Back To CourseEarth Science 102: Weather and Climate
13 chapters | 127 lessons
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Every day we all experience different kinds of weather. Have you ever noticed that there's a pattern to those changes in weather? Depending on where you live, there could be different reasons for that, but if you live anywhere in the mid-latitudes, there is a specific weather phenomenon that is the primary cause of your weather changes. This phenomenon is the mid-latitude cyclone and the result of its formation and decline greatly affects mid-latitude weather. So, how does this happen?
The Polar Front Theory of Cyclogenesis was developed in the early 1900s to explain the formation of mid-latitude cyclones. But what is a mid-latitude cyclone and why is it important? A mid-latitude cyclone is a large-scale low pressure system that travels eastward around the planet between 30 and 70 degrees latitude. It is crucial to day-to-day weather changes and to bringing rain and storms to much of the planet. The polar front is the boundary between the cold winds from the poles and the warm winds from closer to the equator. The cold winds blow from the east between 60 and 90 degrees latitude and are called the polar easterlies. The warm winds blow from the west between 60 and 30 degrees latitude and are called the westerlies. These winds cause mid-latitude cyclones.
Cyclogenesis is when a mid-latitude cyclone goes from origin to maturity to dissipation in a series of stages. It occurs when the cold air blowing down from the poles meets warm air blowing up in the mid-latitudes. These winds of different temperatures meet at the polar front in the mid-latitudes. Cold polar air blows from the east, and warm tropical air blows from the west. Normally, the winds blow right past each other in a straight line, the polar front. This is known as the stationary front stage.
However, certain conditions cause the straight, smooth polar front to develop a buckle or wave shape, usually where there is a sharp temperature difference. These conditions include the air passing by mountains or other irregularities in the landscape, or the air passing over a temperature contrast between ocean and land or over ocean currents. This is known as the wave stage. The wave deepens and causes a low pressure area. The warm air from the tropics and cold air from the poles begins to flow counterclockwise around the low pressure, forming a cyclone. This is the open stage, or mature stage, of cyclogenesis. During the mature stage, a very distinct cold front forms at the front edge of the cold polar air. At the cold front, heavy precipitation and cloud formation occur. Also, a well formed warm front develops at the front edge of the warm mid-latitude air. Light precipitation and cloud development form at the warm front. During this time, there is a sector of warm air wedged between cold air on either side.
The mature stage will not last forever, though, mainly because the cold front and air behind it is moving faster than the warm air. Eventually, the cold front catches up to the warm front to begin the partially occluded stage. Occlusion is when the cold front meets the warm front, pinching off the sector of warm air. Because the cold air is denser than warm air, cold air sinks below the warm air, forcing it higher into the atmosphere. This continues until the warm air is pushed completely off of the surface of earth and a short period of heavy precipitation follows. This burst of activity is the occluded stage. Finally, storm activity slows down and weakens and this is the dissipated stage. Eventually, the cyclone gets completely cut off from the polar front and the life cycle of the cyclone ends.
There are several characteristics of cyclone movement, occurrence and distribution that are pretty consistent. Cyclones or storms move across the mid-latitudes from west to east at about 20-30 miles per hour. This means a storm can cross the United States in about three to four days. However, this does vary with the seasons, since cyclones tend to move faster in winter than in summer. Even though they move west to east, their routes are not in a straight line and tend to be a little erratic. The counter-clockwise movement of cyclones affects wind direction for a large area. Wind all around the center of the storm converges into its center of low pressure. This means that as it passes over you, wind switches direction pretty suddenly. Also, the movement of the cold and warm fronts associated with the cyclone determine what type of precipitation you'll be getting. The warm front advances slowly, and it brings clouds and light rain, so for a longer period of time, that's the weather you'll have as it passes by. The cold front moves faster and brings heavy rains, so the more severe part of the storm moves by fairly quickly.
The location and frequency of cyclones across the planet are also pretty consistent. At any given time, there are 6 to 15 cyclones happening at one time in the Northern Hemisphere and just as many in the Southern Hemisphere. Their positions are scattered and irregular, but all happen within the midlatitudes. During the winter time, the temperature difference between polar and tropical winds is greater, so the cyclones form more frequently, are more developed, and move more quickly. The greater temperature difference just intensifies the process of cyclogenesis and makes it happen more often and faster.
There is also a different between Northern and Southern Hemispheres. The wind blowing in Antarctica is colder all year long compared to the wind from the North Pole. So, cyclones in the Southern Hemisphere happen as often in the summer as in the winter. They also tend to develop a little closer to the South Pole, so they form mostly over water. Even though they are more numerous and stronger, they have less of an effect on people than Northern Hemisphere Cyclones do.
The Polar Front Theory of Cyclogenesis explains the formation of midlatitude cyclones. A midlatitude cyclone is a large-scale low pressure system that forms at the polar front, where cold polar air and warm midlatitude air collide. Cyclogenesis is when a midlatitude cyclone goes from origin to maturity to dissipation in a series of stages. Generally, the cold and warm winds just blow past each other in the stationary front stage. But under certain conditions, a wave forms in the polar front that leads to the wave stage. The wave deepens into a low-pressure center, winds begin to rotate counterclockwise, distinct cold and warm fronts develop, and a sector of warm air gets wedged between cold air. This is the mature stage. The cold front is the fastest moving part of the storm and it eventually catches up to the warm front to begin the partially occluded stage. Occlusion is when the cold front meets the warm front pinching off the sector of warm air. Then all the warm air is forced upward as the fronts meet completely in the occluded stage. The storm finally weakens in the dissipated stage, and the cyclone gets cut off from the polar front and dies.
There are a number of characteristics associated with cyclones. They move from west to east through the midlatitudes and the storms are very big. The counterclockwise rotation affects wind direction over large areas and the cold and warm fronts bring rain and clouds. At any time, there are several cyclones happening at once in both the Northern and Southern Hemispheres. In the winter, the cyclones move faster and form more quickly because the temperature contrast is greater. In the Southern Hemisphere, the storms are mostly over ocean, so they affect people less than in the Northern Hemisphere.
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Back To CourseEarth Science 102: Weather and Climate
13 chapters | 127 lessons