Hi, my name is David Schultz. I'm professor of synoptic meteorology here at the University of Manchester. In this video I want to talk with you about extra tropical cyclones, also called mid-latitude cyclones or simply low pressure systems. These are the dominant weather systems that bring us rain and snow in the mid-latitude regions of the world. In some places over 80% of wintertime precipitation is due to these extra tropical cyclones. More specifically I want to discuss why these cyclones form, their relationship to the jet stream and their structure, specifically from the viewpoint of that associated with their fronts and the air flow through these cyclones. To give you a global perspective on where extra tropical cyclones occur, consider the following image. We're looking at infrared radiation emitted by the earth but viewed by a satellite placed 35,000 kilometers above the equator. The satellite has an instrument that detects infrared radiation and converts it into a gray shading from black, which are the highest temperatures, to white which are the lowest temperatures. So white areas represent high clouds mostly cirrus and deep convective storms. Black areas on the other hand are the earth's surface in the tropics and light gray areas represent the tops of low clouds or cooler land surfaces. When we put these images in motion several aspects are very clear. First, is a band of persistent convective storms along the equator. This is called the Intertropical Convergence Zone and it's caused by the rising warm air in the tropics. North and south of the intertropical convergence zone in the subtropics are darker regions on these images. These regions are relatively cloud free and are associated with the desert regions of the world. These regions are associated with large scale descending motion which is why they tend to be relatively free of clouds. At the surface, we find regions of high pressure the so-called subtropical high pressure systems. Poleward of the subtropical highs lay the mid-latitude regions in both hemispheres. The mid-latitudes are characterized by relatively strong flow from west to east above the surface of the earth called the jet stream. Within the jet stream disturbances form that we call mid-latitude cyclones. These are regions of low pressure and they have circulations that as a result tend to evolve these comma shaped cloud patterns seen here and here. Winds around these mid-latitude cyclones blow around the low counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere. These cyclones tend to move along with the jet stream from west to east with the comma shape becoming more pronounced and wrapped up as the cyclone evolves. The jet stream forms along the strong gradient in temperature between cold polar air and warm tropical air in each hemisphere. Thus the jet stream also is a manifestation of the strong gradient in temperature. So poleward of the jet stream the air tends to be cold and equatorward of the jet stream the air tends to be relatively warm. This temperature gradient is where our story begins. Consider the gradient and temperature in the northern hemisphere; to the north lies the colder air. The boundary between the cold air and the warm air, in reality, is a zone of transition from cold to warm. But in this schematic animation it's shown as a solid surface with a rearward slope over the cold air to the north. This surface is called a front. At some locations a small perturbation to that surface may form often due to an undulation or wave in the jet stream aloft. We call those waves Rossby waves. That perturbation is manifest as a low pressure region at the surface, indicated by the white contours of sea level pressure. Associated with that low pressure region is a counter-clockwise wind flow around the perturbation in the northern hemisphere. This wind causes the sloping frontal surface to deform over time. Where the boundary between the warm and cold air moves we call it a front, where warm air advances poleward, a warm front forms. Where cold air advances equatorward a cold front forms. The winds that blow through extra tropical cyclones occur in conjunction with three separate air streams. The first air streams is the warm conveyor belt. Shown in red, the warm conveyor belt approaches the cyclone from the south typically carrying warm moist tropical air northward. The warm conveyor belt hits the frontal surface and rises up into the jet stream where it splits. Part of the warm conveyor belt turns eastward and part turns westward and wraps around the cyclone. It is this rising branch of the warm conveyor belt that gives the cyclone its coma shape in the satellite imagery. The second air stream is the cold conveyor belt shown in blue in this diagram the cold conveyor belt approaches the cyclone from the east or north. The air is usually cooler than the warm conveyor belt and stays close to the Earth's surface underneath the rising warm conveyor belt. The third air streams is called the dry air stream or dry intrusion. This is descending air from the upper atmosphere and as a result it contains relatively little moisture. The indentation in the comma shape of the clouds associated with the extra tropical cyclone is due to the dry air stream. Putting all three air streams together, the relationship between them becomes apparent. The cold conveyor belt stays close to the surface whereas the warm conveyor belt rides aloft over it. The dry air stream descends from the upper atmosphere behind the cyclone. As the cyclone evolves, the warm air is lifted from the surface shrinking the warm sector; combined with the circulation of air around the cyclone, means that the cold air expands at the surface. The dry air stream becomes more wrapped up in the cyclone as well. Eventually, the surface fronts wrap up and form an occluded front. This is a merger between the cold front and the warm front. So to summarize an extratropical cyclone forms as a result of the temperature gradient that exists between the poles and the equator. That is why fronts are such an important part of their structure and their evolution. As the cyclone intensifies and the circulation about it becomes stronger, the air streams that are flowing through the storm get increasingly wrapped up into the circulation.
