Synoptic situations and temperature gradients in the lower layers of the atmosphere were studied in Québec City over a one year period. Data on climatic conditions resulting there from were subjected to multi-variate analysis which revealed that daily climatic elements could be reduced to two principal components : solar radiation absorbed at the earth's surface and « humidity potential » a direct function of absolute humidity and an inverse function of the saturation deficit. Relationships between synoptic situations and the two principal components which summarize weather conditions are presented as graphs and probability tables which allow for the synthesizing of the two types of daily weather classifications.