Introduction

For well over a century, forecasters have been aware that areas of falling barometric pressures are often accompanied by precipitation and strong winds. However, it wasn’t until the early 1900’s that atmospheric scientists began piecing together a more complete picture of how low pressure systems develop, as well as the weather associated with them.

Recall that a cyclone is an area of low pressure, around which winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. This is due to the fact that winds blow from high to low pressure, but are deflected by the Coriolis force (perpendicular to the right of the motion vector in the Northern Hemisphere, left in the Southern Hemisphere). The focus of this chapter is cyclonic storm systems that form in the mid-to-high latitudes outside of the tropics. These storm systems are either called mid-latitude frontal cyclones, extratropical cyclones, wave cyclones, or simply frontal cyclones. Tropical cyclones will be the focus of a later chapter.

Shortly after World War I, Vilhelm Bjerknes, Jakob Bjerknes, Halvor Solberg, and Tor Bergeron published their Norwegian Cyclone model. This model proposed a life cycle for the development of mid-latitude cyclones, and was mostly based on surface observations. It became known as the Polar Front Theory of a developing wave cyclone. It was eventually modified and today provides a way to describe the structure, weather, and evolution of a moving cyclonic storm system in the mid-latitudes. First we will look at how a mid-latitude cyclone develops at the surface, and then we will look at how the surface evolution is affected by the winds aloft.

Mid-latitude Frontal Cyclones

Following the Norwegian model, the development of a mid-latitude cyclone begins along the polar front. Recall from Chapter 11 that the polar front separates cold polar air from warmer subtropical air at around 60° latitude. It is a semi-continuous boundary and mid-latitude cyclones form and move along it as a series of waves. For this reason, a developing storm is sometimes referred to as a wave cyclone.

Cyclogenesis and Life Cycle

This figure below shows a portion of the polar front as a stationary front, with cold air to the north and warmer air to the south flowing parallel to the front in opposite directions. These winds moving in opposite directions set up rotation, similar to how a pen will turn if you place it between your hands and move them in opposite directions.

Under the right conditions, a frontal wave will begin to form along the front, with a cold front pushing southward and a warm front moving northward. The lowest pressure lies at the junction of the two fronts. The southward-moving cold front pushes warmer, less dense air upward, while the warm front overruns and moves over the colder air ahead of it. This creates rising motion in the column, and a narrow band of precipitation forms. The surface low pressure system is steered by winds aloft, typically moving eastward or northeastward, and it gradually becomes a fully-developed mature cyclone 12 to 24 hours after its incipient stage.

The central pressure lowers and the pressure gradient increases, causing a stronger cyclonic (counterclockwise) flow inward toward the low’s center. The precipitation band widens ahead of the warm front, and narrows ahead of the cold front. The region of warmer air between the cold and warm fronts here is called the warm sector. Here, the weather is generally partly cloudy, with scattered showers possible if the air is conditionally unstable.

Where does the mid-latitude cyclone get its energy from? As the warmer and colder air masses attempt to regain equilibrium, warm air rises over the colder air, which transforms potential energy into kinetic (motion) energy. As warmer air rises, it condenses into clouds, which release latent heat energy into the system. Near the surface, winds converge inward toward the low’s center. Wind speeds may increase as a result of a stronger pressure gradient force near the center, increasing the kinetic energy in the system.

As the cyclone moves eastward, the central pressure continues to decrease and winds increase during its mature stage. The faster-moving cold front closes in on the warm front, decreasing the size of the warm sector. In the Norwegian cyclone model, the cold front overtakes the warm front and the cyclone becomes occluded. Cloud cover and precipitation cover a wide area and the storm is usually most intense at this stage.

The point where the cold front, warm front, and occluded front intersect is called the triple-point. Occasionally, a secondary low may form at this triple point, move eastward, and intensify into another cyclone.

Eventually, as occlusion advances, the low pressure center will begin to dissipate, because cold air exists on both sides of the occluded front. The sector of warm, rising air is removed from the center of the storm, so the storm gets cut off from its primary energy supply. Eventually the old storm dies out and gradually disappears.

This sequence of a developing mid-latitude cyclone is similar to a whirling, spinning eddy in a river that forms behind a stick or log, moves along with the river, and quickly disappears further downstream. This entire life cycle can last from several days to a little more than a week.

Cyclones in various stages of development can be seen all at once along the polar front—this succession of storms is known as a cyclone “family”. As mentioned before, some cyclones form from dying previous cyclones and become a part of the succession.

This polar front model of development for a mid-latitude cyclone is rather simplified and, in fact, very few storms follow this model exactly. However, it is a good foundation for understanding storm structure.

Storm Tracks

The development of a mid-latitude cyclone is a process called cyclogenesis. Certain regions in North America are more favorable for cyclogenesis, including the eastern slopes of mountain ranges like the Rockies and Sierra Nevada, the Atlantic Ocean off the Carolina Coast, and the Gulf of Mexico. When air flows westward across a north-south extending mountain range, the air on the leeward (downwind) side tends to have cyclonic curvature, which adds to the development of a cyclone. This is called lee cyclogenesis, and cyclones that are a result of this are often called lee-side lows/cyclones.

