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Radar Frequently Asked Questions

Why does the image show rain and there is no rain in the area?

Most likely the radar station is in the clean air mode and is echoing off of buildings, trees, and other local obstructions. In the clean air mode, the radar is very sensitive and can pick up a flock of birds or even jet airplanes.
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.
Clear Air Mode

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.

The radar continuously scans the atmosphere by completing volume coverage patterns (VCP). A VCP consists of the radar making several 360 scans of the atmosphere, sampling a set of increasing elevation angles. There are two clear mode VCPs.

In clear air mode, the radar begins a volume scan at the 0.5 elevation angle (i.e., the radar antenna is angled 0.5 above the ground). Once it makes two full sweeps (a surveillance/reflectivity sweep and a Doppler/velocity sweep) at the 0.5 elevation angle, it increases to 1.5 and makes two more 360 rotations. For one of the clear air mode VCPs, two full sweeps are also made at 2.5. Otherwise, at the higher elevations (2.5, 3.5, and 4.5) a single sweep is made (reflectivity and velocity data are collected together).

This process is repeated at 2.5, 3.5, and 4.5. Then the radar returns to the 0.5 elevation angle to begin the next volume scan which will repeat the same sequence of elevation angles. In clear air mode, the complete scan of the atmosphere takes about 10 minutes at 5 different elevation angles.

Precipitation Mode

When precipitation is occurring, the radar does not need to be as sensitive as in clear air mode as rain provides plenty of returning signals. At the same time, meteorologists want to see higher in the atmosphere when precipitation is occurring to analyze the vertical structure of the storms. This is when the meteorologists switch the radar to precipitation mode using one of two volume coverage patterns.

Both precipitation VCP's begin like the clear air mode mentioned above with the same evaluations scans as in the clear air mode. The difference is the radar continues looking higher in the atmosphere, up to 19.5 to complete the volume scan. The time it takes to complete the entire volume scan is also less. In the slower VCP, the radar completes the volume scan of nine different elevations in six minutes. In the faster VCP, the radar completes 14 different elevation scans in five minutes.

Differences in the quality of radar images between the two precipitation mode VCPs are relatively minor. Therefore, during severe weather, the faster VCP is almost always used as it provides the meteorologists with the quickest updates and most elevation slices through the storms.

In summary, when the radar is in clear air mode, radar images will be updated approximately every ten minutes. In precipitation mode, the updates will occur around five to six minutes apart.

What do the colors mean in the reflectivity products?

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.
(inches per hour)
6516+Extreme thunderstorms, possible hail
608.00Very heavy thunderstorms, possible hail
554.00Heavy thunderstorms
471.25Very heavy rain or sleet
410.50Heavy rain or sleet
360.25Moderate rain or sleet
300.10Light rain or moderate snow
20TraceVery light rain or snow

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.

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.

The radar image looks very strange and I am sure it is not ground clutter. What is it?

Most likely the image you are seeing is an anomaly such as a starburst, nuclear blast, or other similiar image. Anomalies are quiet common and usually due to some kind of interference, equipment malfunction, or radar operator error.

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