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Radar Terminology
What do the colors mean?

 

Clear Air dBZ Scale Precipitation dBZ Scale

The colors are the different echo intensities (reflectivity) measured in dBZ (decibels of Z) during each elevation scan. "Reflectivity" is the amount of transmitted power returned to the radar receiver. Reflectivity (designated by the letter Z) 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.

 dBZ   Rainrate 
(in/hr)
65 16+
60 8.00
55 4.00
52 2.50
47 1.25
41 0.50
36 0.25
30 0.10
20 Trace

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 (far left) represents dBZ values when the radar is in clear air mode (dBZ values from -28 to +28). The other scale (near left) represents dBZ values when the radar is in precipitation mode (dBZ values from 5 to 75). Notice the color on each scale remains the same in both operational modes, only the values change. The value of the dBZ depends upon the mode the radar is in at the time the image was created.

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 rainrates 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.


Radar Terminology

Clear Air Mode - (dBZ values from -28 to +28)
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. Note that radar can also detect insects or birds.

Precipitation Mode - (dBZ values from 5 to 75)
When rain is occurring, the radar does not need to be as sensitive as in clear air mode since rain provides plenty of returning signals.

Base Reflectivity (short range)
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).

Composite Reflectivity (long range)
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.

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.

One-hour Precipitation
This is an image of estimated one-hour precipitation accumulation on a 1.1 nm by 1 degree grid. 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 product will not display accumulated precipitation more distant than 124 nm, even though precipitation may be occurring at greater distances.

Storm Total Precipitation
This 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 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.

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.

dBZ levels
correspond to levels of precipitation intensity in terms of inches of rainfall per hour. The higher the dBZ level, the more intense the precipitation; they are directly proportional.

10 DBZ - Very light rain or snow
20 DBZ
-
Drizzle or light rain or snow
30 DBZ -
Light to moderate rain (about 0.2 cm/hr or 1/10 inch/hour) or snow
40 DBZ -
Moderate to heavy rain (about 1 cm/hr or 1/2 inch/hour) or snow
45 DBZ -
Heavy rain (about 2.5 cm/hr or one inch/hour), or snow
50 DBZ -
Very heavy rain or thunderstorms (about 6.0 cm/hr or two to three inches/hour), or snow; hail possible
55 DBZ -
Very heavy rain and hail, some large - severe thunderstorms

Radar information from the National Weather Service

 



US Forecasts are Public Domain and derived from the
National Weather Service - (IWIN) and the National Oceanic Atmospheric Administration (NOAA)
Canadian forecasts are obtained from and are copyright of Environment Canada.

All weather information provided by MarineWaypoints.com should be used with caution.
We cannot guarantee that the data provided here is 100% accurate or up to date!


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