Radial velocity is the component of a target's motion measured directly along the line between the radar and the target, positive when the target is moving away from the radar and negative when it is moving toward the radar.
How the radar measures it
A Doppler radar transmits pulses at a known frequency, and the frequency of the return is shifted slightly by the motion of the target relative to the antenna. Targets moving toward the radar compress the returning wave and shift its frequency up. Targets moving away stretch the wave and shift it down. The radar processor converts that Doppler frequency shift into a wind-component estimate for each range gate and displays it on a velocity image, with a color convention that in most operational systems puts inbound motion in cool colors and outbound motion in warm colors.
The critical limitation is geometric. The radar only measures the component of motion aligned with the beam. A gust of wind blowing at a right angle to the beam produces zero Doppler shift no matter how strong it is. Every velocity image is a projection of the true three-dimensional wind onto the radial direction, and interpretation always has to account for that.
How forecasters read it
The most common read is convergence and rotation. A boundary shows up as a line where inbound velocities on one side sit next to outbound velocities on the other, indicating winds meeting. Rotation shows up as a small couplet of adjacent inbound and outbound values, marking a local shift in wind across a short distance. In supercells that couplet is the mesocyclone signature that anchors most warning decisions.
Radial velocity also gives an estimate of storm-scale winds along the beam. Straight-line damaging winds along the leading edge of a bow echo are read directly off the inbound velocities pointed toward the radar. Downburst signatures show up as small regions of divergent flow near the surface, with inbounds and outbounds arranged around a common point.
Important limits
Beyond the perpendicular-beam blind spot, radial velocity is limited by aliasing. Each radar has a maximum unambiguous velocity set by its pulse rate, commonly around 25 to 30 meters per second at a long-range scan. Real winds above that limit fold back into the image with the wrong sign, producing sharp velocity discontinuities that look like rotation but are actually a processing artifact. Modern radars use dealiasing algorithms, and forecasters learn to spot the residual fold lines that still appear.
Range folding is a second limit. When a distant echo returns after the next pulse has already gone out, the radar cannot tell how far away it really is and often masks that gate on the velocity display. Non-meteorological scatterers such as birds, insects, and ground clutter also contaminate the velocity field. Reading radial velocity well means always tracking which pixels are trustworthy, which are aliased, and which are simply masked out.