The motion of the wind, relative to the radar, is broken down into two components...
- the motion perpendicular to the radar beam and
- the motion along that radial (either directly toward or away from the radar).
All the radar "sees" is that motion along the path of the beam (the radial). This motion is NOT the direction of the wind but the portion of the wind's motion that is moving either directly toward or away from the radar. (The magenta arrows at right).
In the graphic at right, the wind is moving from south to north (indicated by the green arrows). North of the radar (light red shading) the wind is moving away from the radar. As the radar sweeps from position 1 to position 2, the beam becomes more and more inline with the overall wind flow.
As it does, the radar "sees" an increase in the radial velocity away from the radar. At position 2, the radial velocity is the same as the overall wind speed. Then the radial velocity begins to decrease as the radar sweeps to position 3.
At position 4 (and 8) the wind is blowing perpendicular to the radar beam. Since there is no motion toward or away from the radar, it "sees" zero motion. However, the wind IS NOT calm at these points as it is still blowing from the south. This is just the area of zero radial velocity.Positions 5, 6, and 7, in the green shading, are like 1, 2, and 3 in the light red shading except the wind is moving toward the radar. The greatest radial velocity is at position 6 where the wind is blowing directly at the radar.
These observed radial motions are vectors, meaning that the length of the arrows indicates the speed of the wind; the longer the arrow, the faster the speed. The Doppler radar calculates a velocity base on the length of these vectors and creates a color coded graphic for display. In these velocity graphics, red colors indicate wind moving away from the radar with green colors indication wind moving toward the radar.
Two Important Considerations
#1 - Know WHERE the radar is located in the image. The radial velocity, indicated by the colors, only has the proper meaning if you know how it is blowing relative to the position of the radar. Outbound winds on one radar might be inbound winds at an adjacent radar.
In this example (above left) the green colors show the inbound wind with the red colors indicating outbound wind. The brightest colors (the greatest radial velocities = the strongest winds) are located at the center of the image on either side of the radar. Therefore, in this image the wind is blowing from the northeast to the southwest.
#2 - The radar beam increases in elevation as it travels farther and farther from the radar. The observed wind information, while near the ground close to the radar, might be at 20,000 feet or more above the ground at large distances from the radar. This increasing elevation affects all radar images but is especially important when investigating the wind flow.