6.3: 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.

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.

Figure \(\PageIndex{1}\): Profile view of Cold front

Figure \(\PageIndex{2}\): Profile view of warm front

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 \(\PageIndex{4}\). 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.

Front symbolization — Figure \(\PageIndex{3}\): Front Symbols

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 \(\PageIndex{4}\) 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.