What does "Doppler radar" primarily measure?

Doppler radar primarily measures the change in frequency of the radar signal that is reflected off a moving object. This principle is based on the Doppler effect, which refers to the change in frequency or wavelength of waves in relation to an observer moving relative to the source of the waves. When the radar emits a signal towards a moving object, such as a vehicle or precipitation, the frequency of the returned signal is altered depending on the speed and direction of the object relative to the radar.

This capability allows Doppler radar to effectively track the velocity of moving objects, making it particularly valuable in various applications, including weather forecasting, where it can measure wind speed and direction, as well as in law enforcement for speeding detection. The ability to measure these changes in frequency is crucial in interpreting various dynamic events, but specifically, in the context of this question, it emphasizes the measurement of that frequency change, which is integral to the operation of Doppler radar systems. Other options focus on measuring distance, temperature, or wind speed directly, but the core function of Doppler radar is centered around the alterations in frequency due to motion.