In general, the change in frequency of a signal reaching a receiver when the receiver and the transmitting source are in motion relative to one another. This phenomenon was first noted for sound waves by the Austrian physicist Christian Johann Doppler (1803–53) in 1842. In meteorology, this effect is successfully employed with remote sensors, such as Doppler radars and Doppler lidars, in which the receiver (collocated with the transmitter) is fixed and only the scatterers (upon which transmitted power impinges and is reradiated) are moving. The frequency shift, f, induced by a scatterer having a radial component of motion vr relative to the radar may be expressed as

where λ is the wavelength of the transmitter, f is positive for motion toward the radar, and, by the usual convention, vr is positive for motion away from the radar. See also Doppler velocity.