Flaring of gas at upstream oil and gas facilities is a globally significant problem with uncertain emissions. This thesis details a methodology to quantify flare carbon conversion efficiency and emission rates of flares subjected to turbulent crosswind within quantified uncertainties using a closed-loop wind tunnel. Experiments were performed on 1-inch to 4-inch diameter pipe flares burning methane-dominated flare gas mixtures at exit velocities of 0.5-2 m/s in turbulent winds of 2-10 m/s. Flare efficiency was modestly dependent on flare diameter and exit velocity, and highly sensitive to wind speed and flare gas composition. The strong sensitivity to gas composition, even among similar methane-dominated alkane mixtures, is surprising and confounds simple, predictive emissions models. However, the data in this thesis give new insight into emissions of flares subjected to turbulent crosswinds, and developed simple empirical models offer a first-order means to quantifying flare emissions and developing greenhouse gas inventories.