Although perovskite solar cells have demonstrated impressive efficiencies in research laboratories (above 25%), there is a need for developing experimental procedures to fabricate solar cells under ambient conditions to substantially decrease manufacturing costs. Nevertheless, to achieve efficient and highly stable devices in these conditions, the moisture level in the atmosphere must be monitored. Relative humidity (RH) has classically been the parameter of choice; however, in this work, we show that the parameter of relevance is the absolute content of water measured in the form of partial water vapor pressure (WVP). To highlight the importance of this parameter, we demonstrate that small changes in ambient temperature at the same RH result in huge changes in solar cell performance. This is due to the nonlinear dependence of WVP on temperature (according to the Clausius–Clapeyron equation), which explains the dispersion of results found in the literature for devices nominally made at the same ambient RH levels. To illustrate this critical effect, we deposited MAPbI₃ perovskite films at different WVP values, which were derived from the climate parameters, RH and laboratory temperature, present during fabrication (not controlled). Hence, we adapted the fabrication method to the ambient conditions by monitoring the WVP, which allows for the fabrication of MAPbI₃-based devices with efficiencies of up to 18.2% outside the glovebox. In fact, we extended the procedure to accomplish fabrication of high-efficiency FA_0.83MA_0.17PbI₃ devices under ambient conditions by adjusting the dimethyl sulfoxide (DMSO) proportion in the perovskite precursor solution to the WVP.