CLASSIFICATION OF FRONTS

Fronts can be classified geographically and also according to the motion of the air masses involved.

Geographical Classification

The formation of fronts occurs from time to time in widely different regions over the earth's surface.  However, it follows from what has been said about frontogenesis that the development of fronts occurs most frequently in certain geographical regions, particularly, where deformation fields prevail with suitable temperature gradient.  Regions favorable to frontogenesis are usually found along the boundaries of the four major air masses.  The fronts along these boundaries are called artic, polar, and intertropical front according to their mean positions.


Classification According To Motion Of Air

This classification is based primarily on the displacement of fronts and the resultant temperature changes.  Four basic types are recognized by this classification: cold front, warm front, occluded front and stationary front.

A cold front is the boundary along the leading edge of a cold air mass that is pushing out a warm air mass.  As the cold front nears your region, the barometer falls.  The cold air behind the front wedges under the warm air and lifts it sharply off the ground.  Large cumulonimbus clouds appear.  These clouds often bring thunderstorms and rain showers.  As the cold front passes, the wind changes direction.   The weather becomes clear and colder and the barometer rises again.


The diagram shows a typical cold front and its weather map symbol.  Similar to the warm front, the steeper the slope of the front, the more violent are the storms associated with it.  The width of the front is usually 50-100 km; however, on the scale of the diagrams, it looks like a line.

Further differentiation of cold fronts on the basis of vertical velocities is shown in two types.   Type I cold fronts are those observed outside the zone of cyclonic activity and are generally slow moving which may even be quasi-stationary in some cases.  It is defined by a general upglide of warm air over the entire frontal surface so that the cloud system resembles that of a warm front.   Type II cold fronts are observed within the zone of cyclonic activity and as a rule move very rapidly.  Warm air is lifted only along the leading edge of the intruding wedge of cold air.  At higher levels, the warm air is moving faster than the cold air and therefore no ascending motions are observed in the warm air above the frontal surface.


A warm front is the boundary along a warm air mass that is pushing out a cold air mass.  The warm air behind the front glides up and over the cold air. As a warm front approaches, high cirrus clouds appear.  These are followed by stratus and nimbostratus clouds.   The barometer falls and a long, steady rain begins. Gradually, the front passes and the sky clears.  Temperature rises as warm air replaces cold air, and the barometer stops falling.


A warm front and its weather map symbol is shown in the diagram.


A stationary front forms when two air masses remain over a region for several days.  The front formed does not move.


An occluded front forms when a cold front overtakes a slower-moving warm front.  The occluded front is more complicated than the others because two fronts interact.  In the left diagram, colder air is wedging under warm air at the cold front.  Warm air is gliding up and over another cold air mass at the warm front.  The result of what is happening in the left diagram is seen in the right diagram.  The arm air squeezed out and lifted above the ground.  Steady rains falls at an occluded front.


The weather at the occluded front is long, steady rain - similar to warm front weather.  Sometimes the front ends with sharper cold front storms. The map symbol is shown in the left diagram.

During the winter months in the northern hemisphere, cold fronts usually extend to as far as the southern portion of the Philippines.  They are responsible for some of the rainfall along the eastern coasts of the country.

For forecasting purposes, it is not sufficient to know whether one is dealing with a cold front or a warm front.  In order to forecast the weather conditions accurately, it is further necessary to know the stability conditions in both air masses as well as the distribution of vertical velocities.  An estimate of the stability of the respective cold and warm air masses can usually be gained from upper-air sounding and cloud observations.  The determination of vertical currents on a large scale is not feasible so far; however, an estimate of the sign of the vertical velocity can usually be obtained from the distribution of hydrometeors and clouds.