HCCI (Homogeneous Charge Compression Ignition) has the potential for significant fuel efficiency improvements and low engine-out emissions but a major limitation is its relatively small operating range, limited by pressure rise rate at high loads and cyclic variability and incomplete combustion at low loads. Spark Assisted Compression Ignition (SACI) can extend the operating range of HCCI, but since SACI includes both flame propagation and auto-ignition, it experiences higher cyclic variance than HCCI combustion and phasing control can be challenging. This paper investigates the effects of environmental conditions on SACI combustion.The first part of the paper investigates whether CA50 (the location of 50% heat release and the most commonly used combustion parameter for describing combustion phasing) is the best metric to describe combustion phasing and facilitate its control. CA50 and four other combustion phasing metrics are evaluated and compared in this study. To perform this evaluation, IMEP (Indicated Mean Effective Pressure) and RI (Ringing Intensity), key combustion outputs that are directly perceived by a driver, are plotted versus each combustion control parameter and the physical correlation between each parameter and IMEP and RI are examined for individual cycles in the SACI combustion mode. It is concluded that CA50 is the best metric for SACI combustion control because CA50 shows the best correlation with IMEP and RI and does not have high computational requirements.The second and main part of the paper investigates the impact of environmental conditions (i.e. intake air temperature and humidity) on CA50 and the allowable range of CA50 in a production-feasible SACI engine. The allowable operating range is defined by a RI of 5 bar/degree and a coefficient of variance (COV) of IMEP of 4%. Higher ambient temperature is found to advance combustion phasing, while higher humidity is found to retard combustion phasing. The early and late combustion phasing bounds of the operable range are observed to undergo a similar shift.