'Front' is the term given to the zone of contact between two air masses of different qualities. Since air is extremely fluid, fronts are zones of mixing and of unsettled weather. There are two main types of frontal formations. Cold fronts occur when a cold air mass advances into or underruns a warm one, while a warm front is formed when a warm air mass is forced to rise over the edge of a cold air mass. The sinuous fluctuating zone which crosses North America from west to east dividing the polar air masses from those of tropical origin is usually known as the Polar front.
Other fronts may also develop as, for instance, between the Polar Pacific and Polar continental air masses or between a fresh outbreak of polar air and the now modified polar continental air of a previous outbreak.
Southern Canada is often stated to be in the belt of westerly winds and cyclonic storms. Long before the days of air mass studies it was known that our weather was characterized by the passing of alternate high and low pressure areas from west to east across the country. The low pressure areas or cyclones are gigantic eddies in the atmosphere into which air currents spiral from all directions. Precipitation usually accompanies these storms. On the other hand the winds spiral outward from the high pressure areas or anticyclones, the passing of which occasions periods of clear dry weather. This type of weather is more noticeable in the cool half of the year when the cyclonic belt lies farther south. Regardless of where they may first appear on the continent these storms usually leave it by way of eastern Canada.