The dependence of the variance of the normalized radar cross section of the sea, &sgr;¿0, on illuminated area and integration time is investigated. We show that the standard expression for this dependence cannot account for the correlation of power received from different points on the surface which is exhibited by microwave backscatter from the sea. An alternate expression is derived which does not account for these correlations. We show that knowledge of the ocean wave spectrum and modulation transfer function along with the determination of three fundamental parameters, facet variance, the velocity spread of a facet, and correlation area, are necessary to specify the dependence of cross section variance of illuminated area and integration time. The correlation time is not a fundamental parameter but depends on illuminated area. An expression for that dependence is derived here. Issues of system noise and the relationship of received power variance to cross section variance are also addressed. We relate these calculations to the critical problem of determining confidence intervals for &sgr;¿0 measurements at high spatial resolution. ¿American Geophysical Union 1991