Why do I care? These cyclones can bring continuous precipitation for many days covering a large area of the US and during the winter, especially, wintry conditions can spread across much of the US with one storm system, even in the southern regions.
Mid-latitude cyclones are huge weather systems that occur most often during the winter season in the United States. These low pressure areas pop up easily during winter because of the large temperature difference between the equator and the North Pole.
Mid-latitude cyclones can bring severe weather across the entire US with one system. This cyclone formed in mid-March and lasted for 4 days, bringing blizzard conditions to areas all along the East Coast. Figure A. Mid-latitude cyclones form just as other low pressure systems do with the divergence of air high in the atmosphere. The jet stream plays a major role in the location of mid-latitude cyclones.
The jet stream brings down colder air from the north into the southern regions of the United States. A front separates these two air masses. The low pressure system forms to the east of the upper-level trough of the jet stream. Air rises in low pressure systems because of the convergence of air at the surface and diverging air aloft which forms clouds. For the cyclone to intensify, the diverging air aloft has to be greater than the converging air at the surface.
This essentially pulls more air upwards and the surface pressure of the system drops, intensifying the cyclone.
Over the three day period, tornadoes were spawned by thunderstorms associated with this strong low pressure system. So, it's pretty easy to see why weather forecasters would be interested mid-latitude cyclones! Much of what we know about how mid-latitude cyclones work is based on a model called the "Norwegian Cyclone Model" developed by Jacob Bjerknes and Halvor Solberg in , and there's no doubt that this model has shaped the way that meteorologists understand the weather that occurs in the middle latitudes.
While we experience the weather that occurs with mid-latitude cyclones at the surface of the earth, in reality, mid-latitude cyclones are complex, three-dimensional systems.
We'll talk a little bit about some of the mechanisms aloft that drive the formation and evolution of mid-latitude cyclones, but I'll spare you most of the gory details. Instead we'll focus mostly on the aspects of mid-latitude cyclones that people experience on a regular basis. In particular, we'll talk more about air masses and fronts and focus on the types of weather that often occur with various types of fronts.
We'll also explore how mid-latitude cyclones can cause a variety of hazardous winter weather and discuss some important winter weather safety tips.
This lesson will require you to put some "pieces together" from previous lessons, including convergence and divergence, air masses, fronts, gradients, and temperature advection, so we'll do a little reviewing along the way, too.
A difference in temperature or moisture content is the usual reason that fronts form where air masses meet. The difference in temperature or moisture across a front is sometimes very small. The discussion here will focus on stronger more obvious fronts. If one imagines a region of high pressure as a dome or a pile of air on the Earth's surface, it is easy to visualize that where two such dome-like structures touch there occurs a line of pressure lower than at the center of either high.
There are several reasons for unsettled weather where air masses meet: first, both air masses slowly rotate in a clock-wise direction, and where they meet, winds from opposite directions collide; second, when air masses meet, one tends to dominate and the front moves. If cold air is displacing warm air, the contact is said to be a cold front.
Likewise, if warm air is displacing cold air, it is said to be a warm front. A front is identified by an abrupt change in air temperature, dew point, and wind direction. Occasionally only one of these is discernible.
When two air masses and areas of high pressure are in contact with each other, the slightly lower pressure area along the boundary can be thought of as a trough. Along this trough will probably be a front of some sort and the focusing mechanism for more significant weather. A front is defined as the boundary or transition zone between two air masses. Where this boundary meets the ground is where the front is drawn, but the boundary between air masses has a different shape for different air masses.
When two air masses are in contact without one moving against the other, the boundary is called a stationary front. A common stationary frontal situation is pictured in Figure 9.
Continental polar cP air resides north of the stationary front with warm moist mT air to the south. While no major weather events are expected along a stationary front, air is still converging along the front.
Some cloudiness and precipitation can be expected.
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