Stepwise discriminant analysis of eight meteorological variables was used to classify the days from May through September, 1970, on the southern slopes of the San Bernardino Mountains and over the adjacent basin of southern California. The five classes were: 1) hot, dry continental air throughout the day (Santa Ana); 2) relatively dry forenoon, modified marine air in afternoon, very hot (heat wave); 3) moist, modified marine air, hot in afternoon; 4) moist, modified marine air, warm in afternoon; and 5) cool, moist, deep marine air throughout day. Observations of surface temperature, humidity, wind speed and direction, and total oxidant were recorded continuously along the southern slope and crest of the San Bernardino Mountains, and rawinsonde observations were recorded at the base. Vertical profiles of temperature, humidity, and wind measured by rawinsonde document the five classes. Significant differences in fire weather and oxidant air pollution exposure were found on the slope and crest during the five meteorological conditions. Oxidant concentrations are highest on days in classes 2, 3 and 4, when transported up the slope with the marine air during the day.