![]() The precipitation band widens ahead of the warm front, and narrows ahead of the cold front. The central pressure lowers and the pressure gradient increases, causing a stronger cyclonic (counterclockwise) flow inward toward the low’s center. Part 2 of cyclogenesis: the formation of a frontal wave ( Public Domain). 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. This creates rising motion in the column, and a narrow band of precipitation forms. 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. The lowest pressure lies at the junction of the two fronts. 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. Part 1 of cyclogenesis: a stationary front with opposite moving winds on either side ( Public Domain). 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. 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. For this reason, a developing storm is sometimes referred to as a wave cyclone. It is a semi-continuous boundary and mid-latitude cyclones form and move along it as a series of waves. Recall from Chapter 11 that the polar front separates cold polar air from warmer subtropical air at around 60° latitude. Mid-latitude Frontal Cyclonesįollowing the Norwegian model, the development of a mid-latitude cyclone begins along the polar front. Satellite image of an extratropical cyclone over the UK ( CC BY 2.0). 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. 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. It became known as the Polar Front Theory of a developing wave cyclone. This model proposed a life cycle for the development of mid-latitude cyclones, and was mostly based on surface observations. Shortly after World War I, Vilhelm Bjerknes, Jakob Bjerknes, Halvor Solberg, and Tor Bergeron published their Norwegian Cyclone model. ![]() Tropical cyclones will be the focus of a later chapter. ![]() These storm systems are either called mid-latitude frontal cyclones, extratropical cyclones, wave cyclones, or simply frontal cyclones. The focus of this chapter is cyclonic storm systems that form in the mid-to-high latitudes outside of the tropics. 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). 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. 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. Introductionįor well over a century, forecasters have been aware that areas of falling barometric pressures are often accompanied by precipitation and strong winds. Satellite image of a mid-latitude cyclone over North America ( Public Domain). Discuss the relationship between sea level pressure, high and low pressure systems, air columns and mass budgets as a closed system.Understand the hazards associated with mid-latitude cyclones.Sketch the frontal systems involved in a mid-latitude cyclone.Identify areas on a map where mid-latitude cyclones are common, and explain why they move where they do.By the end of this chapter, you should be able to:
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