Radar, in contrast, is hardly new; it’s been around since early in World War II, when the German military began using it to detect Allied fighter aircraft swarming off Great Britain to intercept Nazi bombers headed across the English Channel. Radar is an acronym which stands for RAdio Detection And Ranging, and the modus operandi is relatively simple. Electrical energy in the form of radio waves is trans- mitted by a directional antenna, and some of that energy that strikes reflective objects is returned to the antenna, which is now function- ing as an energy receiver. “Reflective objects” include the surface of the earth, buildings, mountains, and precipitation — that’s the impor- tant reflector for weather avoidance. The return is displayed as a bright area on a cathode-ray tube in the cockpit.
While this isn’t the place for a detailed electronic explanation, imagine the speed of radar’s outgoing radio waves slowed to a crawl; when the antenna emits a burst of energy, the transmitter portion shuts down, turns the antenna into a receiver, and waits a predeter- mined time for an “echo” to return. If something reflects a portion of that energy, the returning radio wave is processed electronically and shows up on the radar scope, which is overlaid with a grid to indicate direction and distance from the aircraft. If nothing is reflected, the radar set switches back to the transmitter mode and repeats the cycle. In reality, all of this happens continuously and at a mind-boggling rate so that an uninterrupted display is provided. It’s analogous to a light bulb powered by the 60-cycle alternating current in your home; the light is blinking on and off 60 times each second as the current reverses, but it’s happening so rapidly that you perceive a steady glow.
A radar antenna sweeps back and forth — left-right, left-right — usually in a 90-degree arc with the airplane position at the apex. As echoes are “painted” (displayed on the scope), they remain illumi- nated through at least one antenna sweep so that you see a constant, updated display; the shape of an area of reflective objects is therefore easy to see.
Radar incorporates a third dimension of weather investigation; in addition to azimuth and several selections of range, the antenna can be tilted up or down as it sweeps and can “look” for reflective objects above and below the aircraft’s flight level.
Atmospheric turbulence is generally associated with high rates of rainfall and large raindrops, both of which are characteristics of 150
thunderstorms. Because water reflects radio energy very well, and because a weather system producing large amounts of rainfall can normally be expected to produce large amounts of turbulence as well, radar systems indicate the “severity” of echoes (in terms of potential airplane problems) in shades of green or by using several colors.
For example, if there are no reflective objects in the “field of vision” of your radar set, the screen will be blank; but let’s generate a cumulonimbus cloud directly ahead, a cloud that is producing light rain. This precipitation would show up as a light green echo, its shape/azimuth/range clearly defined. Imagine this cloud growing to the point at which a core of heavier rainfall develops within the storm. When the reflectivity of the core rainfall increases to a preset level, the radar displays it in a brighter green, or perhaps yellow (for caution) in a multicolor system. There’s usually a third level (even brighter green or red for warning) to indicate the highest preset reflectivity.
Some radar systems incorporate “contouring,” which means that when the rate of rainfall increases rapidly over a relatively short hor- izontal distance the radar “closes its eyes” to that area. Should you be flying toward that storm mentioned earlier, and if the difference in rainfall intensity between the light rain and the core is increasing at a preset rate, the core would appear as a black hole (same as no return) on the radar scope. Pilots unaware of the significance of contouring might plunge directly into the heart of the thunderstorm, thinking they were headed for a no-rain, nonturbulent area. Surprise!
To call your attention to those weather systems that contain strong cores of precipitation and which therefore must be avoided, most radars cause contouring areas to flash on and off (in either green or red) as an additional warning. While some areas of precipi- tation displayed on a radar scope present no problem to most pilots in most airplanes, a “contouring” echo is all but guaranteed to rattle your teeth; you wouldn’t like it one bit in there. The probable pres- ence of very strong updrafts and downdrafts in close proximity to each other is the essence of life-threatening turbulence.
In the absence of very strong storms, radar will normally be capable of pointing out precipitation well in advance of the airplane because some of the electrical energy passes through to be reflected by another rain area up ahead. But can you imagine a big storm straight ahead that is producing so much rain that all of the radar’s energy is reflected back to the antenna? You’d see a very bright echo to be sure, but what you wouldn’t see is the big storm lurking on the other side. This characteristic is called “attenuation” and is a short- coming of many older, less expensive units (some current-generation radars include circuitry that overcomes attenuation, or at least warns
Chapter Eleven
the observer of potential trouble behind the next echo). If nothing else, just knowing that attenuation exists should make it clear that you must have some idea of the extent and severity of a weather sys- tem before putting all your weather-avoidance trust in what you see on the scope.
There is a great deal of interpretation required of the pilot to extract useful, safe information from a weather-radar display. It’s much more than a turn-it-on-and-take-a-look system because the pic- ture changes remarkably with range selection, antenna tilt, airplane altitude and attitude, and storm characteristics. If radar is to be used with safety and efficiency, some well-founded training beyond read- ing the manual is required. There are frequent weather-radar classes and workshops around the country; your best source of information in this regard is the manufacturer.
Airborne weather radar and passive storm detectors are some- times placed in a competitive posture, but any attempt at comparison soon turns into an apples-and-oranges standoff. These two weather detection systems operate on totally different principles and present totally different displays. However, improvements in technology and miniaturization have resulted in on-board weather-detection systems that combine the information from both types of detectors in one dis- play, which is truly the best of both worlds. And if you’d like to spend even more money to stay out of turbulence and storms, you can buy weather-radar systems that use another color (usually magenta) to indicate the presence of turbulence as detected by the movement of reflective particles in the atmosphere. Still haven’t flattened your wal- let? You might look into one of the on-board radars that lets you push a button and look at a vertical slice of a storm; that’s as close to a three-dimensional picture as you’re likely to get.
Wise pilots avoid threatening weather (heavy precipitation and/or potential turbulence) that shows up on the equipment they have to work with. And the key word in that last sentence is avoid. They haven’t yet built a weather-detection system good enough to pick your way through a squall line or to decide with confidence which of those storms up ahead is safe to penetrate. There’s always another route or another day; if you become a faithful follower of the weather-avoidance religion, your chances of becoming an old pilot increase significantly.
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