This paper investigated the effects of changing ambient pressure (standard- and sub-atmospheric conditions) and crossflow air speed on the flame radiation fraction of a pool fire for a size range of pools where no data were available in the literature. Experiments were carried out using either heptane or acetone to produce relatively sooty flames for square pool fires with side dimensions from 10 to 70 cm. Steady cross airflows ranging from 0 to 2.8 m/s were created by wind tunnels in Hefei (ambient pressure: 100 kPa) and Lhasa (altitude: 3650 m; ambient pressure: 64 kPa), while the flame radiation was measured by wide angle radiometers. It was found that the flame radiation fraction decreased with increasing of cross flow air speed with a relatively larger decrease occurring at sub-atmospheric pressure compared to standard pressure conditions, and for smaller pools compared to larger pools. This evolution of flame radiation fraction with cross flow air speed was interpreted theoretically based on the change of soot volume fraction associated with a characteristic flow time (soot residence time) caused by the elongation of the flame. A general non-dimensional model was proposed to describe the decrease in flame radiation fraction in relation to cross flow air speed and pool size at both ambient pressures. This relationship showed that the ratio of flame radiation fraction in a cross airflow to that without cross airflow was a function of a modified Froude number that reflected the change in buoyancy by accounting for the ratio of ambient- to standard atmospheric pressure. Experimental data were shown to correlate well to this proposed function. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.