Manned balloon flights during the nineteenth century attempted to measure temperatures in the upper air but the equipment was generally inadequate for the purpose. Kite measurements were common in the 1890s. During and after the First World War, balloon, kite and aircraft measurements of temperatures and winds were collected in the lower few kilometres of the atmosphere. Forerunners of the modern radiosonde, which comprises a package of pressure, temperature and humidity sensors suspended beneath a hydrogen-filled balloon and transmitting radio signals of the measurements during its ascent, were developed independently in France, Germany and the USSR and first used in about 1929 to 1930. Soundings began to be made up to about 3 to 4 km, mainly in Europe and North America, in the 1930s and the radiosonde was used widely during and after the Second World War. It was improved in the late 1940s when radar tracking of the balloon enabled the calculation of upper-level wind speed and direction; the system was named the radar windsonde or rawinsonde. There are now about 1000 upper-air-sounding stations worldwide making soundings once or twice daily at 00 and 12 hours UTC. In addition to these systems, meteorological research programmes and operational aircraft reconnaissance flights through tropical and extra-tropical cyclones commonly make use of dropsondes that are released from the aircraft and give a profile of the atmosphere below it.
Satellites began to provide a new source of upper-air data in the early 1970s through the use of vertical atmospheric sounders. These operate in the infra-red and microwave wavelengths and provide information on the temperature and moisture content of different layers in the atmosphere. They operate on the principle that the energy emitted by a given atmospheric layer is proportional to its temperature (see Figure 3.1) (and is also a function of its moisture content). The data are obtained through a complex 'inversion' technique whereby the radiative transfer relationships (p. 33) are inverted so as to calculate the temperature (moisture) from the measured radiances. Infra-red sensors operate only for cloud-free conditions whereas microwave sounders record in the presence of clouds. Neither system is able to measure low-level temperatures in the presence of a low-level temperature inversion because the method assumes that temperatures are a unique function of altitude.
Ground-based remote sensing provides another means of profiling the atmosphere. Detailed information on wind velocity is available from upward-pointing high-powered radar (radio detection and ranging) systems of between 10 cm (UHF) and 10 m (VHF) wavelength. These wind profilers detect motion in clear air via measurements of variations in atmospheric refractivity. Such variations depend on atmospheric temperature and humidity. Radars can measure winds up to stratospheric levels, depending on their power, with a vertical resolution of a few metres. Such systems are in use in the equatorial Pacific and in North America. Information on the general structure of the boundary layer and low-level turbulence can be obtained from lidar (light detection and ranging) and sonar (sound detection and ranging) systems, but these have a vertical range of only a few kilometres.
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