In situ measurements of water vapor and temperature from recent aircraft campaigns have provided evidence that the upper troposphere is frequently supersaturated with respect to ice. The peak relative humidities with respect to ice (RHI) occasionally approached water saturation at temperatures ranging from -40 ¿C to -70 ¿C in each of the campaigns. The occurrence frequency of ice supersaturation ranged from about 20% to 45%. Even on flight segments when no ice crystals were detected, ice supersaturation was measured about 5--20% of the time. A numerical cloud model is used to simulate the formation of optically thin, low ice number density cirrus clouds in these supersaturated regions. The potential for scavenging of ice nuclei (IN) by these clouds is evaluated. The simulations suggest that if less than about 5¿10-3 to 2¿10-2 cm-3 ice nuclei are present when these supersaturations are generated, then the cirrus formed should be subvisible. These low ice number density clouds scavenge the IN from the supersaturated layer, but the crystals sediment out too rapidly to prevent buildup of high supersaturations. If higher numbers of ice nuclei are present, then the clouds that form are visible and deposition growth of the ice crystals reduces the RHI down to near 100%. Even if no ice clouds form, increasing the RHI from 100% to 150% between 10 and 10.5 km results in a decrease in outgoing longwave radiative flux at the top of the atmosphere of about 8 W m-2. If 0.02--0.1 cm-3 IN are present, the resulting cloud radiative forcing reduces the net radiative flux several watts per square meter further. Given the high frequency of supersaturated regions without optically thick clouds in the upper troposphere, there is a potential for a climatically important class of optically thin cirrus with relatively low ice crystal number densities. The optical properties of these clouds will depend very strongly on the abundance of ice nuclei in the upper troposphere. ¿ 2001 American Geophysical Union