Anemometer beam exits reflect, or backscatter, the light back into a detector, where it is measured relative to the original laser beam. When the particles are in great motion, they produce a Doppler shift for measuring wind speed in the laser light, which is used to calculate the speed of the particles, and therefore the air around the anemometer. Ultrasonic anemometers, first developed in the 1950s, use ultrasonic sound waves to measure wind velocity. They measure wind speed based on the time of flight of sonic pulses between pairs of transducers. Measurements from pairs of transducers can be combined to yield
Radon measure regular, and locally finite, and inner regular for open sets. It coincides with "m" on compact and open sets, and "m" can be reconstructed from "M" as the unique inner regular measure that is the same as "M" on compact sets. The measure "m" is called moderated if "M" is σ-finite; in this case the measures "m" and "M" are the same. (If "m" is σ-finite this does not imply that "M" is σ-finite, so being moderated is stronger than being σ-finite.) On a strongly Lindelöf space every Radon measure is moderated. An example of a measure "m" that is