Table 3.1*
Chronology of changes in U.S. radiosondes
Date Change
1943 Lithium chloride humidity element replaced hair hygrometer.
1943 Dark ceramic resistance sensor replaced glass-tube electrolytic temperature element.
1948 Relative humidity computed using saturation relative to water instead of ice
1948 Change observing times from 2300, 1100 UTC to 0300, 1500 UTC.
1949 Smaller temperature sensor to reduce response time.
1950 Correction for solar radiation introduced (until 1960).
1957 Change observing times from 0300, 1500 UTC to 0000, 1200 UTC.
1960 Introduced white-coated temperature elements.
1965 Carbon humidity element, began reporting low humidities.
1972 Redesigned humidity ducts to reduce solar effects.
1973 Stopped reporting relative humidity below 20%.
1980 New carbon hygristors, new relative humidity transfer equation.
1988 Precalibrated hygristor replaced type requiring preflight calibration.
1988 New VIZ sonde with new humidity duct.
New Space Data Division (SDD) manufactured radiosonde at some stations. Differences between VIZ and SDD noted.
1993 Relative humidity to be reported over broader range, values <20% and up to 98% (instead of 95%) in cloud.
1993 Gravitational constant used to define geopotential height from geopotential changed from 9.8 to 9.80665 m s .
* From Trenberth, K.E., 1994: Atmospheric Circulation Climate Changes, 21, 427-453.