In the midlatitudes, cold, polar air masses meet warm, tropical air masses. It is here that fronts are generated. Officially, a front is a transition zone between two air masses of different densities or temperature. Frontogenesis is a term describing the formation of a weather front or the regeneration and intensification of existing, decaying fronts. The necessary conditions for frontogenesis are:
Temperature Gradient - There must be a temperature difference between two air masses. For example, no frontogenesis takes place around the Equator where air masses are of similar temperature. The temperature difference or temperature gradient must be enhanced or concentrated. Isotherms on a temperature map would be close together.
Convergence of air masses - Air masses must be converging, attempting to occupy the same space.
Convergence of air masses
In the 1920s, meteorologists of the famed Scandinavian School developed the theory of the Polar Front, a semi-permanent, semi-continuous undulating front encircling the globe and separating polar and tropical air masses. As areas of low pressure form along this front in the northern hemisphere, air flows towards the low. Because of the rotation of the Earth, the winds will flow in an anti-clockwise direction around and into the low. Warm air from the south rushes northward ahead of the low, and cool air from the north is pulled southward behind the low. The boundary between the two air masses is a front. A warm front is formed to the east of the low while a cold front forms initially to the west and then southward.
The sharpening of the front
The effect of convergence towards a front is generally to narrow the transitional zone between the adjacent air masses. In other words to sharpen the front. Ageostrophic wind components directed towards the front on both sides, brings the isotherms closer together. The intervening air is squeezed upwards and the transitional zone becomes narrower and the front sharper. The sharpness of the front is maintained against the natural tendency towards a diffuse mixing of the air from each airmass.