Airborne Weather Radar — Page 208, Lesson 198

I want to walk you through the airborne weather radar now. This is a big one, because it's the primary tool you'll use to thread your way through convective weather. Let's start with the fundamental question: why does the radar use the frequency it does? The optimum radar frequency is one that has a wavelength comparable to the size of the objects we wish to detect. And what are we trying to detect? Not the cloud itself, but the large water droplets and wet hail that are associated with severe turbulence. Those droplets are about 3 centimetres across. So the radar is tuned to see things that size. The typical frequency adopted by most commercial systems is 9375 MHz, plus or minus 30 MHz. That frequency produces the best returns from those large water droplets and wet hail found in convective clouds. It also lets us produce narrow, efficient beams. Let's work out the wavelength. The formula is wavelength, lambda, equals 300 divided by the frequency in MHz, giving metres. So 300 divided by 9375 gives us 3.2 centimetres. That matches the droplet size we're after. Now, why not go higher or lower? A frequency higher than 9375 MHz would produce returns from smaller droplets and cause unnecessary clutter on the display. A lower frequency would fail to produce sufficient returns to highlight the area of turbulence. So 9375 MHz is the sweet spot. And just to give you a sense of the beam geometry, at 3 degrees of tilt, the beam covers different altitudes at different ranges — at 0 nautical miles you're looking at 0 feet, at 30 NM you're covering 9000 feet, at 80 NM it's 24,000 feet, and at 180 NM it's 54,000 feet. That's the radar beam coverage at varying ranges. Now, there's a practical problem you need to know about: water and ice in the radome. The radome is the protective cover over the antenna. Some of the radar wave energy is absorbed by water and ice — exactly the same principle as a microwave oven. If there's water inside the radome, or ice on the outside, the absorbed energy causes the water to evaporate and the ice to melt. That sounds helpful, but here's the catch: that energy is being used to heat the water and ice, so less energy is transmitted in the forward direction. The result is weaker returns and a degradation of performance. So a wet or icy radome quietly robs you of radar capability. Let's move on to weather depiction — how the radar actually shows you what's out there. The equipment is designed to do three things: detect those clouds likely to produce turbulence, highlight the areas where turbulence is most severe, and indicate safe routes to avoid them where possible. The key indicator is the size and concentration of water droplets in the cloud. That's an indication of turbulence — but note this carefully: it does not detect clear air turbulence, CAT. CAT has no water droplets, so the radar is blind to it. Also, in continuous rainfall, the shorter the distance between light and strong returns, the steeper the rainfall gradient, and the greater the likelihood of turbulence. So a tight transition from weak to strong echo is a danger sign. For a given transmission power, a 3 centimetre wavelength gives the best returns from large water droplets. Wavelengths of 10 centimetres and above produce very few weather returns — that's why we don't use those for weather detection. Now, the colour coding. In colour weather radar systems, targets are colour-coded by rainfall intensity. Let me give you the exact thresholds. BLACK means very light or no returns — that's less than 0.7 millimetres per hour. GREEN is light returns — 0.7 to 4 millimetres per hour. YELLOW is medium returns — 4 to 12 millimetres per hour. RED is strong returns — greater than 12 millimetres per hour. And MAGENTA indicates turbulence, due to rainfall intensity. One important detail: on colour systems without magenta, the RED areas may have a CYCLIC function. That means they alternate between RED and BLACK on the display, specifically to draw the pilot's attention to the worst areas. So where's the danger? The areas of greatest potential turbulence occur where the colour zones are closest together — that's the steepest rainfall gradient. And there are specific shapes on the display that are associated with turbulence: U-shapes, Fingers, Scalloped edges, and Hooks. These are all areas to avoid. If you see those shapes, you don't fly through them — you route around. So to tie it all together: the radar is tuned to 9375 MHz to see 3-centimetre droplets, it's degraded by a wet or icy radome, and it paints a colour-coded picture where tight gradients and specific shapes mark the turbulence you must avoid. That's the core of airborne weather radar.