Precipitable water at Purcell, Oklahoma (PURC) as measured by GPS, radiometer and radiosonde from 0000 UTC 22 May to 0000 UTC 26 May 1993. The small blue dots represent radiometer measurements along the line-of-sight to GPS satellites, mapped to the zenith. The red dots represent 30-minute averages of precipitable water (PW) calculated from the least-squares fits of GPS wet delay. Twelve hourly radiosonde data from Norman, OK are shown by the green dots. The vertical arrow denotes the time of the observed surface cold front passage at Purcell. Time of thunderstorm activity is indicated with conventional symbols.