How Is a Cold Front Formed?

Air Masses

• The most basic ingredients of all the Earth's weather, including cold fronts, are air masses. Air masses are large bodies of air that have uniform characteristics, namely their temperature and moisture content. In order for a large body of air to develop uniformity, the air mass requires a large, flat geographical region with uniform characteristics. Air masses tend to linger over these regions, enabling them to take on the region's surface characteristics before moving on. It is a certain kind of air mass that leads to the formation of cold fronts.

Types of Air Masses

• Air masses are categorized by the regions they form over. These regions are broken down by their latitudinal location. Tropical air masses typically form close to the equator, within 25 degrees of latitude. They are characterized by warm air. Polar air masses typically form in regions higher than 60 degrees of latitude and are characterized by cold air. Air masses are also classified by the characteristics of their source regions. Continental air masses are dry and form over large land masses while maritime air masses are moist and form over oceans. Cold fronts are formed by polar air masses. Cold fronts over the central United States are typically continental in nature, lacking moisture.

Collision of Air Masses

• Polar air masses tend to move in a southeasterly direction over the United States. However, tropical air masses typically move in a northerly direction. This means that air masses with different properties are continually colliding over the country. The boundary between these colliding air masses is called a front, or frontal boundary. When the cold air is advancing, the front is called a cold front. When the warm air is advancing, the front is called a warm front. The front always represents the leading edge of the advancing air mass.

Cold Fronts

• Polar air masses are generally driven by an area of low pressure, with a counterclockwise circulation. The cold air is located to the west of the area of low pressure, meaning it is pushed southward. At the same time, the area of low pressure draws warm air northward. This results in a collision between the cold and warm air masses that typically occurs to the south or southwest of the area of low pressue. Here, the heavier cold air plows under the lighter warm air, forming a cold front. Cold fronts have a steep leading edge, like a giant wedge. This steep gradient forces the warm moist air to rise rapidly, producing thunderstorms and severe weather along the cold front. Furthermore, the cold front brings colder temperatures as it passes a given location.