Monthly Ku band &sgr;0 and significant wave height (SWH) histograms from the NASA altimeter on the TOPEX/POSEIDON satellite are presented from January through June 1993 for three latitude bands between ¿60 degrees. The data are compared to distributions from the Geosat mission for the same months in 1987--1989. Generally, the distributions agree quite well, although there are some seasonal/hemispherical differences. The &sgr;0 comparison reveals an overall bias between the two altimeters with the TOPEX &sgr;0 higher by about 0.7 dB, which is consistent with algorithmim improvements for TOPEX. The SWH distributions show strong hemispherical/seasonal changes. The seasonal/hemispherical differences between TOPEX and Geosat are consistent for SWH and &sgr;0.

The joint distribution of &sgr;0 and SWH is extremely stable from month to month. The typical SWH is independent of &sgr;0 for &sgr;0 greater than 11.3 dB. The minimum SWH grows exponentially with wind speed. This joint distribution may be useful for understanding electromagnetic bias in altimeter measurements. Finally, altimeter data are compared to buoy values from 21 overflights of the NASA verification site near Pt. Conception, California. Wave heights agree well with an RMS difference of only 0.2 m. Altimeter &sgr;0 values are compared to buoy wind speeds. The results are consistent with the -0.7 dB &sgr;0 offset from the histogram comparisons. ¿American Geophysical Union 1994