How Airborne Weather Radar Shows Storms Ahead
An airliner's nose radar sends microwave pulses at rain and hail and paints the echoes in colour. Here is what it shows, what it misses, and how pilots allow for that.
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Behind the nose cone of most airliners sits a small radar that looks ahead for rain. It sends out short pulses of radio energy, listens for the echoes that bounce back from water in the air, and shows the strength of those echoes as coloured patches on a cockpit screen. The main point is that it detects precipitation, not turbulence itself, so pilots read it as a guide to where the worst weather probably is rather than as a perfect map.
The basic idea: pulse and echo
Airborne weather radar works like other radar. A transmitter sends a very brief pulse of microwaves through an antenna. Raindrops and wet hail scatter a little of that energy back. The set measures how long the echo takes to return, which gives distance, and how strong it is, which gives an idea of how intense the precipitation is. The Wikipedia article on weather radar describes ground-based radars that use pulses about one microsecond long and wavelengths of 1 to 10 centimetres, because that range suits detecting droplets. The aviation article on the Aerocorner site says most airborne sets use X-band, a wavelength of about 3 centimetres, which allows a compact antenna that fits in a nose.
The antenna sweeps from side to side so the screen shows a fan-shaped view ahead of the aircraft. SKYbrary, the aviation safety knowledge base, notes that the antenna is typically in the nose and that a computer turns the signals into colour-coded patterns for the pilots.
What the colours mean
Colours rise with echo strength. Wikipedia's weather radar article gives a three-level aviation scale: level 1 (green) usually means light precipitation, level 2 (yellow) means moderate precipitation and possible moderate turbulence, and level 3 (red) means heavy precipitation with possible thunderstorms and severe turbulence. According to that article, aircraft try to avoid level 2 where they can and always avoid level 3. Aerocorner adds that modern sets may use magenta to mark turbulence detected through Doppler shifts.
- Green: light rain
- Yellow: moderate rain, some bumpiness possible
- Red: heavy rain, thunderstorms and severe turbulence possible
- Magenta (on some sets): turbulence flagged by Doppler measurement
Echo power grows very quickly with drop size. Aerocorner states that it rises with the sixth power of the drop's diameter, so a small increase in drop size makes a large jump in the returned signal. That is why big drops and wet hail stand out sharply.
Doppler and wind shear
SKYbrary says modern airborne weather radars are mostly Doppler radars. A Doppler radar compares the signal of successive pulses to find how fast the targets are moving toward or away from the aircraft. This lets the set pick out strong air movement inside a storm, and SKYbrary adds that some systems can predict wind shear, a sudden change of wind speed or direction over a short distance. Wikipedia's article explains the same principle for ground radars: an inbound and outbound velocity pair side by side is a sign of strong wind changes such as downbursts.
What the radar misses
Because the radar sees liquid water, several things can fool it. Aerocorner lists these limits:
- It does not detect clouds, fog, snow or dry ice crystals well, so frozen storm tops can fade from the display, especially if the antenna is tilted too high.
- Heavy rain absorbs the beam, which can leave a 'radar shadow' that looks clear but may hide more weather behind it.
- The beam is wide, so two nearby cells can merge into one patch until the aircraft gets closer.
- Doppler turbulence detection relies on precipitation, so it cannot find clear-air turbulence.
Wikipedia makes a related point about attenuation: shorter wavelengths resolve small particles but are weakened faster as they pass through heavy rain, which is why ground services prefer longer wavelengths where they can afford them.
Tilt and keeping a margin
Pilots can tilt the antenna up or down to sample different heights. Aerocorner advises tilting up and down from time to time, because one fixed angle can miss the top or the base of a storm. It also says that hazardous turbulence may extend up to 20 miles from the edge of an echo, and gives guidance of avoiding severe echoes by at least 20 miles and passing between two of them only if they are at least 40 miles apart. These are figures from that article; operators set their own procedures.
Pilots also use other sources. The same article says air traffic control precipitation data can be up to 6 minutes old, and ground radar pictures sent by datalink can lag by 5 to 15 minutes or more, so they show past conditions, not the present. Airline dispatchers can pass on new storm information during a flight.
Is it required?
Rules vary by region. SKYbrary reports that the European operating rule EU-OPS 1.670 requires airborne weather radar on pressurised aeroplanes, and on certain unpressurised ones, when flying at night or in cloud where thunderstorms or other radar-detectable hazards may be expected along the route. Small propeller-driven pressurised aeroplanes up to 5,700 kg with no more than nine passenger seats may use other approved equipment instead.
A recent reminder of the limits
Aerocorner also mentions a June 2024 case in which an Austrian Airlines A320 flew into hail that its radar did not show. According to that article, no final report had been published as of February 2026, so the cause is not settled and should not be assumed.
In short, weather radar turns echoes from rain and hail into a picture of where the heaviest weather lies. It is powerful, but it sees only part of the sky, so pilots combine it with forecasts, ground information and generous distances. This is general information, not operating guidance.
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