Lidars are capable of measuring a variety of atmospheric properties as a function of height or range on a continuous or semi-continuous basis. Commonly measured quantities include temperature, water vapour content and aerosols. Continuous measurements give lidars an important advantage over radiosondes (weather balloons), which provide information only as often as they are launched, which is typically at intervals of several hours. This is of benefit in several fields, including meteorology and radiowave propagation research.
The ground-based Raman lidar system at Chilbolton measures elastic backscattering and water vapour profiles in the troposphere. The lidar transmits an expanded beam vertically into the atmosphere from a high power Nd:YAG pulse laser operating at 355 nm. The lidar measures how much the laser radiation is scattered by the molecules in the atmosphere, as a function of height. The beam is scattered both elastically (at the same wavelength) and inelastically (at a changed wavelength). Inelastic scattering occurs via the process of Raman scattering.
The elastic scattering measurements give information on clouds and aerosols. The Raman scattering measurements are used to determine water vapour and temperature profiles. The high power of the laser makes water vapour and temperature measurements possible during daylight. This is often difficult as the Raman scattering signals are relatively weak compared with those from elastic and without a high power laser they will be too small to detect above solar radiation.
particularly in the lower part of the troposphere, affect its
refractivity, which in turn determines the extent to which radiowaves
deviate from a straight path. This is an important effect in radiocommunications.