Radiowaves that pass through the lowest part of the earth's atmosphere
are strongly affected by its refractivity, that is how much it bends the
rays. Researchers attempting to model how radiowaves behave in different
types of atmospheric conditions need detailed information about how refractivity
varies at different heights above the ground. Weather balloons can measure
these conditions, but only every few hours and researchers have to make
statistical guesses about the times in between flights.
Optical radar, or lidar, can measure refractivity from the ground continously,
which is a significant improvement over instrument-carrying balloons.
This is because lidar can detect smaller particles than conventional radars,
since it uses shorter wavelengths. Lidars pick up scattering from the
molecules in clear air, rather than the clouds and rain droplets detected
by conventional radar. Raman lidars transmit a signal into the atmosphere
and pick up return signals at different frequencies. The shift in frequency
between the transmitted and returned frequencies is characteristic of
certain types of molecules. By looking at ratios between different returned
signals, Raman lidars continously measure the temperature and humidity
of the air from the ground. Refractivity is related to both these quantities.
The lidar measurement project at RCRU's Chilbolton Observatory has set
up a Raman lidar system capable of measuring refractivity continuously.
The system incorporates a powerful near-ultraviolet laser as well as analysis
software developed by RCRU. The lidar system at Chilbolton complements
the weather radars already onsite and is able to determine meterological
and environmental parameters for a range of projects.
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