FIGURE 8.27  The development of mid-latitude cyclones and their associated high-pressure systems on the Earth’s surface is related to jet stream processes in the upper troposphere. Flow in the jet stream can often undergo divergence or convergence due to changes in wind speed or direction. Diverging flow in the jet stream creates a vacuum that draws air from the Earth’s surface to fill the void. This process also leads to the formation of a mid-latitude cyclone. Upper-air convergence in the jet stream creates a buildup of air in the upper atmosphere. To compensate for this build-up, the air in the upper atmosphere begins to move down to the Earth’s surface, creating a high-pressure system. Image Copyright: Michael Pidwirny.

Description and Characteristics

            Mid-latitude or frontal cyclones are large traveling atmospheric cyclonic storms up to 2000 km (1250 mi) in diameter with centers of low atmospheric pressure. An intense mid-latitude cyclone may have a surface pressure as low as 970 mb, compared to an average sea level pressure of 1013 mb. Frontal cyclones are the dominant weather systems in the mid-latitudes of the Earth, forming along the polar front (Figure 8.23). 

            Mid-latitude cyclones result from the dynamic interaction of warm tropical and cold polar air masses at the polar front. This interaction causes the warm air to be cyclonically lifted vertically into the atmosphere, where it combines with colder upper-atmosphere air. This process also helps to transport excess energy from the tropics and subtropics to the middle and high latitudes.

            The mid-latitude cyclone is rarely motionless and typically travels about 1200 km (750 mi) in a single day. The direction of movement of these storms is generally in an eastward direction (Figure 8.24). The precise movement of this weather system is controlled by the orientation of the Polar Jet Stream in the upper troposphere. An estimate of the future movement of a mid-latitude cyclone can be determined from wind speeds immediately behind the cold front. For example, if the winds are blowing at 70 kph (44 mph), the cyclone can be projected to continue its movement along the ground surface at this velocity.

            Figure 8.25 describes the patterns of wind flow, surface pressure, fronts, and zones of precipitation associated with a mid-latitude cyclone in the Northern Hemisphere. Around the low, winds blow counterclockwise and inwards (clockwise and inward in the Southern Hemisphere). West of the low, cold air traveling from the north and northwest creates a cold front extending from the cyclone's center to the southwest. Southeast of the low, northward-moving warm air from the subtropics produces a warm front. Precipitation is located at the center of the low and along the fronts where the air is being uplifted. Mid-latitude cyclones can have a wide variety of precipitation types. Precipitation types include rain, freezing rain, hail, ice pellets (sleet), snow pellets, and snow. Frozen forms of precipitation (except hail) are common in winter storms. Hail tends to be associated with severe thunderstorms forming along or in front of cold fronts during spring and summer. 

            Figure 8.26 describes a vertical cross-section through a mature mid-latitude cyclone. In this cross-section, we can see how air temperature changes as we move from behind the cold front to a position ahead of the warm front. Behind the surface position of the cold front, the forward-moving cold, dense air causes the uplift of the warm, lighter air ahead of the front. Because this uplift is relatively rapid along a steep frontal gradient, the condensed water vapor quickly organizes itself into cumulus and then cumulonimbus clouds. Cumulonimbus clouds produce heavy precipitation and can develop into severe thunderstorms if conditions are just right. Above the gradually sloping warm front, the lifting of moist air produces firstnimbostratus clouds,followed by altostratus and then cirrostratus. Precipitation is less intense along the warm front. It ranges from light to moderate rain showers, some distance ahead of the warm front's surface position.

            Frontal cyclone development is related to Polar Jet Stream processes. Within the jet stream, air outflow areas can be localized due to upper-level divergence. This outflow type creates a vacuum (Figure 8.27). To compensate for the void in the upper atmosphere, surface air flows cyclonically upward into the outflow to replenish the mass lost. This process stops when the upper air vacuum is filled with surface air. The end of this event also causes the demise of the mid-latitude cyclone.

            Mid-latitude cyclones cause far less damage than tropical cyclones (hurricanes). Hurricanes involve much greater amounts of atmospheric energy exchange. As one goes away from the equator, the energy available to fuel a weather system decreases as the amount of solar radiation and heat declines. Yet, an intense mid-latitude cyclone can have winds as strong as a weak hurricane. Frontal cyclones tend to be most disruptive to human activity during the winter months. Winter storms can produce heavy snowfall or freezing rain, slowing transportation, knocking out power lines, and damaging or killing vegetation. In January 1998, a winter storm in eastern North America resulted in more than 20 human deaths, billions of dollars in damage, the loss of electrical power in some areas for up to 2 weeks, and the destruction of many deciduous trees due to ice damage (Figure 8.28). 

Mid-Latitude Cyclone Life Cycle

            Figure 8.29 illustrates the typical life cycle or cyclogenesis of the mid-latitude cyclone. The cyclone begins as a weak disturbance somewhere along the frontal zone (stationary front), where cold air from the poles meets warm air from the south (Stage 1). The collision of these two air masses results in the uplift of the warm air into the upper atmosphere, creating a cyclonic spin around a low-pressure center (Stages 2 and 3). This center of atmospheric circulation is associated with moving cold and warm fronts. During the middle stages of cyclogenesis, the storm intensifies, and the pressure at the storm's center drops (Stages 4 and 5). The warm air south of the low's center and between the two fronts is known as the warm sector. Cold fronts usually move along the Earth's surface faster than warm fronts. As a result, the late stages of cyclogenesis occur when the cold front overtakes the warm front, lifting the air in the warm sector into the upper atmosphere (Stage 6). The resulting boundary between the cold and cool air masses is called an occluded front. A day or two after occlusion, the occluded front dissipates, winds subside, and the stationary front forms on the surface of the Earth again (Stages 7 to 9).

FIGURE 8.26  Vertical cross-section of the line A - B in Figure 8.25. This cross-section cuts through the two major fronts typically associated with a mid-latitude cyclone. The cold front has a narrow band of clouds just behind its surface location. Cumulus and cumulonimbus clouds are produced when the steep cold front quickly pushes warm moist air to higher altitudes. The warm front has a relatively wide band of cloud development associated with it. At this boundary, the warm, moist air is pushed up a gentle gradient, creating clouds farther ahead of the front. Image Copyright: Michael Pidwirny.

FIGURE 8.28  (A). GOES false-color satellite image of the ice storm of January 1998. In the image, the center of the mid-latitude cyclone is located over the Great Lakes. This system pulled moisture from the Gulf of Mexico and the Atlantic Ocean, which was converted into freezing rain and snow that fell from the northeastern United States to southeastern Canada. (B). The weight of ice collapsed many power transmission towers, leaving some areas without electricity for almost two weeks. (C). Freezing rain from the 1998 ice storm also took its toll on many trees.  Image Source: Human Resources Development Canada - Ice Storm '98 Emergency: A Study in Response.

FIGURE 8.29  The life cycle of a mid-latitude cyclone as portrayed in a time series of surface weather maps. Image Copyright: Michael Pidwirny.