8.2: Fronts

Surface Fronts

Surface fronts are the boundaries or transition zones between air masses at the Earth’s surface. Changes in temperature, humidity, wind, pressure, and visibility, as well as specific cloud and precipitation patterns, are often observed at fronts. There are four main types of fronts.

1. Cold fronts
2. Warm fronts
3. Occluded fronts
4. Stationary fronts

Fronts are named based on the characteristics of the air mass that is replacing the prior air mass. For example, if a cold air mass is moving toward a warm air mass, the boundary between them will be called a cold front because the cold air is effectively replacing the warm air from the perspective of a stationary point on Earth’s surface. Fronts are usually associated with low-pressure systems. Frontal boundaries on a map are labeled in the location where the temperature gradient of the front meets the Earth’s surface.

Cold Fronts

Cold fronts are transition zones in which the advancing cold air mass replaces the retreating warm air mass. On weather maps, cold fronts are depicted as blue lines with blue triangles pointing toward the warmer air mass, indicating the direction of frontal movement, as illustrated in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): A vertical cross-section of a cold front shows cold air behind the cold front (dark blue lines) advancing into warmer air ahead of the front. Where the two air masses meet, convergence often occurs, resulting in the upward motion of air parcels. If the air contains enough moisture, rain can occur. (CC BY-SA 4.0; Ravedave via Wikimedia Commons).

Because warm air is less dense than cold air, the cold air stays at the bottom, and warm air is forced to rise above the advancing cold air. This forced lifting produces typical cloud patterns ahead of a cold front, including cirrus and cirrostratus. The number of cumulus clouds in the warm air mass increases as the frontal boundary approaches. Because the warm air mass is forced to rise, atmospheric instability occurs along the cold front and results in towering cumulus and cumulonimbus clouds, which may produce heavy rain and thunderstorms along the frontal boundary. During the passage of a cold front, the wind direction generally shifts from south or southwest (in the warm sector) to west or northwest (in the cold sector) in the northern hemisphere. After a cold front’s passage, fair weather returns with the appearance of cumulus and stratocumulus clouds.

Warm Fronts

Warm fronts are transition zones in which the advancing warm air mass replaces the retreating cold air mass. On weather maps, warm fronts are drawn as red lines with red semicircles pointing toward the colder air mass in the direction of the frontal movement, as illustrated in Figure \(\PageIndex{2}\).

Figure \(\PageIndex{2}\): A vertical cross-section of a warm front (dark red lines) shows how surface warm air behind the warm front flows upward in a slantwise manner over top low-level cool or cold air ahead of the position of the surface warm front. This causes clouds and precipitation ahead of the surface warm front when the air is sufficiently moist. (CC BY-SA 3.0; Ravedave via Wikimedia Commons).

Warm air advancing over the retreating cold, dense air is forced to rise. Again, because of this forced lifting, typical cloud patterns are common ahead of a warm front. These include upper-level clouds, such as cirrus and cirrostratus, before clouds thicken, and lower-level clouds, such as altostratus, nimbostratus, and fog, near the frontal boundary. As the air mass rises along the warm front, clouds form, and steady precipitation may occur. During the passage of a warm front, wind direction shifts from east or southeast (in the cold sector) to southwest (in the warm sector) in the northern hemisphere. After a warm front’s passage, cloud cover and precipitation decrease, with only scattered cumulus clouds remaining.

Occluded Fronts

Occluded fronts are a frontal boundary that forms when a cold front catches up to a warm front, as illustrated in Figure \(\PageIndex{3}\). Cold fronts move faster than warm fronts, so cold fronts can often catch up to warm fronts, but not the other way around. When this happens, the warm air is separated (occluded) from the cyclone center at the surface. The cold air from the cold front and cooler air ahead of the warm front cover the surface near the front, while warm, moist air is convected to higher elevations. The point where the warm front becomes the occluded front is called the triple point.

Figure \(\PageIndex{3}\): A vertical cross-section through an occluded front shows that the cold air mass is moving faster than the cool air mass ahead. As the two fronts converge, the cold air undercuts the warmer air mass. (Public Domain; JWIZMAN96 via Wikimedia Commons).

There are two types of occlusions: cold occlusions and warm occlusions. Cold occlusions, as illustrated in Figure \(\PageIndex{3}\), occur when the advancing air mass is colder than the retreating air mass. Warm occlusions occur when the advancing air mass is warmer than the retreating air mass. Warm air occluded fronts are more common on the West Coast of continents, while cold air occluded fronts are more common on the East Coast of continents. On weather maps, occluded fronts are drawn as purple lines with purple triangles and purple semicircles, as illustrated in Figure \(\PageIndex{4}\). You’ll notice that this symbol is a combination of the cold front and warm front symbols, which isn’t surprising because an occluded front is effectively a combination of the two. Occluded fronts are the final stage of a frontal boundary because the warm air above the cool air is stable.

Figure \(\PageIndex{4}\): Frontal symbol for an occluded frontal boundary (Public Domain; -xfi- via Wikimedia Commons).

Stationary Fronts

Stationary fronts are a type of frontal system in which the winds flow nearly parallel to one another, from opposite directions on each side, separated by the front. On weather maps, stationary fronts are depicted as alternating blue and red lines, with blue triangles indicating the direction of the warmer air mass and red semicircles indicating the direction of the colder air mass, as illustrated in Figure \(\PageIndex{5}\). This is the only scenario in which the direction of the symbols does not indicate movement.

Figure \(\PageIndex{5}\): Frontal symbol for a stationary frontal boundary (Public Domain; -xfi- via Wikimedia Commons).

Other Frontal-like Features

Not all weather features have frontal characteristics, such as a trough and a dryline. While troughs feature changes in cloud and precipitation patterns, they lack the sharp temperature and moisture gradients observed in fronts. Drylines are a boundary between warm, moist air and warm, dry air. Because both air masses are warm, a dryline cannot be classified as either a warm front or a cold front. Drylines commonly occur during spring and summer in the southwestern United States, particularly in Texas. Warm, humid air from the Gulf of Mexico meets with the warm, dry air from the desert plateau. During the afternoon, convective clouds and even thunderstorms can develop in drylines as moist air rises over the denser dry air.