Geoscience Reference
In-Depth Information
THE HISTORY OF UPPER AIR MEASUREMENTS
box 7.1
significant
20th-c. advance
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.
1 The vertical variation of pressure systems
compare the heights of the 1000 and 700 mb pressure
surfaces, warming of the air column will lower the
height of the 1000 mb surface but will not affect
the height of the 700 mb surface (i.e. the thickness of the
1000 to 700 mb layer increases).
The models of Figure 7.1 illustrate the relationships
between surface and tropospheric pressure conditions.
A low-pressure cell at sea-level with a cold core will
intensify with elevation, whereas one with a warm core
will tend to weaken and may be replaced by high
pressure. A warm air column of relatively low density
The air pressure at the surface, or at any level in the
atmosphere, depends on the weight of the overlying air
column. In Chapter 2B, we noted that air pressure is
proportional to air density and that density varies
inversely with air temperature. Accordingly, increasing
the temperature of an air column between the surface
and, say, 3 km will reduce the air density and therefore
lower the air pressure at the surface without affecting
the pressure at 3 km altitude. Correspondingly, if we
Search WWH ::




Custom Search