Soiling significantly impacts PV systems’ performance, but this can be mitigated through optimized frequency and timing of cleaning. This experimental study focused on the conditions leading to soiling. It utilized a novel method to evaluate the effectiveness of different cleaning frequencies. The transmittance of horizontally mounted glass coupons exposed outdoors in a warm and humid location was measured weekly and these measurements were used (i) to evaluate the variability of soiling and its seasonal correlations with environmental factors using linear regression models and (ii) to assess the effectiveness of the different cleaning cycles using statistical (F- and t-test) analysis. The minimum transmittance loss occurred during the season with the most frequent rainfall, which acted as the dominant natural cleaning agent. The experimental campaign showed that rainfalls do not completely clean soiling; a minimum intensity threshold has to be achieved to have a cleaning effect. The threshold rainfall was the highest for the weekly cleaned glass coupon and lowest for a coupon that was never cleaned. Based on the statistical analysis, it is suggested that weekly cleanings during winter and post-monsoon seasons and monthly cleanings during pre-monsoon and southwest monsoon seasons are optimal for areas in the Köppen–Geiger Cwa climate classification category. The correlation between soiling and environmental parameters was found to be highly dependent on the season. It may therefore not be possible to develop a simple, universal predictive relationship for soiling losses. The presented methodology is applicable to additional locations, even outside of the study area of India, to contribute to the understanding and mitigation of soiling.