11.3: Surface Fronts

Surface fronts are the boundaries or transition zones between air masses at the Earth’s surface. Changes in temperature, humidity, wind, pressure, visibility, as well as particular 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

Diagram showing a vertical cross section through a cold front (CC BY-SA 4.0).

Cold fronts are a transition zone in which the advancing cold air mass replaces a retreating warmer air mass. On weather maps, cold fronts are drawn as blue lines with blue triangles pointing toward the warmer air mass, in the direction of frontal movement, as seen in the above image.

Because warm air is less dense than colder air, the cold air stays on the bottom and warm air is forced to rise above the advancing cold air. This forced lifting results in typical cloud patterns ahead of a cold front, which include cirrus and cirrostratus clouds. 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

Diagram showing a vertical cross section through a warm front (CC BY-SA 3.0).

Warm fronts are a transition zone in which the advancing warm air mass replaces a retreating colder 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.

Advancing warm air is forced to rise above the retreating cold dense air. 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 clouds before clouds thicken and lower-level clouds like altostratus, nimbostratus, and fog near the frontal boundary. Because the air mass is rising 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 decreases with only scattered cumulus clouds remaining.

Occluded Fronts

Diagram showing a vertical cross section through an occluded front (Public Domain).

Occluded fronts are a frontal boundary that forms when a cold front catches up to a warm front. Cold fronts move faster than warm fronts, so cold fronts can sometimes catch up to warm fronts, but not the other way around.

There are two types of occlusions: cold occlusions and warm occlusions. Cold occlusions (shown above) 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. On weather maps, occluded fronts are drawn as purple lines with purple triangles and purple semi circles. 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.

Frontal symbol for an occluded frontal boundary (Public Domain).

Occluded fronts are the final stage of a frontal boundary because warm air above cool air is a stable scenario.

Stationary Fronts

Stationary fronts are a type of frontal system that are almost stationary with the winds flowing nearly parallel and from the opposite paths in each side separated by the front. On weather maps, stationary fronts are drawn as alternating blue and red lines with blue triangles pointing toward the warmer air mass and red semicircles pointing toward the colder air mass. This is the only scenario where the direction of the symbols does not indicate a direction of movement.

Frontal symbol for a stationary frontal boundary (Public Domain).

Other Frontal-like Features

Not all weather features have frontal characteristics, such as a trough and a dryline. While troughs feature a change in cloud and precipitation pattern, it lacks the sharp change in temperature and moisture as 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 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.

Chapter 12: Questions to Consider

1. What is an air mass? What are some different types of air masses and where do they originate?
2. Label the cold front cross-section:

Query \(\PageIndex{1}\)

3. Label the warm front cross-section:

Query \(\PageIndex{2}\)

4. Match the front name with the correct symbology:

Query \(\PageIndex{3}\)

5. What cloud and precipitation patterns are typically associated with warm and cold frontal passage?

Selected Practice Question Answers:

Query \(\PageIndex{4}\)