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NAS 125: Meteorology Weather Systems of Middle Latitudes Defoe and Franklin • Daniel Defoe was the first to propose that storms in the middle latitudes usually tracked from west to east. – Defoe based his conclusions on study of a storm that struck the British Isles on 7-8 December 1703. • He had heard that a similar storm had run up the East Coast of North America a few days earlier. – Benjamin Franklin came to a similar conclusion after a storm in Philadelphia caused him to miss a 21 October 1743 lunar eclipse. • His brother (in Boston) told him the eclipse was visible, but that it was stormy in Boston the next day. Rev. 20 April 2006 Weather Systems of Middle Latitudes 2 Storms and the landscape, part 1 • Storms are phenomena that are more limited than the broad-scale wind and pressure systems. – They are transient and temporary. • Storms involve the flow of air masses as well as a variety of atmospheric disturbances. Rev. 20 April 2006 Weather Systems of Middle Latitudes 3 Storms and the landscape, part 2 • They have short-run and long-run impacts. – In some parts of world, have major influence on weather, some on climate. – Long-run includes both positive and negative impacts on landscape. • Positive: promote diversity in vegetative cover, increase size of lakes and ponds, and stimulate plant growth. Rev. 20 April 2006 Weather Systems of Middle Latitudes 4 Air masses, part 1 • An air mass is a large parcel of air that has relatively uniform properties in the horizontal dimension and moves as an entity. Such extensive bodies are distinct from one another and compose the troposphere. • Characteristics – An air mass must meet three requirements: • It must be large (horizontal and vertical). • Its horizontal dimension must have uniform properties (temperature, humidity, and stability). • It must be distinct from surrounding air, and when it moves, it must retain that distinction (not be torn apart. Rev. 20 April 2006 Weather Systems of Middle Latitudes 5 Air masses, part 2 • Origin – Air mass formation occurs if air remains over a uniform land or sea surface long enough to acquire uniform properties. • Source regions are parts of Earth’s surface that are particularly suited to generate air masses because they are – Extensive – Physically uniform – Associated with air that is stationary or anticyclonic. Rev. 20 April 2006 Weather Systems of Middle Latitudes 6 Air masses, part 3 • Classification – Because the source region determines properties of air masses, it is the basis for classifying them. – Classifications use a one- or two-letter code. – The following table provides a simplified classification of air masses, along with the properties associated with each. Rev. 20 April 2006 Weather Systems of Middle Latitudes 7 Rev. 20 April 2006 Weather Systems of Middle Latitudes 8 Air masses, part 4 • Movement and modification – Some air masses remain in source region indefinitely. – Movement prompts structural change: • Thermal modification – heating or cooling from below; • Dynamic modification – uplift, subsidence, convergence, turbulence; • Moisture modification – addition or subtraction of moisture. – A moving air mass modifies the weather of the region it moves through. Rev. 20 April 2006 Weather Systems of Middle Latitudes 9 Rev. 20 April 2006 Weather Systems of Middle Latitudes 10 Air masses, part 5 • North American air masses – The physical geography of U.S. landscape plays a critical role in air-mass interaction. • There are no east-west mountains to block polar and tropical air flows, so they affect U.S. weather and climate. • North-south mountain ranges in the west modify the movement, therefore the characteristics, of Pacific air masses. Rev. 20 April 2006 Weather Systems of Middle Latitudes 11 Air masses, part 6 • North American air masses (continued) – Maritime tropical (mT) air from the Atlantic, Caribbean/Gulf of Mexico strongly influences climate east of the Rockies in the United States, southern Canada, and much of Mexico. • Primary source of precipitation. Also brings periods of uncomfortable humid heat in summer. – Continental tropical (cT) air has insignificant influence on North America, except for bringing occasional heat waves and drought conditions to the southern Great Plains – Equatorial (E) air affects North America only through hurricanes. Rev. 20 April 2006 Weather Systems of Middle Latitudes 12 Rev. 20 April 2006 Weather Systems of Middle Latitudes 