An analysis of boundary layer cumulus clouds and their impact on land surface–atmosphere exchange is presented. Seasonal trends indicate that in response to increasing insolation and sensible heat flux, both the mixed-layer height (zi) and the lifting condensation level (LCL) peak (∼1250 and 1700 m) just before the growing season commences. With the commencement of transpiration, the Bowen ratio falls abruptly in response to the infusion of additional moisture into the boundary layer, and zi and LCL decrease. By late spring, boundary layer cumulus cloud frequency increases sharply, as the mixed layer approaches a new equilibrium, with zi and LCL remaining relatively constant (∼1100 and 1500 m) through the summer. Boundary layer cloud time fraction peaks during the growing season, reaching values greater than 40% over most of the eastern United States by June. At an Automated Surface Observing System (ASOS) station in central Massachusetts, a growing season peak is apparent during 1995–98 but reveals large variations in monthly frequency due to periods of drought or excessive wetness. Light–cloud cover regression relationships developed from ASOS ceilometer reports for Orange, Massachusetts, and Harvard Forest insolation data show a good linear fit (r2 = 0.83) for overall cloud cover versus insolation, and a reasonable quadratic fit (r2 = 0.48) for cloud cover versus the standard deviation of insolation, which is an indicator of sky type. Diffuse fraction (the ratio of diffuse to global insolation) shows a very good correlation (r2 = 0.79) with cloud cover. The sky type–insolation relationships are then used to analyze the impact that boundary layer clouds have on the forest ecosystem, specifically net carbon uptake (), evapotranspiration (ET), and water use efficiency (WUE). During 1995, afternoon was 52% greater on days with boundary layer cumulus clouds than on clear days, although ET was the same, indicating greater light use efficiency and WUE on partly cloudy days. For 1996–98, afternoon was also enhanced, especially during dry periods. Further analysis indicates that the vapor pressure deficit (VPD) was significantly greater (>8 hPa) during 1995 and parts of 1996–98 on clear days as compared with partly cloudy days. A long-term drought combined with abnormally warm weather likely contributed to the high VPDs, reduced , ET, and the dearth of clouds observed during 1995. In general, the presence of boundary layer cumulus clouds enhances net carbon uptake, as compared with clear days.