Chapter 9 Weather Patterns & Midlatitude Cyclones
Frontal Weather Fronts are boundary surfaces between different air masses. Warm front Cold front Stationary front Occluded front Drylines
Frontal Weather Warm fronts: A warm front occurs when warm air moves into colder air. It is shown on a weather map by a red line with red semicircles on the line in the direction of travel. Warm fronts have relatively gentle slopes.
Warm fronts Frontal Weather
Warm fronts Frontal Weather
Cold fronts: Frontal Weather When cold air advances into warm air, the zone of discontinuity is called a cold front. It is represented on a weather map by a blue line with blue triangles pointing in the direction of travel. Their slopes are about twice as steep as warm fronts.
Cold fronts Frontal Weather
Cold fronts Frontal Weather
Frontal Weather Backdoor cold front (nor easter): Rather than moving from west to east, this type of cold front moves from east to west in the northeast U.S. They occur primarily in the spring. The weather goes from warm to cool and moist.
Cold front Frontal Weather
Man cross-country skiing in Boston nor easter Dec. 2009 Credit: REUTERS/Brian Snyder
Stationary front: Frontal Weather This occurs when airflow is neither toward cold air nor a warm air mass. On a weather map, it is represented by red semicircles and blue triangles alternating sides. They may remain over an area for a long time.
Occluded front: Frontal Weather When rapidly moving cold air overtakes a warm front, an occluded front occurs. These are represented by a purple line with alternating semicircles and triangles pointing in the direction of movement. Occlusion: As cold air wedges a warm front upward, a new front forms between the advancing cold air and the warm air.
Occluded front Frontal Weather
Frontal Weather Drylines: Drylines are boundaries, which develop most often over the southern Great Plains. They occur when dry (ct) air meets moist, (mt) air from the Gulf of Mexico. They most often generate thunderstorms.
Midlatitude Cyclones and the Polar Front Theory Midlatitude cyclones: These are low pressure systems. Their diameters often exceed 1000 km. These cyclones usually travel west to east. Most have a warm front and a cold front extending from center. They frequently produce precipitation.
1. In general, Fronts are boundary surfaces that separate air masses of different densities. 2. A polar front separates cold Arctic air from warm subtropical air. 3. Fronts are normally between 15 and 200 km wide - where the two air masses fight each other.
Mid-Latitude Cyclone 1. Development & intensification of a midlatitude cyclone is explained in terms of the "polar-front" theory. 2. Cyclones form along fronts. Life cycle last about 3 to 7 days.
Mid-Latitude Cyclone - 2 1. Six stages: 1. Formation (cyclogenesis) 2. Development of wave in the front 3. Cyclonic circulation established 4. Occlusion begins 5. Occluded front develops 6. Cyclone dissipates
Mid-Latitude Cyclone - 3 Stage 1. Two air masses of different densities & temperatures are moving parallel to the front, and in opposite directions. Typically - cp associated with polar easterlies on the north of the front, mt driven by westerlies on the south. (This causes counterclockwise rotation of the air mass.) Stage 2. Under suitable conditions, the front takes on a wave shape that is usually several hundred km long.
Mid-Latitude Cyclone 4 Stage 3. For a cyclone to develop, the wave must steepen and "break" (just like surfing beaches). The warm moves pole-wards into the cold air, establishing a warm front, while cold air moves equator-wards, establishing a cold front. This sets up a circular flow pattern in a counterclockwise direction - the cyclone. Stage 4. Cold front normally travels about 50% faster then the warm front, and overtakes it, causing occlusion. Ends of the fronts in center of cyclone occluded first.
Mid-Latitude Cyclone - 5 Stage 5. The size of the occluded front grows in length, displacing the warm front aloft. The storm usually intensifies, the central pressure falls, and wind speeds increase. During Winter, can get heavy snowfalls and blizzard-like conditions (because of the high winds). Stage 6. Once all the warm air has been displaced, we have just the cold air and no temperature gradient. There is thus no energy left to drive the cyclone, which therefore dissipates.
Idealized Weather Map of a Midlatitude Cyclone
Using winds as a forecasting tool.
Midwest Floods of 2008 & 1993 June 2008 floods The Great Flood of 1993
Rainfall based on 10 days of satellite data
Cedar River, Cedar Rapids, OH
Monroe County, 1993
Flash Floods are exactly what they sound like. You cannot out run rushing water. http://wmp.gsfc.nasa.gov/projects/project_flashflood.php
Violent Spring Weather
Case Study of a Midlatitude Cyclone March 21 The storm reached the west coast of U.S. March 23 It was centered over the Kansas-Nebraska border. It was well developed. Air pressure was at 985 millibars.
Case Study of a Midlatitude Cyclone March 24 Air pressure deepened to 982 millibars. The storm curved northward through Iowa. An occluded front developed when the cold front overtook the warm front. Severe weather was produced.
March 25 Pressure moved up to 1000 millibars. The storm was situated over the Great Lakes. It dissipated within 24 hours.
Travelling Cyclones 1. Cyclones generally move over the surface of the Earth at about 25 to 50 kph. 2. The path of a cyclone is controlled to a large extent by the air flow aloft (500 mb level). The upper level flow "steers" the surface cyclone. 3. It is important to be able to predict the path of a surface cyclone. 4. We must understand and be able to predict the wavy flow in the jetstream.
Travelling Cyclones - 2 1. In general, cyclones form where large temperature contrasts occur in the lower troposphere. 2. Main sites for cyclone formation: lee side of Rockies; along Atlantic coast, east of the Appalachians; North Pacific; North Atlantic.
Major sites of cyclone formation
Patterns of Movement 1. Cyclones that form east of the Rockies tend to migrate to the east, and then northeast. 2. Cyclones that influence western North America originate over the Pacific, and move northeastwards towards the Gulf of Alaska, where they merge with the Aleutian low.
Typical cyclone tracks
Anticyclones & Blocking Highs 1. Although high pressure systems generally produce clear skies and calm conditions, they are not always a "good thing". 2. Large anticyclones often develop over the Arctic in winter. If these migrate south over the plains (no blocking mountains), their dense frigid air can bring atypical cold weather.
Anticyclones & Blocking Highs 1. Occasionally, large midlatitude anticyclones will stagnate, staying at the same location for 2 or more weeks. 2. They deflect the normally eastwards flow of air towards the poles. 3. If they stay at the same location, they are called Blocking Highs.
Anticyclones & Blocking Highs 1. Blocking highs cause good weather to exist around themselves, but can cause extensive rain by not allowing cyclones to continue to the east as usual. 2. Blocking highs can also cause air pollution episodes. The subsidence within an anticyclone can produce a temperature inversion (with warm air lying above cooler air) that acts like a lid to trap pollutants. Winds near the center of the anticyclone are light, so do not disperse the pollutants.
End Chapter 9 Chapter 9 Homework: GIST: 2, 4, 5, 8 Problems 3 & 4