Weather radar

Weather radar systems made simple

Tim Takeoff
5 pictures
6 minutes

As everyone knows, the aeroplane is the world’s safest means of transport. There are, nevertheless, events that can jeopardise flight safety, with unforeseen occurrences being among the most serious hazards. One of the biggest issues is weather, which is why pilots use on-board weather radar systems to support their decision-making processes. But how do such systems work?

The world’s weather is a continuous global process. A wide range of high and low pressure areas continuously create an incredible diversity of cloud patterns and precipitation events. Aviators, of course, are excited to see clear blue skies, which make for perfect flying conditions. In commercial aviation, however, there are fixed schedules which the weather, unfortunately, does not obey.

The greatest dangers for an aircraft in the air are, of course, the extremes. In particular, strong windshear and downdraughts can create major problems, especially around storms. Precipitation in itself is not a hazard – until it’s combined with low temperatures, high wind speeds and turbulence. During the day, these “regular” storms – known in weather parlance as cumulonimbus – can usually be detected with the naked eye and simply flown around.

At night or in low visibility, however, the pilot’s visual perception is no longer sufficient. Sometimes storms also become integrated into other cloud layers. In such cases, they are known as “embedded cumulonimbus”. When these arise, it is no longer possible for the naked eye to distinguish between a harmless “dry” cloud and a highly developed storm cell. The latter usually combine extreme updraughts and downdraughts with strong turbulence and the risk of ice.

Nose cones as radar windows

In order to detect such storm cells, all aircraft have built-in weather radar systems. These are usually located in the aircraft’s nose cone, in what is known as the radome. Even if the bulk of the aircraft is made of metal, the radome is constructed of a specialised plastic. This works like a kind of “window”, enabling radar waves to pass unobstructed through it.

The weather radar itself emits special microwaves in what is known as a radar beam. These reflect off obstacles, including raindrops, hail, ice, snow and the surrounding landscape. A radar receiver captures these reflections and directs them to one of two computer systems. These operate independently from one another to ensure redundancy.

It all starts with a water droplet

As clouds are made up of collections of water droplets, they have reflective surfaces. As well as detecting the presence of water droplets, a Doppler radar is used to measure their movements. If increased turbulence is present, this becomes “visible” due to marked changes in the speed and direction of the raindrops’ movement. The weather radar can detect these and provide the pilot with a detailed image of the intensity, size and direction of the storm.

The radar therefore scans the skies ahead of the aircraft for such anomalies. It searches both horizontally and vertically. As radar systems cannot search along the curve of the earth’s surface, but only in straight lines, the radar beam emitted is constantly adjusted. It can be set several degrees higher or lower using the “tilt” function. This is essential during takeoff and landing, where the aircraft is tilted at a much higher or lower angle than during cruising.

Microwaves allow greater visibility

In addition to the radar beam’s direction, its strength is crucial in obtaining a clear picture. This is not always easy to obtain, however, due to the droplets in the clouds themselves. An ice crystal – like that, for example, which is contained at high altitudes in the familiar cirrus clouds – has a much smaller reflective surface than a large raindrop, or even a hailstone. The weather radar must react accordingly and increase the intensity (or “gain”) appropriately for very dry clouds. The gain must, on the other hand, be reduced in cases of larger storm fronts in order to ensure a clear image of the weather situation ahead.

Things can get problematic when one large moist cloud conceals another. In such cases, the microwaves are reflected off the cloud in front, while the other remains hidden in a kind of “radar shadow”. Pilots must always be prepared for such instances.

Turbulent affairs – windshear

Weather radar systems are even capable of detecting strong windshear. This is distinguished by rapid, dramatic changes in wind direction and strength and is particularly dangerous during takeoff and landing, when the aircraft is flying at lower speeds. A sudden strong tailwind would immediately and drastically reduce the plane’s airspeed (the speed relative to the air around it). By the same token, an extremely strong gust from the front would push the plane to its mechanical limits. In order to prevent such incidents, the weather radar also detects windshear. The windshear detection function and the autopilot then work together, with the autopilot using the output from the weather radar and other available information to guide the plane safely through the windshear.

User-friendly information

The radar image received is ultimately evaluated by a complex software program which is capable of filtering out ground reflections to give the pilot a realistic assessment of the clouds in front of and below the aircraft. Storms often build up under the plane and quickly grow upwards. The weather radar continues to update the navigation display approximately every four to ten seconds in order to provide the crew with information, a process which is usually fully automated. The pilot can, however, manually access the weather radar at any time. He or she can also control the radar’s tilt and gain functions separately in order to generate a different image or to gather additional information.

Effect on the flight path

Using the information collected, the pilot and air traffic control staff work together to select a new route. The sooner the pilot can determine the need for a change of route, the less of a detour he or she will have to make. Night flights require particularly rapid reactions in such cases.

In the best-case scenario, passengers remain unaware of the situation. Often all you will see is a storm in the distance, one which the crew has systematically and successfully skirted. It is only the aircraft’s state-of-the-art equipment, like weather radar systems, which keep it in pole position as the world’s safest mode of transport. After all, there will always be some things for which humans will need a little help from technology.

Want to know more about aircraft radar? Read our article on in-flight radar.

Pictures © Tim Takeoff

by Tim Takeoff

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