To provide a more detailed description of the forest light environment useful in interpreting the growth response of herbs and tree seedlings in the forest understory, we present light measurements taken under different sky conditions, seasons and spatial locations both vertically and along transects. Cloudy skies reduced the heterogeneity in both PPFD (photosynthetically-active photon flux density) and R.FR (red to far red ratio, 656-664 pm/ 726-734 nm) in shade and forest gaps. Under cloudy conditions, R:FR was relatively stable across different PPFD. The mean light intensity in some forest sites was higher under cloudy skies than under clear skies because of increased penetration of diffuse light. In areas where overcast conditions are frequent, growth of understory plants may show strong correspondence with the cloudy PPFD and R:FR conditions. Percent canopy opening was stable during the summer but daily sunfleck duration varied considerably because of the shifting solar track. Because of the presence of ground vegetation, light gradient above the forest floor is greatest from 0 to 1 m in the shade. Light environment may become dimmer with increasing heights beyond 1 m as shading from low tree branches imposes a greater influence. Across a forest transect, percent canopy opening showed marginal to five-fold increases from 0 to 1 m indicating higher PPFD on cloudy days. On clear days, plants at 1 m could experience more sunflecks of higher intensity and longer individual duration. Carbon balance of forest understory plants may vary significantly with vertical location and weather conditions.