So far, we have examined the basic elements of weather as well as the dynamics of atmospheric motions. We are now ready to apply our knowledge of these diverse phenomena to an understanding of day-to-day weather patterns in the middle latitudes. For our purposes, middle latitudes refers to the region between southern Florida and Alaska. The primary weather producers here are midlatitude cyclones, or middle-latitude. On weather maps, they are shown by an L, meaning low-pressure system. The following figure (▼) shows two views of a large idealized midlatitude cyclone with probable air masses, fronts, and surface wind patterns.

Midlatitude cyclones are large centers of low pressure that generally travel from west to east. Lasting from a few days to more than a week, these weather systems have a counterclockwise circulation, with an airflow inward toward their centers. Most midlatitude cyclones also have a cold front extending from the central area of low pressure and, frequently, a warm front as well. Convergence and forceful lifting initiate cloud development and often cause abundant precipitation in the vicinity of the low-pressure center.

As early as the 1800s, it was known that midlatitude cyclones were the bearers of precipitation and severe weather. But it was not until the early part of the 1900s that a model was developed to explain how cyclones form. A group of Norwegian scientists formulated and published this model, created primarily from near-surface observations, in 1918.

Years later, as data from the middle and upper troposphere and from satellite images became available, modifications were necessary. However, this model is still a useful working tool for interpreting the weather. If you keep this model in mind when you observe changes in the weather, the changes will no longer come as a surprise. You should begin to see some order in what once appeared to be disorder, and you might even occasionally “predict” the impending weather.

Idealized Weather of a Midlatitude Cyclone

The midlatitude cyclone model provides a useful tool for examining the weather patterns of the middle latitudes. The following figure (▼) illustrates the distribution of clouds and, thus, the regions of possible precipitation associated with a mature system.

Compare this drawing to the satellite image shown in the following figure (▼). It is easy to see why we often refer to the cloud pattern of a midlatitude cyclone as having a “comma” shape.

Guided by the westerlies aloft, cyclones generally move eastward across the United States, so we can expect the first signs of their arrival in the west. However, often in the region of the Mississippi River Valley, cyclones begin a more northeasterly path and occasionally move directly northward. A midlatitude cyclone typically requires 2 to 4 days to move completely across a region. During that brief period, abrupt changes in atmospheric conditions may be experienced. This is particularly true in the winter and spring, when the largest temperature contrasts occur across the middle latitudes.

Using the following figure as a guide, we will now consider these weather producers and what we should expect from them as they move over an area. To facilitate our discussion, ▼ includes two profiles along lines A—E and F—G:

A very different set of weather conditions prevails in the regions north of the storm’s center along profile F—G of ▲. Temperatures progress from cool at F to cold at G. The first hints of the approaching low-pressure center are decreasing air pressure and increasingly overcast conditions that bring varying amounts of precipitation. This section of the cyclone most often generates snow during the winter months and heavy rain during warmer months.

Once the formation of an occluded front begins, the character of the storm changes. Because occluded fronts tend to move more slowly than other fronts, the entire wishbone-shaped frontal structure of the storm rotates counterclockwise. As a result, the occluded front appears to “bend over backward.” This effect adds to the misery of the region influenced by the occluded front because it lingers over the area longer than the other fronts.

The Role of Airflow Aloft

When the earliest studies of midlatitude cyclones were made, little was known about the nature of the airflow in the middle and upper troposphere. Since then, a close relationship has been established between surface disturbances and the flow aloft. Airflow aloft plays an important role in maintaining cyclonic and anticyclonic circulation. In fact, more often than not, these rotating surface wind systems are actually generated by upper-level airflow.

Airflow around a surface cyclone (low-pressure system) is inward, a fact that leads to mass convergence, or coming together (▼). The resulting accumulation of air must be accompanied by a corresponding increase in surface pressure. Consequently, we might expect a low-pressure system to “fill” rapidly and be eliminated. However, this does not occur. On the contrary, cyclones often exist for a week or longer. For this to happen, surface convergence must be offset by a mass outflow at some level aloft (refer to ▼). As long as divergence (spreading out) aloft is equal to or greater than surface inflow, the low pressure and its accompanying convergence can be sustained.

Because cyclones are bearers of stormy weather, they have received far more attention than anticyclones. Anticyclones on a weather map represent the air masses that were discussed earlier. For example, the surface high-pressure system shown in ▲ is likely a continental polar air mass. Because a close relationship exists between surface highs and lows, it is difficult to separate any discussion of these two types of pressure systems. The surface air that feeds a cyclone, for example, generally originates as air flowing out of an anticyclone. Consequently, cyclones and anticyclones typically are found adjacent to each other. Like a cyclone, an anticyclone depends on the airflow far above to maintain its circulation. Divergence at the surface is balanced by convergence aloft and general subsidence of the air column (refer to ▲).

• The primary weather producers in the middle latitudes are large centers of low pressure that generally travel from west to east, called midlatitude cyclones. These bearers of stormy weather, which last from a few days to a week, have a counterclockwise circulation pattern in the Northern Hemisphere, with an inward flow of air toward their centers.

• Most midlatitude cyclones have a cold front and frequently a warm front extending from the central area of low pressure. Convergence and forceful lifting along the fronts initiate cloud development and frequently cause precipitation. The particular weather experienced by an area depends on the path of the cyclone.

• Guided by west-to-east-moving jet streams, cyclones generally move eastward across the United States. Airflow aloft (divergence and convergence) plays an important role in maintaining cyclonic and anticyclonic circulation. In cyclones, divergence aloft supports the inward flow at the surface.

• midlatitude cyclone: Also referred to as a middle-latitude cyclone.  A large center of low pressure and cyclonic airflow with an associated cold front and frequently a warm front, found in areas between 30°  and 60°   latitude.  These drive much of the stormy weather in the continental United States.