Metrics that estimate the beginning of the growing season in semi-arid monsoonal ecosystems have become a central part of both crop models that are used to monitor crop production using satellite remote sensing and early warning indicators of possible future reductions in yield due to growing season length. This paper presents a new phenological model which is tuned to the semi-arid, monsoonal ecosystem of the West African Sahel and that uses humidity instead of calendar dates to identify the beginning of the season. We implement this model on vegetation index data from AVHRR, SPOT-Vegetation and MODIS data, and evaluate the results along with a rainfall-based start of season estimate for the region. Contributions of the paper include evaluating existing methods using ground observations of sowing date, comparing the same metric across multiple sensors and to rainfall-based SOS, and the development of a metric based on NDVI and relative humidity.