Before occlusion there are two fronts in a midlatitude cyclone

ONCE FORMED, MID-LATITUDE CYCLONES migrate across the surface and commonly evolve through a series of steps, due to the way that winds circulate around the area of low pressure. Circulation around the cyclone will steer the trailing fronts in a counterclockwise direction (in the Northern Hemisphere), while west-to-east motion of the westerlies and Rossby waves shift the entire storm system from west to east. Here, we examine the life stages of a mid-latitude cyclone as it moves west-to-east across North America.

What Are the Three Stages in the Life Cycle of a Mid-Latitude Cyclone?

Formation (Cyclogenesis)

The map on the left shows several fronts as decorated lines, areas of high and low pressure, and isobars as thin lines. A mid-latitude cyclone begins when a relatively cold air mass and a warmer one meet along a frontal boundary. Here, cold and warmer air masses meet along a stationary front labeled “Cyclone A.”

At some point along this boundary, a small amount of surface convergence occurs because of local circulation features that push one air mass into the other, because of topographic influences, or by some other local mechanism. This convergence causes the low pressure to intensify.

This satellite image shows clouds and precipitation at the same time as represented by the weather map on the left. Note that the clouds and precipitation are associated with both cyclones on this map (cyclone A over Texas, and cyclone B over New England).

Maturity

At a later time, the low pressure has moved east and intensified with a stronger pressure gradient, causing more surface air to move toward it. As this air begins to converge from all directions, the Coriolis effect causes the air to have an apparent deflection to the right. This results in counterclockwise flow that pushes cold air southward, turning one segment of the stationary front into an eastwardadvancing cold front.

On the other side (east of the low), warm air pushes northward, forming a north-moving warm front. The movement of the two air masses causes a clear bend in the front, an indication that cyclone A has matured into a mid-latitude cyclone. Note that cyclone B, now east of the continent, has a more curved shape than cyclone A, an indication that it is even more mature.

Both cyclones are visible in the satellite image above. Although not obvious on this image, cyclone A now has distinct cold, warm, and cool sectors of air. The “cold” sector is behind (west of ) the cold front, whereas the “warm” sector is behind (south of ) the warm front. The cool sector is in front of (north of ) the warm front — neither the warm front nor the cold front have passed through this sector. On the satellite image, dense cumuliform clouds (along the south edge of the image) form along the cold front, while indistinct stratiform clouds form along the warm front and in the cool sector. Advancing cold air can easily displace warmer air, but the warmer air has more difficulty moving the cool air; with time the cold front advances faster than the warm front, as in cyclone B.

Occlusion

1. With time, the westerlies push both weather systems toward the east or northeast. Eventually, the cold fronts will catch up with, and even overtake, the warm fronts as both flow counterclockwise around the low-pressure areas. When this happens, the warmer air mass is lifted above the surface, because it is less dense than both the cold air and the “cool” air. This process is called occlusion, and a front that has experienced occlusion is an occluded front. In occlusion, the moisture is moved counterclockwise around the low, so that the heaviest precipitation may be northwest of the low.

2. Occluded fronts, like that near cyclone B on the map above, are purple on weather maps with the triangles and semicircles pointing in the direction that the system is moving. Though many mid-latitude cyclones never occlude, occlusion often, but not always, signals the end of the mid-latitude cyclone's life, because by this time the cold air has warmed and the warm air has cooled. Some occluded fronts can continue to strengthen for some time.

3. Notice on the satellite image that the sky over the occluded front is filled with clouds. The cumuliform cloudiness associated with the cold front more or less merges with the stratiform clouds associated with the warm front. Occluded fronts, however, can have a wide variety of cloudy features.

4. This figure contains three cross sections that capture different stages in the process of occlusion. Occlusion begins nearest to the low-pressure core because there the cold front has the least distance to catch up to the warm front, and it proceeds away from the low over time. To visualize what occurs, move through the three cross sections, which represent different places, but also successive stages in the occlusion process.

5. In this cross section, the cold front is moving to the east and the warm front is moving to the north, but the cold front is moving faster and so it is getting closer to the warm front.

6. Here, the cold front has caught up with the warm front, trapping the warm, moist air above the mass of cold air and the mass of cool air, which are in the process of joining together.

7. At segment EF, the two air masses are fully beneath the warm air, lifting it higher. As the warm air is forced up, it cools, forming clouds and precipitation. The air north of the warm front is cool but not cold because it is blocked from receiving a fresh supply of cold air from the air mass to the west.

