# Phase Changes and Velocity Measurement

## Using Phase Change To Measure Velocity

Light, radio, microwaves and x-rays are all forms of electromagnetic waves, which is also the type of energy emitted by weather radars. All of these types of electromagnetic waves have different wavelengths (the distance between two crests of the wave), but it is most common for weather radars to use wavelengths of five or ten centimetres.

Figure 1. The wavelength is the distance between the crests of the wave.

The phase describes the current position of the wave in its sinusoidal cycle. That is, the wave may be at a crest, a trough, or somewhere in between when it returns to the radar, and a Doppler radar is capable of measuring this for each wave that is returned from a reflecting particle. The difference in phase of two waves is known as the phase shift.

Figure 2. The difference in phase of two waves is known as the phase shift.

If the reflecting particle is not moving, successive pulses will have travelled exactly the same distance, and there will be no phase shift. However, if the particle has a velocity with a component towards or away from the radar, successive pulses will return to the radar with a different phase. The resulting phase shift is therefore caused by the movement of the reflecting particle, with larger velocities resulting in larger phase shifts. Hence by measuring the phase of each returning pulse, the Doppler weather radar is able to measure the velocities of the reflecting particles.

Parent page: About Doppler Wind Images