Cyclones may also develop near Cape Hatteras, North Carolina, where warm moist air from the Gulf Stream can increase the north-south air mass temperature/moisture contrast to the point where cyclogenesis might occur. These cyclones are called northeasters (or nor’easters) and normally move northeast along the Atlantic Coast. These storms can bring heavy rain or snow and high winds to areas along the East Coast.

Typical cyclone storm tracks are named after the region in which they form, like the Hatteras low, Alberta Clipper, or Colorado low. Alberta clippers and Colorado lows form or re-develop on the lee-side of the Rockies. Mid-latitude cyclones always move toward the east due to the prevailing westerlies.

What influences the strength of a mid-latitude cyclone, and determines how long it will persist? There are some surface conditions that influence cyclogenesis, but the real key to mid-latitude cyclone development lies in the winds aloft. How are mid-latitude cyclones influenced by upper-level flow?

Mid-latitude Cyclone in Three Dimensions

Developing surface lows are usually more intense with height and appear on upper-level charts as a trough or a closed low. However, the low in the upper-levels usually exists to the west of the surface low (again, in the Northern Hemisphere). This is a necessary condition for a low pressure system to continue to develop and intensify. If the upper-level low were directly over the surface low, the surface low would quickly dissipate. This is because winds converge inward toward the low, but only at the surface. This convergence at the surface causes the air mass to “pile up” and air density to increase just above the surface low. The increase in air mass causes surface pressures to rise, and the low fills in and dissipates. How then do cyclones intensify and develop? The air that piles up at the surface must have an “exit path” out of the column so that the surface air pressure can continue to decrease and the cyclone can strengthen. The vertical structure of the atmosphere must allow for air to rise out of a surface low pressure.

The following figure shows an idealized model of the vertical structure of a cyclone and anticyclone in the Northern Hemisphere. A surface low and a surface high are accompanied by an upper level trough and ridge respectively.

On the right hand side is a Northern Hemisphere frontal cyclone with a warm and cold front. The cold air behind the cold front at the surface also extends upward aloft. Recall that the layer between two pressure surfaces is thinner when the air temperature of the layer is cold (more dense), and thicker when the layer is warm (less dense). When pressure levels are packed closer together, pressure decreases more rapidly with height in a column of cold air. Because of this, low pressure is found aloft a body of cold air, just as you find behind a cold front because the constant pressure surfaces are squeezed closer to the Earth’s surface. Thus an upper low is often found in the cold air aloft to the west of, or behind, the surface low. The surface low tilts toward the northwest moving up from the surface. Directly above the surface low, the airflow spreads out and diverges. This allows the converging surface air to rise and flow out of the air column at the tropopause, reinforcing vertical motion. When the divergence in the upper levels is stronger than convergence at the surface, surface pressures will lower further, and the low will intensify and deepen. Put another way: when air exits the column more rapidly aloft than it enters the column at the surface, then the amount of air in the column will reduce, the surface pressure will lower, and the cyclone will intensify.

Notice that there is convergence directly aloft of the high pressure system. Here, the air mass increases aloft and piles up, while air flows clockwise (in the Northern Hemisphere) and out of the anticyclone at the surface. If air were able to flow freely out of the anticyclone, the air pressure would rapidly drop and the anticyclone would dissipate. Therefore, to maintain or strengthen the high pressure system, air has to continually be added to the anticyclone. This happens when there is convergence above a surface high. The air that piles up aloft sinks in the column increasing surface pressure. If the convergence aloft is stronger than the divergence at the surface (more air is added than is removed), then the surface pressure will increase.

Winds at the 500-mb pressure level tend to steer surface low and high pressure systems. Generally speaking, surface storm systems tend to travel at about 16 knots in summer, and roughly 27 knots in winter. This is due to stronger jets and upper-level flow in the winter, a result of stronger north-south temperature differences.

Redistribution of Heat

Mid-latitude frontal cyclones are both a vital part of global circulation and a result of global circulation. They’re also an important pattern in the climatology of regions in the mid-latitudes.

The temperature gradients that cause frontal cyclones form as a result of the colliding surface air from the polar and Ferrel cells. The strong temperature gradient with cold air from the polar region and warm air from the tropics is the energy source that drives the frontal storms. On the flip side of the same token, frontal cyclones are a huge contributor to the redistribution of heat globally. Warm air moving poleward in a warm front and cold air moving equatorward in a cold front are later stabilized after the temperature gradient balances itself by forcing the cold air aloft (occluded front). Frontal cyclones are thus both a result of and a contributor to global circulation and the redistribution of heat from the equator to the poles.

The beginning of the chapter showed images of these storm systems extending over scales the size of continents and land-masses.  It also described how these storm systems last from days to over a week. Instabilities along the polar front are always growing and dying, and passing over fixed points on Earth. The point is that if you live somewhere along the storm track in the Northern or Southern hemisphere, in the wintertime, these storm systems dictate your weather. For example, if you live in Boston, Massachusetts, your winter weather may look something like this: a few days of warm clear weather, a quick change (passing cold front) resulting in a large drop in temperatures and some heavy rain, then cold dry weather for a few days. This cold weather then transitions slowly to warm by some light rain and warming temperatures (warm front). While variable, this pattern repeats itself week after week. Depending on the stage of the frontal storm as it passes over you, it may be more or less severe, and you may receive more or less rain, snow, or other wintery weather. The precipitation and temperature variations resulting from frontal cyclones are an important part of the climatology of mid-latitude weather.