Geoscience Reference
In-Depth Information
7.1 The history of upper-air measurements
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 World War I (1914-1918), balloon, kite and aircraft
measurements of temperatures and winds were collected in the lower few kilometers 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 1929-1930. Soundings began to be made up to about
3-4km, mainly in Europe and North America, in the 1930s and the radiosonde was widely used
during and after World War II. 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
wind sonde or rawinsonde. There are now about 1000 upper-air sounding stations worldwide
making soundings once or twice daily at 00 and 12 hours UTC, and sometimes more frequently.
In addition to these systems, meteorological research programs and operational aircraft
reconnaissance flights through tropical and extra-tropical cyclones commonly make use of drop-
sondes 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 sounders are especially valuable in providing data in areas
where rawinsonde coverage is sparse, such as Antarctica, the Arctic Ocean and large areas of the
global ocean. They operate in the infrared 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. 41) are inverted
so as to calculate the temperature (moisture) from the measured radiances. Infrared 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 10cm (UHF) and 10m (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 meters. 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 may be obtained from lidar (light detection and ranging)
and sodar (sound detection and ranging) systems, but these have a vertical range of just a few
kilometers.
