An onboard weather radar is a Primary Surveillance Radar (PSR) that computes areas of precipitation by determining the range and azimuth of transmitted and reflected radio frequency energy. The primary use of weather radar is to aid the pilot in avoiding thunderstorms and associated turbulence. It is used for avoiding, not penetrating severe weather. Weather radar detects only precipitation, it does not detect minute cloud droplets. Therefore a clear area on a radar screen does not indicate that an area is also free of cloud. The best radar reflectors are raindrops and wet hail, the larger the raindrop the better it reflects. Extreme weather can usually be identified by certain patterns displayed on a radar screen, these include:
Tilt management is the single most important factor for effective use of an airborne weather radar system. Failure to properly manage tilt is the most misused function of weather radar systems. Too low of a tilt setting results in excessive ground returns and the inability to distinguish weather from ground clutter. With tilt set too high, the beam will scan over the top of the weather. Either way, the pilot will not distinguish crucial weather data. Correct tilt angle is directly dependant on the storm’s distance from the aircraft and upon height and intensity.
Raindrops absorb and scatter radar signals, so less energy reaches the target and even less returns to radar as an echo. Hence, precipitation in general and rainfall, in particular, affect the propagation of electromagnetic waves (radar signals) in two ways:
Airborne or ground based weather radar will normally reflect areas of precipitation. The frequency and severity of turbulence associated with the areas of high water content generally increases the radar return. No flight path, through an area of strong or very strong radar echoes separated by 40 NM or less, can be considered free of severe turbulence.
REFERENCES AIM COM 2.7 Flight Operation Near Thunderstorms