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Radiosondes have for several decades been the primary means of
obtaining atmospheric vertical profile data from the surface to the
lower stratosphere. They are routinely used as input to the
operational meteorological analyses that are used in numerical
weather prediction and meteorological diagnostics. In the absence
of other in situ measurements, radiosonde observations have
recently been used to assess trends of atmospheric conditions above
the surface, even though they were not designed for this purpose.
The instrument packages carried aloft by balloons are generally
equipped with temperature, humidity, and pressure sensors, whose
measurements are radio-transmitted to a ground receiving station.
Wind data are also obtained by tracking the position of the
instrument during ascent. Temperature sensors vary according to the
manufacturer and model of the radiosonde; most contemporary
instruments carry a thermocapacitor, wire resistor, thermocouple,
or bimetallic sensor. An excellent overview of radiosonde
instruments, including discussion of measurement error
characteristics, is provided by the World Meteorological
Organization (WMO, 1996).
Currently, the global radiosonde network nominally includes
about 900 upper-air stations, of which about two-thirds make
observations twice daily (at 0000 and 1200 Coordinated Universal
Time (UTC)). The network is predominantly land-based and favors the
Northern Hemisphere (Figure 2.6). Radiosondes can achieve heights
of about 35 km, although many soundings terminate below 20 km
because less expensive balloons burst at a lower altitude. There
has been some deterioration in the radiosonde network in recent
years. The loss of navigational systems used to track the sondes
has led to at least temporary closing of some stations,
particularly in Africa. Efforts to reduce operating costs have led
to station closures and reduced observing schedules in some parts
of the former Soviet Union and elsewhere.
Measurements are made and transmitted with approximately
10–50 m resolution during ascent, but archived sounding data
may contain only about 20 data levels per sounding.
Radiosonde-based data sets for climate monitoring come from two
basic data products: individual soundings containing all reported
data (Angell, 1988), or monthly mean data (known as CLIMAT TEMP
reports) at mandatory pressure levels only (Parker et al., 1997).
Only about 45% of stations provide CLIMAT TEMP reports in addition
to the daily sounding data. Missing daily datacontinue