The air masses differ in their temperature and in their moisture content. They also differ in their densities and atmospheric pressure.
When two different air masses with sharp contrasts in their physical characteristics such as temperature, humidity, pressure, density etc. are brought together by converging movements in the general atmospheric circulation, they do not mix readily.
In fact, they come in contact with one another along sloping boundaries. These sloping boundaries are actually a transition zone across which a sharp contrast in weather conditions occurs.
Weather conditions within an air mass are comparatively uniform, but in leaving an air mass boundary, there is an abrupt discontinuous change in the weather elements previously encountered.
This is why the term ‘line of discontinuity’ is applied to this transition zone. The term ‘front’ is synonymous with the ‘line of discontinuity’. A front may thus be defined as “an interface or transition zone between two air masses of different density”.
The concept of front or frontal surfaces was introduced in meteorology by the Norwegian school of meteorologists in about 1918.
The three distinguished Norwegian meteorologists whose intensive investigations laid the foundation of this concept are V.Bjerknes, J. Bjerknes and H. Solberg.
The term ‘front’ was borrowed by analogy from the military front during World War I. Just as a military front implies the presence of two enemy forces, so a front in meteorology represents the meeting ground of two contrasting air masses.
Since air masses are three-dimensional, i.e. they have a vertical as well as horizontal extent, the surface separating adjacent air masses in vertical plane is known as the frontal surface.
Therefore a frontal surface may be defined as the three-dimensional transition zone between contrasting air masses. Frontal surfaces are zones of discontinuities in the air mass properties.
Weather charts and maps depict the air masses and fronts as two dimensional. It is, therefore, advisable not to lose sight of the three dimensional aspect while dealing with air masses and fronts.
The air masses are of vast size covering tens of thousands of square kilometers. Therefore the frontal zones of discontinuity about 15 to 200 kilometers wide are relatively narrow.
Soon the weather maps they are represented by only a thick line. The line formed by the intersection of the frontal surface with the ground is referred to as the ground front or simply the front.
The narrow frontal zones are marked by abrupt changes in various weather elements such as temperature, air pressure, humidity, etc.
On occasions, in a narrow frontal zone a temperature change of 10° to 20° Celsius may be observed over a short distance of 3 kilometers.
In an air mass the changes in different climatic elements are only gradual. But the boundary lines of air masses are distinguished by rapid changes in the weather elements.