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UNIT 4—LESSON 2

FRONTS

OVERVIEW

Describe the specific parts that make up a front and describe how a front is classified as cold, warm, occluded, or quasi-stationary.

Describe the relationship of fronts to air masses and cyclones.

Describe frontogenesis and frontolysis and iden-tify the frontogenetical zones.

Describe the frontal elements and general char-acteristics of fronts.

OUTLINE

Definitions and classification

Relation of fronts to air masses

Relation of fronts to cyclones

Conditions necessary for frontogenesis

Frontolysis

World frontogenetical zones

Frontal elements

General characteristics of fronts

FRONTS

A front, generally speaking, is a zone of tran-sition between two air masses of different density and temperature and is associated with major weather changes, some of which can be violent. This fact alone is sufficient reason for an in-depth study of fronts and their relationship to air masses and cyclones.

Learning Objective: Describe the specific parts that make up a front and identify how a front is classified as either cold, warm, occluded, or quasi-stationary.

DEFINITIONS AND CLASSIFICATIONS

A front is not just a colorful line drawn on a surface chart. A front is a three-dimensional phenomena with a very specific composition. Since a front is a zone of transition-between two air masses of different densities, there must be some sort of boundary between these air masses. One of these boundaries is the FRONTAL SURFACE. The frontal surface is the surface that separates the two air masses. It is the surface next to the warmer air (less dense air). In reality, however, the point at which two air masses touch is not a nice, abrupt separation. This area is a zone of a large density gradient. This zone is called the FRONTAL ZONE. A frontal zone is the transi-tion zone between two adjacent air masses of different densities, bounded by a frontal surface. Since the temperature distribution is the most important regulator of atmospheric density, a front almost invaribly separates air masses of different temperatures. At this point you should be aware of the various types of fronts. The question in your mind should be how a front is classified as either cold, warm, stationary, and so on. A front is classified by determining the instantaneous movement. The direction of movement of the front for the past 3 to 6 hours is often used.. Classification is based on movement relative to the warm and cold air masses involved. The criterion is as follows:

1. COLD FRONT. A cold front is one that moves in a direction in which cold air DISPLACES warm air at the surface. In other words the cold (or cooler) air mass is moving toward a warmer air mass. The cooler, denser air is sliding under the warmer, less dense air displac-ing it upward.

2. WARM FRONT. A warm front is one along which warmer air REPLACES colder air. In this case, a warmer air mass is moving toward a cooler retreating air mass. The warmer, less dense air moves only toward and replaces the colder, more dense air if the colder air mass is also moving.

3. QUASI-STATIONARY FRONT. This type front is one along which one air mass does not appreciably replace the other. These fronts are stationary or nearly so (speed under 5 knots). They can move or undulate toward either the cold or warm air mass.

4. OCCLUDED FRONT. An occluded front is one where a cold front overtakes a warm front, forcing the warm air upward. The occluded front may be either a WARM FRONT or a COLD FRONT TYPE. A WARM FRONT TYPE is one in which the cool air behind the cold front over-rides the colder air in advance of the warm front, resulting in a cold front aloft. A COLD FRONT TYPE is one in which the cold air behind the cold front underrides the warm front, resulting in a warm front aloft.

Learning Objective: Describe the relation-ship of fronts to air masses and stable and unstable wave cyclones.

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