This paper describes a study focusing on the effect of inclement weather conditions on macroscopic traffic behaviour. It is hypothesized that traffic stream patterns would vary significantly under different weather conditions (i.e. rainy and snowy weather). To test this hypothesis and quantify the underlying relationships, weather data from RWIS and traffic data from loop detectors on a selected segment of Queen Elizabeth Way (QEW) in Ontario were collected and analyzed. Multiple regression analyses were utilized to estimate the speed-flow and speed-density relationships and assess the impacts caused by the two inclement weather conditions. The results of the empirical investigation showed 2.5%-4.7% and 5%-7% reductions of free flow speed and capacity, respectively, for light rain weather. Moreover, light snow weather was found to cause a drop of 21%-24% in capacity and 17%-19% in free flow speed when compared to clear weather. Lastly, these findings were compared with those recommended by the Highway Capacity Manual (HCM 2010), and it was revealed that HCM underestimated the effect of inclement weather conditions. By analyzing the speed-density relationships, it was found that drivers tend to drive slowly and leave a larger headway distance under inclement weather conditions because they tend to be more cautious and alert about the unexpected road conditions.