Chapter 11, Air Masses, Fronts, and the Wave Cyclone Model 11-7
Figure 11-6. Cold Front
11.3.3 Stationary Front
When the forces of two air masses are relatively equal, the boundary or front that separates them remains
stationary and influences the local weather for days. This front is called a stationary front. Stationary frontal
slope can vary, but clouds and precipitation would still form in the warm rising air along the front
(see Figure 11-7). The weather associated with a stationary front is typically a mixture that can be found in
both warm and cold fronts.
Figure 11-7. Stationary Front
Chapter 11, Air Masses, Fronts, and the Wave Cyclone Model 11-8
11.3.4 Occluded Front
Cold fronts typically move faster than warm fronts, so in time they catch up to warm fronts. As the two
fronts merge, an occluded front forms (see Figure 11-8). At the occluded front, the cold air undercuts the
retreating cooler air mass associated with the warm front, further lifting the already rising warm air. Clouds
and precipitation can occur in the areas of frontal lift along, ahead of, and behind the surface position of an
occluded front. As the occluded front approaches, warm front weather prevails but is immediately followed
by cold front weather.
There are two types of occluded fronts that can occur, and the temperatures of the colliding frontal systems
play a large part in defining the type of front and the resulting weather.
A cold front occlusion occurs when a fast-moving cold front is colder than the air ahead of the slow-moving
warm front. When this occurs, the cold air replaces the cool air and forces the warm front aloft into the
atmosphere. Typically, the cold front occlusion creates a mixture of weather found in both warm and
cold fronts, providing the air is relatively stable.
A warm front occlusion occurs when the air ahead of the warm front is colder than the air of the cold front.
When this is the case, the cold front rides up and over the warm front. If the air forced aloft by the warm
front occlusion is unstable, the weathe r is more severe than the weather found in a cold front occlusion.
Embedded thunderstorms, rain, and fog are likely to occur.
Figure 11-8. Occluded Front
Chapter 11, Air Masses, Fronts, and the Wave Cyclone Model 11-9
11.4 The Wave Cyclone Model
A wave cyclone 3 is a low-pressure circulation that forms and moves along a front. The circulation about
the cyclone center tends to produce a wavelike kink along the front. Wave cyclones are the primary weather
producers in the mid -latitudes. They are large lows that generally travel from west to east along a front.
They last from a few days to more than a week.
A wave cyclone typically follows a predictable evolution. Initially, there is a stationary front separating
warm air from cold air (see Figure 11-9).
Figure 11-9. Wave Cyclone Model—Stage 1
A low-pressure wave forms on the front (see Figure 11-10). The front develops a kink where the wave
develops. Precipitation develops with the heaviest intensity (dark green) located in the zone of lift along
the front.
Figure 11-10. Wave Cyclone Model—Stage 2
As the wave intensifies, both the cold and warm fronts become better organized (see Figure 11-11).
3 A wave cyclone should not be confused with the alternative name for a tornado. They are quite different.
Chapter 11, Air Masses, Fronts, and the Wave Cyclone Model 11-10
Figure 11-11. Wave Cyclone Model—Stage 3
In the fourth stage, the wave becomes a mature low (see Figure 11-12). The occluded front forms as the
cold front overtakes the warm front.
Figure 11-12. Wave Cyclone Model—Stage 4
As the cold front continues advancing on the warm front, the occlusion increases and eventually cuts off
the supply of warm moist air (see Figure 11-13). This causes the low to gradually dissipate.
Figure 11-13. Wave Cyclone Model—Stage 5
Chapter 11, Air Masses, Fronts, and the Wave Cyclone Model 11-11
11.5 Dryline
A dryline is a low-level boundary, hundreds of miles long, and separating moist and dry air masses. In the
United States, it typically lies north-south across the southern and central High Plains during the spring and
early summer, where it separates moist (mT) air from the Gulf of America to the east and dry desert (cT)
air from the southwestern states to the west (see Figure 11-14).
Figure 11-14. Dryline Example
The dryline typically advances eastward during the afternoon and retreats westward at night. However, a
strong wave cyclone can sweep the dryline eastward into the Mississippi Valley, or even f arther east,
regardless of the time of day. Low-level clouds and early morning fog often prevail in the moist air, while
generally clear skies mark the dry side. Severe and sometimes tornadic thunderstorms often develop along
a dryline or in the moist air just to the east of it, especially when it begins moving eastward.
A typical dryline passage results in a sharp drop in humidity (hence the name), clearing skies, and a wind
shift from south or southeasterly to west or southwesterly. Blowing dust and rising temperatures also may
follow, especially if the dryline passes during the daytime. These changes occur in reverse order when the
dryline retreats westward.