Most modern airborne weather radars operate in the frequency band of 8 to 12… — Page 266, Lesson 242

I want to walk you through the basics of airborne weather radar as it relates to your role as a professional pilot. This is a complex subject, and while I'll give you a solid general overview, the manufacturer's instructions for the specific radar fitted to your aircraft are the final authority. Let's start with the operating frequency. Most modern airborne weather radars operate in the frequency band of 8 to 12 GHz. That corresponds to wavelengths between 2.5 and 4 centimetres. This band is often called the 'X' band, and it was chosen specifically for weather radar because it is highly sensitive to wet precipitation — that is, rain or other liquid water droplets. Since most weather systems that a pilot needs to avoid contain wet precipitation, the X-band gives us a strong return from those targets. Now, a critical point: airborne weather radars do not detect turbulence directly. They detect precipitation. However, turbulent air — especially inside a thunderstorm — very often contains water. So the radar can indicate where turbulence is likely to be, because it shows you where the heavy precipitation is. Some more advanced radars go a step further: they measure a change in frequency in the reflected radar signal, called a Doppler shift, caused by the movement of the precipitation. That Doppler measurement is then used to give an indication of likely turbulence. Next, let's talk about what reflects radar signals best. Wet precipitation is the most reflective. But other forms of water also reflect radar energy, just to a lesser degree. I want you to remember the order, from most reflective to least reflective: first is wet hail, then rain, then hail, then ice crystals, then wet snow, then dry hail, and finally dry snow at the bottom. So if you see a strong return, it's likely wet precipitation or wet hail; a weaker return might be ice crystals or dry snow. The strength of the returned radar signal — what we call the echo — is not just about what the precipitation is made of. It is also affected by three other factors: the range of the aircraft from the precipitation, the gain setting — that's the amplification of the signal being used by the receiver — and the aerial tilt setting, which is the vertical angle of the antenna. So you have to interpret the display with those settings in mind. Finally, a very important operational note: with weather radars, the significance of a radar return of a given intensity usually increases with altitude. In other words, a medium-strength echo at high altitude may be more significant than the same strength echo at low altitude. But — and this is crucial — the strength of the echo is not an indication of the strength of any associated turbulence. A weak echo can still be in very turbulent air, and a strong echo does not guarantee severe turbulence. The radar tells you where the water is, not how rough the ride will be.