Frontogenesis and Frontolysis; Cold, Warm and Occluded Fronts

Air mass is a volume of air defined by its temperature and water vapour content. An air mass may be of many hundreds or thousands of square miles, and adopt the characteristics of the surface below them. An air mass can be so extensive that it may cover the large portion of a continent below it and may be vertically so thick that may cover the troposphere. The vertical distribution of the temperature in an air mass and moisture content of the air are the two properties of air air mass which control the weather conditions of an area under that particular air mass. The air mass is considered to be cold air mass if its temperature is lower than the underlying surface, while an air mass is terms warm air mass when its temperature is higher than the underlying surface. The boundary between the two air masses is called the front.

Air masses are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. Continental and superior air masses are dry while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density (temperature and/or moisture) characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character.

Frontogenesis and Frontolysis

The boundary between the two air masses is called the front.  A temperature difference is essential in the definition of a front because it implies a density difference. The air masses of different densities don’t mix readily and tend to retain their identity as far as we care for the moisture. The front represents a transition zone between two air masses of different density. Generally, an air mass from one region moves to the other region which is occupied by some other air mass. When a warmer and lighter air mass moved against a cold and denser air mass, the former rides over the other and it is called warm front. If the cold air mass forces its way under a warm air mass, it is called cold front. When new fronts are created or old fronts are regenerated, it is called Frontogenesis. Please note that fronts don’t appear all of a sudden. They appear only after a process of Frontogenesis which is there in place for quite some time. When winds converge towards a point it would lead to Frontogenesis.

Frontogenesis takes place only when two conditions are met. First, two air masses of different densities must exist adjacent to one another; and second, a prevailing wind field must exist to bring them together.   There are three basic situations, which are conducive to Frontogenesis and satisfy the two basic requirements. The wind flow is cross isothermal and flowing from cold   air   to   warmer   air.   The   flow   must   be   cross isothermal, resulting in  a  concentration  of  isotherms (increased  temperature  gradient).  The  flow  does  not have to be perpendicular; however, the more perpendicular the cross isothermal flow, the greater the intensity of Frontogenesis.

On the other hand, the dying of a front is called Frontolysis. Frontolysis also does not happen all of a sudden. The process of Frontolysis must happen for quite some time to destroy the existing front.

Types of Fronts

Cold Front

When a cold air invades the warm air, it remains at the ground and forcibly uplifts the warmer and lighter air mass. This is known as Cold front. This upward motion causes lowered pressure along the cold front and can cause the formation of a narrow line of showers and thunderstorms when enough moisture is present. Cold fronts can move up to twice as fast as warm fronts and can produce sharper changes in weather. Since cold air is denser than warm air, it rapidly replaces the warm air preceding the boundary. Cold fronts are usually associated with low-pressure areas. Cold front usually causes a shift of wind from southeast to northwest, and in the southern hemisphere a shift from northeast to southwest.

Warm front

When a warmer and lighter air mass moved against a cold and denser air mass, the former rides over the other and it is called warm front.  Being lighter, the warm air mass is unable to displace the cooler air mass and instead is forced upward along the upper boundary of the colder air in a process known as overrunning. The boundary between the two air masses has a gradual slope of 130 and lifting is slow but persistent. As the air mass rises into regions of lower pressure, it expands and cools. As it cools, water vapour condenses and forms extensive cloud coverage. The first clouds to form along the sloping surface of the cold air are high cirrus, which thicken to cirrostratus and altostratus.

Occluded front

An occluded front is a front that is formed when a cold front overtakes a warm front. The cold front moves rapidly than the warm front. Ultimately, the cold front overtakes the warm front and completely displaces the warm air at the ground.