Recent changes in the NWS upper-air observing network are listed and an analysis is presented of the change in 1995 from VIZ to Vaisala radiosonde instruments. Results are shown for 14 stations at 850, 700, 500, 100, and 50 hPa and at both 0000 and 1200 UTC. This change in radiosonde type resulted in detectable shifts in the means of temperature, height, dewpoint, and relative humidity at the time of the change. The largest differences in temperature occur at the 100- and 50-hPa levels and are likely related to radiation effects on the sensors. However, the magnitude and, in some cases, the sign of the difference was found to vary depending on pressure level, time of day, region, and season. Thus, no single networkwide adjustment to eliminate artificial shifts seems possible.

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Air Resources Laboratory, NOAA, Silver Spring, Maryland

Office of Operational Systems, National Weather Service, NOAA, Silver Spring, Maryland