Quebec has a great variety of climatic conditions due to its geographical position, large area and complex physiographic relations.
The main factors regulating the distribution of climatic influences are not to be sought in latitude or proximity to the ocean only. The position of the great masses of cold and warm air over the North American continent have a direct influence on the Quebec weather. Fox: instance, the succession of cold waves in winter, and that of warm and humid ones in summer are logical results of air mass movements. The St. Lawrence valley happens to be one of the regular paths followed by cyclonic storms between the Great Lakes and the Atlantic Ocean. Those are disturbances that account for the irregularity of our climate, for the rapid changes in the weather and for the many departures from "normal conditions". In fact, as far as temperature is concerned, Quebec has a rather continental climate, in spite of its nearness to the sea board. The climate is very severe in wintertime, even in the south, and very warm and humid in summertime.
Amongst the various elements of the climate, let us first consider temperature. The mean annual temperature ranges between 42° around Montreal to 36° in the northmost inhabited parts except on the North Shore of the Gulf. Of more significance are the isotherms for the extreme months of January and July. In January the coldest inhabited regions are located in the Laurentian Upland. The average temperature there stands between 8° in the Ottawa Valley and 4° in the region of Northwestern Quebec and in the Lake St. John area. In the south, January mean temperatures range between 12° in Montreal and 10° at Quebec and in the Eastern Townships. The marine influence also raises the average temperature to 10° on the Gaspé shores and to 12° in Anticosti and Magdalen Islands. Some features worthy of observation on the January map are the following: the 8° isotherm skirting the outside edge of the Laurentians from the Ottawa Valley to Tadoussac, the contrast of lower temperature on bolder reliefs (Laurentide Park and Shickshocks) with the higher temperature in the low Lake St. John district. These temperatures are easy to explain. The inland parts of the province are more likely to be covered by polar continental air than the southeast, where low pressure storms bring milder temperatures in wintertime.
In July, none of the inhabited parts of Quebec fall below 60°. Montreal averages 70° and the whole St. Lawrence Lowland nearly as much. Most of the other settled parts average between 64° and 66°. The coolest parts are on the shores of Gaspé (62°), except Chaleur Bay (64°), and the North Shore of the Gulf (60°).
In summer Southern Quebec is invaded by warm air currents from the southwest. Temperatures over 90° are frequent; and if the air mass has a high relative humidity, as is often the case, the weather is not pleasant for city dwellers. They find cooler places along the shores down the St. Lawrence where breezes of more northern latitudes dissipate the heat, and where a cold marine current keeps the water at a lower temperature than that of the air.
The number of frost-free days is very important for agriculture. Around Monttreal the average is 150 days per season; it is 130 at Quebec and along the south shore as far as Matane. Here the influence of the wide estuary is important. The average length of the frost-free season is well over 100 days in all sections where agriculture is practised. This correlates rather closely with the isotherm of 60°F. average temperature for the four months of June, July, August and September. It extends from Abitibi in Western Quebec to the Isle of Orleans and along the south shore to Lake Temiscouata. It also forms a closed circle around the Lake St. John district.