The sub-ambient clear sky daytime radiative cooling has been achieved recently by applying advanced nano fabrication technologies, which attracted significant research attention in this area. However, there are also several researches which reported that radiative coolers with excellent thermal radiative spectrum failed to achieve sub-ambient radiative cooling. To elucidate the difference, in this work, we perform a detailed investigation of ambient conditions, in particular, water vapor density, on the performance of radiative coolers. We first present the details of how to estimate the performance of radiative coolers under different water vapor concentration and simulate the transmittance and emitted radiation of atmosphere as well as the cooling performance of the ideal selective emitter, which finds the cooling power decreases by 86.6 W/m(2) with increasing total water vapor column. Furthermore, the cooling performance of an integrated photonic solar reflector and thermal emitter with atmospheric profiles in California and Hong Kong were calculated and compared with experiment result. To verify our calculation, nighttime on-site cooling performance of a reference film (Al foil) and two selective emitters (DESR-M and double-layer coating) was measured in several ambient conditions. Our research reveals how the ambient humidity affects the radiative cooling performance by both theoretical and experimental analysis and is important for the further application development of radiative coolers.