How Do the Great Lakes Interact with Cyclones?

The Great Lakes in the north-central part of the U.S. are famous for interacting with winter storms, producing intense snowfall called lake-effect snows. The largest amounts of lake-effect snow occur after mid-latitude cyclones and associated cold fronts have moved eastward across the Great Lakes, and higher pressure behind them begins to approach the area. The circulation over the water causes moist air to flow onto land and dump huge amounts of snow on land south and east of the lakes, including the cities of Syracuse and Buffalo in upstate New York.

As a cyclone moves across one of the lakes, it encounters water temperatures that are somewhat warmer than the air or the surrounding land. This is largely due to water changing temperature more slowly than air or land from fall to winter. In winter, especially early winter, the Great Lakes are much warmer than the air above them or the land beside them. This causes steep environmental lapse rates, destabilizing the atmosphere and strengthening a cyclone passing over the Great Lakes.

Having flowed across the Great Lakes, the moist, destabilized air is pushed inland by an approaching anticyclone, depositing snow on adjacent downwind land, especially where topography, such as the mountains of upstate New York, cause orographic enhancement.

Introduction

Mid-latitude or frontal cyclones are large traveling atmospheric cyclonic storms up to 2000 kilometers in diameter with centers of low atmospheric pressure. An intense mid-latitude cyclone may have a surface pressure as low as 970 millibars, compared to an average sea-level pressure of 1013 millibars. Normally, individual frontal cyclones exist for about 3 to 10 days moving in a generally west to east direction. Frontal cyclones are the dominant weather event of the Earth's mid-latitudes forming along the polar front. (Figure 7s-1).

Mid-latitude cyclones are the result of 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 lower latitudes to the higher latitudes.

The mid-latitude cyclone is rarely motionless and commonly travels about 1200 kilometers in one day. Its direction of movement is generally eastward (Figure 7s-2). Precise movement of this weather system is controlled by the orientation of the polar jet stream in the upper troposphere. An estimate of future movement of the mid-latitude cyclone can be determined by the winds directly behind the cold front. If the winds are 70 kilometers per hour, the cyclone can be projected to continue its movement along the ground surface at this velocity.

Figure 7s-3 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 air is being uplifted.

Mid-latitude cyclones can produce a wide variety of precipitation types. Precipitation types include: rain, freezing rain, hail, sleet, snow pellets, and snow. Frozen forms of precipitation (except hail) are common with storms that occur in the winter months. Hail is associated with severe thunderstorms that form along or in front of cold fronts during spring and summer months.

Figure 7s-4 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, forward moving cold dense air causes the uplift of the warm lighter air in advance 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 right (see section 7t). Along the gently sloping warm front, the lifting of moist air produces first nimbostratus clouds followed by altostratus and cirrostratus. Precipitation is less intense along this front, varying from moderate to light showers some distance ahead of the surface location of the warm front.

Frontal cyclone development is related to polar jet stream processes. Within the jet stream, localized areas of air outflow can occur because of upper air divergence. Outflow results in the development of an upper air vacuum. To compensate for the vacuum in the upper atmosphere, surface air flows cyclonically upward into the outflow to replenish lost mass. The process stops and the mid-latitude cyclone dissipates when the upper air vacuum is filled with surface air.

Mid-latitude cyclones cause far less damage than tropical cyclones or 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. Mid-latitude cyclones can have winds as strong as what is associated with a weak hurricane. But, this is a rare occurrence. Frontal cyclones tend to be most disruptive to human activity during winter months. Winter storms can produce heavy snowfalls or freezing rain which slows down transportation, snaps powerlines, and kills vegetation. In January 1998, a winter storm in eastern North America resulted in more than 20 human deaths, billions of dollars of damage, the loss of electrical power in some areas for up to two weeks, and the destruction of many deciduous trees because of the weight of ice (Figures 7s-5, 7s-6, and 7s-7).

Cyclogenesis

The animation in Figure 7s-8 illustrates the 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 polar regions 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). Associated with this center are the cold and warm fronts described in topic 7r. 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 at velocities greater than the warm front. As a result, the late stages of cyclogenesis occur when the cold front overtakes the warm front causing the air in the warm sector to be lifted into the upper atmosphere (Stages 6 and 7). 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 a stationary front forms on the surface of the Earth again (Stages 8 to 10).