8.3: Fronts
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Fronts are boundaries between contrasting masses of air. Atmospheric scientists recognize fronts of different spatial scales. These range from the quasi-stationary fronts along which cyclones form to "weather" fronts embedded in cyclones. Fronts are three-dimensional features. They are not only a boundary between contrasting air masses running along the surface, but extend upwards into the troposphere as we will later learn.
Quasi-stationary fronts
At the global scale are quasi-stationary fronts found migrating within a particular latitudinal zone throughout most of the year. The polar front is the boundary between polar-type air and tropical-type air. The polar front migrates between about 35o and 65o, following the annual cycle of earth surface heating (Figure \(\PageIndex{1}\)). Above the polar front is found the polar front jet stream, a high velocity corridor of wind that controls the development and movement of mid-latitude cyclones.
During the winter, the polar front slides equatorward along with invading cold air. During the summer, the polar front retreats northward. This seasonal migratory pattern moves cyclones into and out of the middle latitudes giving them quite variable weather conditions over the seasons.
Synoptic Scale Fronts
At a smaller or synoptic scale are the "weather" fronts e.g., cold, warm, occluded, and stationary. Cold fronts are those where cold air replaces warm air. A warm front is where warm air replaces cold air. A noticeable difference between warm and cold fronts is the slope of the front above the surface. The frontal surface, the portion extending upward above the surface, is much steeper for the cold front than warm front. The steepness of the frontal surface directly impacts the type of weather one experiences along these fronts.
An occluded front forms when a cold front catches up with a warm front. Air is often converging at a front producing a trough of low pressure along it. A decrease in pressure is often experienced with their passage. A stationary front is where no change in air masses or movement of the front occurs. The weather associated with these fronts is discussed later.
Weather Map Depiction
Meteorologists use both symbols and color to distinguish between synoptic scale fronts on weather maps. If printed in color, warm fronts are shown as a line of red semi-circles pointing in the direction of movement. Cold fronts are depicted as a line of blue triangles. Occluded fronts appear in purple with both warm and cold front symbols on the same side. The symbols point in the direction of the front is moving. Stationary fronts are alternating warm and cold front symbols on opposite sides, indicating no movement. A portion of a simplified weather map is shown in Figure \(\PageIndex{5}\). The map depicts a wave cyclone as it is starting to occlude. We see an occluded front trailing southeast from the center branching into cold and warm fronts.
The location of air masses on weather maps are identified by their letter abbreviation, e.g., mT, cP, mP. Shading is used to show where areas of precipitation occur. Looking at the local environmental setting can give a clue as to what mechanism caused uplift for precipitation to form. Note that the areas of precipitation in Figure \(\PageIndex{5}\) either occur ahead of a front (frontal lifting) or to the north of the center of the low (convergent lifting).
The distribution of air pressure is shown by isobars, lines connecting points of equal air pressure. Isobars are drawn in increments of 4 millibars on surface weather maps. Recall that it is the pressure gradient that controls wind speed. Strong pressure gradients and hence faster winds occur where the isobars are closely spaced. Weak pressure gradients and slow winds occur where the isobars are widely spaced.