Warm Fronts, Occluded Fronts & Stationary Fronts

Warm Fronts

A front in which a warmer air mass is advancing and replacing a retreating colder air mass is a warm front.
A horizontal temperature discontinuity exists across the frontal surface, with colder air ahead of the frontal surface and warmer air behind it.

Warm Front Structure

The surface front is located on the warm side of the transition zone.

A moisture discontinuity exists across the frontal surface. Air behind the warm front typically is more moist (in terms of an absolute measure of moisture) than air ahead of the warm front. In other words, the dew point increases with FROPA. However, the air ahead of the warm front is normally nearer to saturation than that behind the warm front because fallen precipitation is evaporating. So, the relative humidity decreases with FROPA. While the warm air behind the front holds more moisture, it is farther from saturation.

The surface and 850mb fronts are located in frontal troughs. Strong pressure falls (PRESFR) are observed ahead of the surface front. This is caused by the warm- air advection in the transition zone. A slower change in pressure occurs after FROPA. The pressure behind the surface front may fall slowly, rise slowly, or remain steady because of weak temperature advection.

Winds turn cyclonically across the frontal trough and veer with frontal passage.

Vertically, the frontal surface slopes, over the colder air mass. The frontal surface is identified as the warm side of the transition zone. The upper front at 850 mb marks the intersection of the frontal surface with the 850 mb constant- pressure surface. Appreciable cyclonic turning is usually found in this region. Above the 850-mb level, a well-defined frontal trough may not be found. Strong fronts are clearly defined up through 700 mb. Weak fronts are normally best defined at 850 mb and below. The slope of a warm front is typically between 1/100 and 1/300. Fronts with steep slopes generally are visible up to 700 mb, but may be reflected up to 500 mb. Shallow fronts normally extend only up to 850 mb, not above.

An upper-air sounding originating on the cold air side of the surface front will exhibit veering winds in the vertical within the stable layer. (Recall: The wind veering with increasing height suggests warm-air advection).

Warm Front Characteristics

Warm fronts generally move more slowly than cold fronts. The speed with which the colder air mass retreats determines the speed at which the warm front advances. The movement of the warm front and the strength of the upward vertical motion caused by warm air gliding up the frontal surface (overrunning) are closely related. If the colder air mass is retreating as rapidly as the warmer air mass is advancing, then there is little lift along the frontal surface, thus little or no precipitation occurs. If the colder air mass is retreating slowly, but the warmer air mass is advancing rapidly, then the warm air ascends the frontal surface and widespread precipitation occurs.

The cloud sequence ahead of a warm front with significant overrunning follows a normal routine. As the surface front approaches, CI, CS, AS and finally NS (ST) clouds appear.

The precipitation sequence as the warm front approaches will depend on the time of year and whether the temperature of the retreating colder air mass is below freezing. If the temperature of the colder air mass is below freezing, then the precipitation sequence as the warm front approaches is as follows:

Snow falls well ahead of the front, where the air is below freezing throughout the column.
Ice pellets fall where the warm layer aloft is deep enough to partially melt the falling precipitation.
Freezing rain occurs where the warm layer aloft is deeper and surface temperatures remain below freezing.
Rain and/or drizzle occurs near the front where the warm layer extends to the ground.

If the temperature of the cold air is well above freezing, then a large area of light rain will occur ahead of the front. Fog is often observed near the front.

Not every warm front will have the weather patterns described above. If the air is very dry, or the upward vertical motion is weak, only high cloudiness may be present. Next, we will examine occluded fronts.

Occluded Fronts

Cold Type Occlusion

Cold type occlusions occur where the coldest air is found behind the occlusion. They are normally located within the thermal ridge on the thickness chart. This type of occlusion will appear on the surface as an extension of the cold front poleward on the warm front and wrapping into the baroclinic low. Cold occlusions form when the cold front overtakes the warm front, lifting both the warm air behind the warm front and the cool air ahead of it.

Cold occlusions are quite common over the eastern portions of continents and western ocean areas in the winter. In the summer, they may be found along the west coast of continents where the ocean air is colder than the relatively warmer air over the continent.

Warm Type Occlusion

Warm type occlusions occur where the coldest air is located ahead of the occlusion. They are normally located behind the thermal ridge on the thickness chart. On the surface, this type of occlusion will appear as an extension of the warm front, poleward of the surface cold front, wrapping into a baroclinic low. They form where the cold front and the cool air behind it are forced aloft, over the warm frontal surface.

Warm occlusions are quite common over the western portions of continents and eastern ocean areas during the winter months. In the summer, they may also be found along the east coasts of continents and western ocean areas, where the continental air is relatively warmer than that of the air found over ocean areas

Occlusion Characteristics

When identifying occlusions, one must keep in mind the following:

• Temperature structure varies with each type of occlusion.
• Winds veer with the passage of the occlusion.
• Winds ahead of the occlusion are typically from the east or southeast, whereas winds behind the occlusion will be from the southwest or west.
• The occlusion will be found in or near the thickness ridge.
• Occlusion structure can vary greatly. Vertical motion and latent heat release can substantially affect the thermal pattern associated with the occlusion.

Stationary Fronts

Stationary Front Structure. A front which exhibits little or no movement is a stationary front. This front is not truly stationary but more quasi-stationary (movement less than 5 knots) It is located in regions where there is no further appreciable movement of the cold air equatorward or the warm air poleward. A horizontal temperature discontinuity exists across the frontal surface, with warm air on one side, and cold air on the other.

The surface front will be found on the warm side of the transition zone on the thickness chart. On the surface chart it lies in a frontal trough.

The winds aloft are nearly parallel to the front (and hence the transition zone) so little temperature advection occur.

Small-scale influences concentrate frontal lift near the surface front. The frontal surface slopes up over the colder air mass and is identified as the warm side of the transition zone.

An upper-air sounding originating on the cold side of the surface front will exhibit light winds within the transition zone. Light winds suggest weak temperature advection.

Weather Associated With Stationary Fronts

Stratiform cloudiness and precipitation are generally confined to the cold air side of the front. Cumuliform cloudiness and precipitation can occur on either side of the front. Whether it is present or not depends on the stability of the warm air mass.