I assume you are talking of the airborne weather radars. They work on the principles of a basic primary radar. That is, the radar uses its own reflected signals to create an image on the radar display. Most antennas located in the nose compartment of aircraft are able to create two separate types of beams. One is a conical beam and the other is a cosecant beam. The conical beam is used for long-range weather and ground mapping, while the cosecant beam is used for short-range ground mapping.
Weather radars use very high frequencies for their operation ( around 9400 Mhz). The advantage of this is the associated low wavelength. The lower the wavelength the lower is the beamwidth of the radiation. This allows better resolution imagery. Think of this, a fat beamwidth at a longer distance might show two clouds as one single huge return but once you are closer, the reduced beamwidth shows the clouds as two separate entities. The frequency gives a wavelength of around 3 degrees which is the diameter of large water droplets and hail.
The radar display shows areas of turbulence in different colours:
- Black = no returns,
- Green = light returns,
- Yellow = medium returns,
- Red = strong returns,
- Magenta = very strong returns.
The decided points where the colors are close together show a sharp rainfall gradient with associated turbulence.
The pilot can tilt the radar up and down using the radar control panel. As a general rule, the lower you are the higher should be the tilted angle to get better weather coverage. Similarly, as the range increases for ground mapping, the tilt should increase. The opposite is the case as you fly higher.
In mapping mode, the radar is used to find terrain conditions. For this, a cosecant beam is used for short-range and the conical beam is used for longer range mapping just like using it for weather detection. In-ground mapping, the radar power is reduced as distance decreases to the surface. This is because the reduction of power with distance is a function of cosecant of the depression angle. The power is altered with the gain switch of the control panel.