Cold fronts are formed when air masses that consist of cooler heavier air move in to displace warmer air. An air mass is a large body of air that shares a relatively uniform temperature and level of humidity. Fronts are the atmospheric boundaries between two air masses.
Cold air masses are formed above polar regions and areas that have relatively low surface temperatures. Air masses can shift and move depending on the influence of wind patterns, changes in atmospheric density and even the earth's rotation. Frontal systems occur when an air mass of different temperature, density and humidity moves into a region to displace an existing air mass.
Cold fronts often produce precipitation, especially when displacing an air mass of greater humidity. Water vapor within a warm air mass is condensed by the colder heavier air contained within a frontal system until rain or frozen precipitation begins to form. Cold fronts often trigger thunderstorms and may produce severe weather, especially when there is a large temperature and humidify difference between the air mass that is being displaced. Cold fronts can bring about rapid temperature changes because they have a steep slope which causes the air in front of them to be forced upwards along the leading edge.