Weather radar

The antenna sends a short burst of microwave energy in a narrow beam. Airliner weather radars use X-band (about 9.3 GHz, wavelength ≈ 3.2 cm). Raindrops and hailstones scatter a tiny part of it back. The delay gives the range: range = c × t ÷ 2, which works out to 12.36 µs of round-trip time per nautical mile. The antenna sweeps left and right and every sweep refreshes the picture, which is why the scope shows a moving sweep line and fading echoes.

Echo power rises steeply with drop size (roughly with the sixth power of the diameter), so heavy rain and wet hail return far more than cloud droplets. Reflectivity is measured in dBZ and shown in colours. Typical scheme: green light rain (≈ 25–35 dBZ), yellow moderate (35–45), red heavy (45–52), magenta very heavy rain or hail (above 52 dBZ), and in WX+T mode also turbulence. Cloud without precipitation, clear air and dry snow give little or no echo.

Beamwidth ≈ 70 × λ ÷ D degrees, so a 24 in antenna gives about 3.7° (change the antenna size to see it). At 100 nm that beam is about 6.5 nm across, nearly 40 000 ft tall. Beam centre height ≈ aircraft altitude + range × tan(tilt) − range² ÷ 1.5 (ft, range in nm), and the last term is the Earth's curvature. Tilt too high: the beam passes over the tops of storms, so they look weak or vanish (overscanning). Tilt too low: the lower edge hits the ground and patches of ground clutter appear. A common technique is to lower the tilt until a little ground clutter shows at the top of the display, then raise it slightly. Watch the side view below the scope to see this.

X-band energy is absorbed by heavy rain. A strong cell therefore weakens the beam behind it, and a storm hiding behind another one can look small or be missed completely (a radar shadow). Switch "Rain attenuation" off to see the difference. Take any red cell as a warning that there may be something worse behind it.

Doppler processing measures how much the speed of the raindrops varies inside the beam. Strongly varying speeds mean turbulence, shown in magenta. It only works within about 40 nm and only where there is precipitation, so it does not detect clear-air turbulence.

Range sets the scale. Tilt aims the beam (see 3). Gain raises or lowers the displayed reflectivity: calibrated (0 dB) is normal, and extra gain can turn green areas yellow or red. WX / WX+T / MAP switch between weather, weather plus turbulence, and ground mapping, where the ground returns are shown in cyan, yellow and magenta (water, land, built-up areas). Real radars also have auto-tilt, multi-scan and predictive windshear functions.

Common guidance is to stay at least 20 nm clear of severe cells (red and magenta, especially with sharp colour gradients), never fly under an anvil, and expect hail to be thrown well downwind. Decide on a deviation early, while the storm is still 40–80 nm away. Remember that radar shows precipitation, not lightning, and must not be used on the ground near people or fuelling.

It cannot see cloud without rain, clear-air turbulence, lightning or dry volcanic ash. Ground clutter can look like weather, attenuation hides storms behind storms, small cells at long range are diluted by the wide beam, and rain or ice on the radome weakens the signal.