A Weather Radar emits microwave energy at a known frequency:

• S band ~ 2 800 MHz
• C band ~ 5 600 MHz

The emitted energy is absorbed, then re-emitted by target. The radar receives the re-emitted energy and if the target is moving, frequency of returned radiation intercepted by a radar will be changed. The Doppler effect is used in weather radars primarily to measure the velocity of detected objects and to remove clutter.

Any rain drops or snow flakes in motion affect the frequency of the returned radar beam according to the Doppler effect. With velocities of less than 70 m/s for weather echoes and radar wavelength of 10 cm, it amounts to only 10^-5%. This difference is too small to be noted by electronic instruments. As discussed earlier, most weather radars are pulsed, that is they emit a pulse of energy and then listen for the return signal, measuring the time taken in between. However, as the targets move slightly between each pulse, the returned wave has a noticeable phase difference or phase shift from pulse to pulse. Doppler weather radars are using this phase difference (pulse pair difference) to calculate the precipitation's motion.

• The phase difference between transmitted and received pulses can be measured
  
• The change in this phase difference from one pulse to the next is dependent on the motion of the target
  
• Doppler weather radars use this phase difference to estimate velocity

Doppler radar typically calculates

• total reflectivity R
  
• mean velocity v
  
• spectral width w

of the ensemble of scatterers in the sample volume