NEXRAD derived product which attempts to identify storm cells.
The table contains the following fields:
- ID - This is the ID of the cell. The ID is also printed on the radar image to enable you to reference the table with storms on the radar image. If a triangle is shown in this field, it indicates NEXRAD detection of a possible tornadic cell (this "detection" is called the tornado vortex signature). If a diamond appears in this field, NEXRAD algorithms detect the storm is a mesocyclone. If a yellow-filled square appears, the storm has a 70% or greater chance of containing hail.
- Max DBZ - This is the highest reflectivity found within the storm cell.
- Top (ft) - Storm top elevation in feet.
- VIL (kg/m²) - Vertically Integrated Water. This is an estimation of the mass of water suspended in the storm per square meter.
- Probability of severe hail - Probability that the storm contains severe hail.
- Probability of hail - Probability that the storm contains hail.
- Max hail size (in) - Maximum hail stone diameter.
- Speed (knots) - Speed of the storm movement in knots.
- Direction - Direction of storm movement.
On the radar image, arrows show the forecast movement of storm cells. Each tick mark indicates 20 minutes of time. The arrow length indicates where the cells are forecast to be in 60 minutes.
When choosing the top 5 or top 10 storms from the "Show Storms" select box, the top storms are based on Max DBZ.
This should not be used for protection of life and/or property. It is a new feature, and currently in an experimental phase of integrating StrikeStar into The Weather Underground's NEXRAD Radar product.
StrikeStar is a network of Boltek lightning detectors around the United States and Canada. These detectors all send their data to our central server where the StrikeStar software developed by Astrogenic Systems triangulates their data and presents the results in near real-time.
Please note: Because of errors in sensor calibration and large distances between some sensors, lightning data may display skewed or be missing in certain regions.
If you have a Boltek detector and run Astrogenic's NexStorm software then we would like to hear from you. There are a small number of simple criteria you need to fulfill to join the network. You can email us at firstname.lastname@example.org for further details.
Figure 1. A Terminal Doppler Weather Radar (TDWR). Image credit: NOAA.
Figure 2. View of a tornado taken by conventional WSR-88D NEXRAD radar (left) and the higher-resolution TDWR system (right). Using the conventional radar, it is difficult to see the hook-shape of the radar echo, while the TDWR clearly depicts the hook echo, as well as the Rear-Flank Downdraft (RFD) curling into the hook. Image credit: National Weather Service.
Figure 3. View of a squall line (left) taken using a TDWR (left column) and a WSR-88D system. A set of three images going from top to bottom show the squall line's reflectivity as it approaches the TDWR radar, moves over the TDWR, than moves away. Note that when the heavy rain of the squall line is over the TDWR, it can "see" very little of the squall line. On the right, we can see the effect a strong thunderstorm with hail has on a TDWR. The radar (located in the lower left corner of the image) cannot see much detail directly behind the heavy pink echoes that denote the core of the hail region, creating a "shadow". Image credit: National Weather Service.
Figure 4. Typical errors seen in the velocity data (left) and reflectivity data (right) when range folding and aliasing are occurring. Image credit: National Weather Service.
Plymouth State College offers single-site radar images of all radar products going back 7 days.
A portion of this help file was supplied courtesy of The National Weather Service.