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Which of the following phenomena is least likely to be detected by radar?
  • A
    Clear air turbulence
  • B
    Wet snow and turbulence in cloud that has precipitation
  • C
    Turbulence in cloud that has precipitation
  • D

Conventional weather radar cannot detect wind speed or turbulence directly, it only detects solid and liquid objects above a threshold size. The most turbulent areas of thunderstorms are also those areas that contain the most water and ice. Note that the areas in which the concentration of droplets decreases rapidly (which are marked on the radar screen by a sharp fall in signal intensity) are normally areas in which the most severe turbulence can be encountered. These are areas in which the strong up-going air currents are in close proximity to downward flowing currents and there is a strong risk of severe vertical wind shear. These areas will be identified on the radar screen by a rapid change in appearance of the cloud return - steep change in colors of the return (steep color gradient). If you identify this area on your radar screen, avoid that area!

* Green: light precipitation, slight turbolence
* Yellow (or amber): medium precipitation,light turbolence
* Red: heavy precipitation, medium to severe turbolence
* Magenta: Extremely high precipitation, severe turbolence

So the weather radar is merely detecting the presence of water. It is the pilot interpreting the picture who is really detecting the turbulence.
Clear air turbulence (CAT) does not have condensated water in it and is therefore undetectable by conventional radar. There are systems such as LIDAR that can detect CAT but they generally not installed on aircraft.

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