• What does the key on a Velocity image mean?
• What Is Velocity Data And How Is It Used?
• How Can I Determine The Actual Wind Direction From a Velocity Image?
• What Does The Purple Color ("RF") Indicate?
• How Can Base Velocity Data Be Used to Detect Rotation in Thunderstorms?

What does the key on a Velocity image mean?

AccuWeather provides access to Base Reflectivity Tilt 1 images for the entire network of 143 NEXRAD Doppler Radars in the Continental US and San Juan, Puerto Rico. The format of the key on the right of the images is as follows:

Doppler Radar
Product (Base Velocity)
Date / Time
Site ID / Range
Tilt (Elevation Angle - 0.5 degrees)
Radar Mode
Velocity Key (Positive = Towards, Site Negative = Away)
Maximum Velocity (in knots)

What Is Velocity Data And How Is It Used?

This product shows radial wind speeds, that is wind speeds towards and away from the radar along the radar beam. The negative values (down the left side of the radar graphic key) represent wind speeds towards the radar and the positive values (down the right side of the key) represent wind speeds away from the radar. Wind speeds are represented in knots. Multiply the wind speed in knots by 1.15 to arrive at the corresponding wind speed in miles per hour.

As with Base Reflectivity data, Radial Wind Velocity data is gathered at all elevation angles ( tilts ) surveyed in each volume scan, but only four of those elevation angles, Tilts 1-4, are available to users outside the National Weather Service. Table 6I below shows how the elevation angles correspond to each tilt number, and how they correspond to the mode of the radar. These are the elevation angles planned for most NEXRAD sites. Elevation angles at some locations with unique geography may differ somewhat.

For example, for Tilt 1, the radar makes one 360 degree sweep at 0.50° above the local horizon and the wind speed data gathered from this sweep is what is displayed on the Radial Wind Velocity, Tilt 1 product. The elevation angles corresponding to Tilts 2-4 are listed in the table above. These are the same elevation angles as those used for the Base Reflectivity , Tilts 1-4 products.

Because the NEXRAD Doppler radar measures only radial wind speeds (wind speeds towards and away from the radar along the radar beam) the component of the wind that is perpendicular to the radar beam cannot be measured at all. NEXRAD only measures the component of the wind that blows along the radar beam either toward or away from the radar site. The boundary between the gray colored inbound velocities of 0 to 10 knots and the white outbound velocities of 0 to 10 knots is the zero wind speed line. Recognizing this boundary is the key to interpreting NEXRAD Radial Wind Velocity data.

Zero knot wind speeds on NEXRAD base velocity data are caused by one of two things. Either the wind speed is actually zero knots (no wind) or the wind direction is perpendicular to the radar beam at that location and is thus not detectable by the NEXRAD Doppler radar. When looking at Radial Wind Velocity data, it is important to remember that as you look farther away from the radar site in any direction, the radar beam is looking higher in the sky. Close in to the radar site, the radar beam is looking very near the Earth's surface. However, due to the curved surface of the Earth, the Earth literally runs out from underneath the radar beam as you get farther away from the radar site, the result being that the farther from the radar site you go, the higher up in the atmosphere you are looking. Refer to Figure 4 to find the corresponding range and height coordinates for each Base Velocity tilt angle. (These are the same as for Base Reflectivity.) These range distances can be drawn as concentric circles on the display, each with a radius corresponding to the ranges listed above. Everything appearing on the base velocity product in the vicinity of a range ring of this type is at the same height above the ground. For example, on the Tilt 1 product, everything appearing on the display at a range of 30 miles from the center of the image (the radar site) in all directions (at a 30 mile range ring) is occurring at 2,000 ft.

Figure 4: Base Velocity Tilt Numbers and Range Distance

Base Reflectivity Tilt #	Height at 30 miles	Height at 60 miles	Height at 90 miles	Height at 120 miles	Height at 143 miles
Tilt 1	2,000 ft.	5,000 ft.	8,000 ft.	13,000 ft.	16,000 ft.
Tilt 2	5,000 ft.	10,000 ft.	16,000 ft.	24,000 ft.	28,000 ft.
Tilt 3	8,000 ft.	15,000 ft.	24,000 ft.	35,000 ft.	40,000 ft.
Tilt 4	11,000 ft.	20,000 ft.	32,000 ft.	46,000 ft.	52,000 ft.

How Can I Determine The Actual Wind Direction From a Velocity Image?

To interpret the proper wind direction at any location on the display, follow these steps.
1) Draw a straight line from the radar site (located at the center of the image) to any point along the zero wind speed line (the boundary between the gray and white colors on the display)
2) Draw an arrow perpendicular to the line drawn in Step 1 at the point on the zero wind speed line to which you drew the line in Step 1, the arrow head pointing from inbound velocities to outbound velocities.
3) This arrow represents the wind direction at that point on the display.
The true wind speed is represented by the maximum inbound and outbound velocities found on the display at the range your arrow is drawn.

What Does The Purple Color ("RF") Indicate?

Radial Wind Velocity data may be missing in places on any given display if there are no scatterers in the atmosphere to return the radar's signal from which the velocity data is derived. There needs to be either precipitation, cloud cover, smoke, sea spray or some particle in the atmosphere large enough to reflect the radar's signal back to the radar in order to get wind speed data. This is why when precipitation is scattered across an area, velocity data will only appear in the areas where the precipitation is located and will be missing in other areas. For this reason, it is very useful to compare what is seen on a given Wind Velocity display with what is seen on the Base Reflectivity display of the same tilt angle. RF , the purple color in the key, stands for Range Folding or bad data. In these areas, the radar detected something but was unable to detect the true wind speed or was unable to determine the proper location of the wind speed that was detected. Rather than display misinformation, the radar ignores the data and replaces it with range folding. This is not a flaw in the design of the NEXRAD Doppler radar, rather it occurs as a by-product of the operational characteristics of the radar.

How Can Base Velocity Data Be Used to Detect Rotation in Thunderstorms?

Circulations, small scale rotations, within thunderstorm can be detected using NEXRAD Doppler radar Base Velocity data. Thunderstorm rotations that persist over several volume scans, extend vertically throughout the storm (i.e. circulations that can be detected at more than one tilt angle of the Base Velocity products, say at both tilts 1 and 2) and are of sufficient strength are called mesocyclones . Thunderstorms with mesocyclones frequently produce severe weather, consisting of large, damaging hail (technically hail greater than three-quarters of an inch in diameter), high, damaging winds (technically wind speeds greater than 50 knots or 57 miles per hour) or tornadoes. Only about 40 percent of all rotating thunderstorms ever produce tornadoes. Base Velocity data should always be used in conjunction with Base Reflectivity, Vertically Integrated liquid, Echo Tops, and other NEXRAD and weather data products in determining the severity of individual thunderstorms.

Rotations are identified in AccuWeather's Base Velocity data by a very small area of strong inbound velocities (dark green or dark blue) right up against a very small area of strong outbound velocities (dark orange or red). The inbound and outbound velocities are usually oriented such that the rotation is cyclonic or counterclockwise. So for a thunderstorm that existed to the north of a radar site, the left side of the circulation would show strong inbound velocities, and the right side of the circulation would show strong outbound velocities.