11  How Weather Radar Works

Before you can read radar, it helps to know what the radar is actually measuring. This chapter is a plain-language primer; later chapters dig into each product.

11.1 The basic idea

A weather radar (in the United States, the network of WSR-88D / NEXRAD sites) is a rotating dish that sends out pulses of microwave energy and listens for the tiny fraction that bounces back. Anything in the air that can scatter microwaves sends a little energy back to the antenna: raindrops, snow, hail, insects, birds, dust, even the boundary between two air masses.

From each returned pulse the radar measures three fundamental things:

  1. How much energy came back tells you how many and how large the scatterers are. This becomes reflectivity.
  2. How long the round trip took tells you the range (distance) to the target.
  3. How the wave’s phase shifted tells you whether the target is moving toward or away from the radar. This becomes velocity.

Modern radars are dual-polarization (“dual-pol”): they transmit both horizontally and vertically oriented pulses, which adds information about the shape and uniformity of the targets. That powers products like correlation coefficient and differential reflectivity.

11.2 Volume scans, tilts, and elevation angles

The antenna completes a 360° sweep, tips up slightly, sweeps again, and repeats, building a stack of cone-shaped “tilts” called a volume scan. Each tilt has an elevation angle (e.g. 0.5°, 0.9°, 1.5°…).

The lowest tilt (typically 0.5°) is the one you’ll look at most, because it samples closest to the ground. This single fact drives most of the limitations covered in Artifacts & Limitations.

Diagram of the radar beam rising above the curving Earth with distance

As the beam travels outward in a straight line, the Earth curves away beneath it, so the beam samples higher above ground with increasing range.
ImportantThe beam rises with distance

The radar beam travels in a nearly straight line while the Earth curves away beneath it. So the farther a storm is from the radar, the higher up the beam is sampling it. At 60 nmi the 0.5° beam is already roughly a kilometer above ground; at long range it can completely overshoot low-level features like a developing tornado or shallow snow.

11.3 Range, azimuth, and the radar’s point of view

Everything the radar reports is relative to its own location:

  • Range: distance outward from the radar.
  • Azimuth: compass direction from the radar.

This radar-centric geometry matters a great deal for velocity, which only measures motion along the beam (toward or away from the radar), never motion across it. Keep the radar’s position in mind when you interpret a scan. NeoWeatherwall shows you the active radar site so you always know where the beam is coming from.

11.4 What a single pixel really is

Each colored pixel you see is the average return from a small radar sample volume, a chunk of atmosphere that grows larger with distance. Near the radar a pixel might represent a volume a few hundred meters across; far away it can be several kilometers across. Fine structure visible close in gets smeared out at long range.

With that model in mind (energy back, time of flight, phase shift, sampled in rising cones of growing pixels), the individual products in the next chapters will make a lot more sense.