Fog makes a significant contribution to the hydrology of a wide range of important terrestrial ecosystems. The amount and frequency of fog immersion are likely to be affected by rapid ongoing anthropogenic changes but the effects of these changes remain relatively poorly understood compared to changes in rainfall. Here, we present the design and performance of a novel experiment to actively manipulate low lying fog abundance in an old-growth tropical montane cloud forest (TMCF) in Peru - the Wayqecha Amazon Cloud Curtain Ecosystem Experiment (WACCEE). The treatment consists of a 30m high, 40m wide mesh curtain suspended between two towers and extending down to the ground, and two supplementary curtains orientated diagonally inwards from the top of each tower and secured to the ground upslope. The curtains divert and intercept airborne water droplets in fog moving upslope, thereby depriving a ~ 420 m2 patch of forest immediately behind the curtains of this water source. The treatment caused a strong reduction in both air humidity and leaf wetness, and an increase in vapour pressure deficit, above the canopy compared to a nearby unmodified control plot. This effect was most pronounced during the nighttime (20:00 - 05:00). Below-canopy shifts within the treatment were more subtle: relative humidity at 2 m height above the ground was significantly suppressed during the daytime, while soil moisture was apparently elevated. The treatment caused a small but significant increase in air temperature above the canopy but a decrease in temperature in and near the soil, while mixed effects were observed at 2 m height above the ground. Above-canopy radiation was slightly elevated due to the treatment, which was mainly caused by a notable increase relative to the control during the dry season. We employ the WACCEE infrastructure to understand in situ impacts of fog reduction within a pristine TCMF, but the basic principle of the method is extremely versatile. Further application of the method in other systems where fog plays a major role in ecosystem processes could improve our understanding of the ecological impacts of this important but understudied climate driver.