13 Fronts, part 1 • Fronts are zones of discontinuity between unlike air masses where properties of air change rapidly. – They are narrow but three-dimensional. – The yare typically several kilometers wide (even tens of kilometers wide). – Fronts function as a barrier between two air masses, preventing their mingling except in this narrow transition zone. – Frontogenesis occurs when density contrast between air masses strengthen. – Frontolysis occurs when density differences weaken. Rev. 20 April 2006 Weather Systems of Middle Latitudes 14 Fronts, part 2 • Fronts (continued) – Though all primary physical properties are involved in a front, temperature provides the most conspicuous difference. – Fronts lean, which allows air masses to be uplifted and adiabatic cooling to take place. • Fronts lean so much, closer to horizontal than vertical. • Fronts always slope so that warmer air overlies cooler air. – Fronts move in association with the direction of the more active air mass, which displaces the less active. Rev. 20 April 2006 Weather Systems of Middle Latitudes 15 Fronts, part 3 • Warm fronts – A warm front is the leading edge of an advancing warm air mass. • It brings warm air. • It results in clouds and precipitation, usually broad, protracted, and gentle, without much convective activity. • Unstable rising air can result in showery and even violent precipitation. • Weather maps show ground-level position of warm front; precipitation usually falls ahead of this position. • Frontal fogs may develop ahead of the front. Rev. 20 April 2006 Weather Systems of Middle Latitudes 16 Rev. 20 April 2006 Weather Systems of Middle Latitudes 17 Rev. 20 April 2006 Weather Systems of Middle Latitudes 18 Fronts, part 4 • Cold fronts – A cold front is the leading edge of a cool air mass actively displacing warm air mass. • It brings cold air. • It leads to rapid lifting of warm air, which makes it unstable and thus results in blustery and violent weather along the front; if the front is fast moving, a squall line may form. • Weather maps show ground-level position of cold front (usually has a protruding “nose”); clouds and precipitation tend to be concentrated along and immediately behind the ground-level position. Rev. 20 April 2006 Weather Systems of Middle Latitudes 19 Rev. 20 April 2006 Weather Systems of Middle Latitudes 20 Fronts, part 5 • Stationary fronts – A stationary front is the common “boundary” between two air masses in a situation in which neither air mass displaces the other. • Precipitation and clouds along the front may form a broad or narrow band. • Warm air blowing up and over stationary fronts cools by expansion, leading to condensation and possibly precipitation; this is called overrunning. Rev. 20 April 2006 Weather Systems of Middle Latitudes 21 Rev. 20 April 2006 Weather Systems of Middle Latitudes 22 Fronts, part 6 • Occluded front – An occluded front is a complex front formed when a cold front overtakes a warm front. • A cold-type occlusion occurs when the air behind the advancing cold front is colder than the cool air ahead. • A warm-type occlusion occurs when the air behind the advancing cold front is warmer than the air ahead. • A neutral occlusion features insignificant temperature differences between the air behind the advancing cold front and the air ahead. Rev. 20 April 2006 Weather Systems of Middle Latitudes 23 Rev. 20 April 2006 Weather Systems of Middle Latitudes 24 Rev. 20 April 2006 Weather Systems of Middle Latitudes 25 Fronts, part 7 • Occluded front – An occluded front is a complex front formed when a cold front overtakes a warm front. • A cold-type occlusion occurs when the air behind the advancing cold front is colder than the cool air ahead. • A warm-type occlusion occurs when the air behind the advancing cold front is warmer than the air ahead. • A neutral occlusion features insignificant temperature differences between the air behind the advancing cold front and the air ahead. Rev. 20 April 2006 Weather Systems of Middle Latitudes 26 Rev. 20 April 2006 Weather Systems of Middle Latitudes 27 Rev. 20 April 2006 Weather Systems of Middle Latitudes 28 Midlatitude cyclones, part 1 • A midlatitude cyclone is a large migratory lowpressure system that occurs within the middle latitudes and moves generally with the westerlies; midlatitude cyclones are also called lows or wave cyclones, depressions. – Midlatitude cyclones are probably most significant of all atmospheric disturbances. – They are basically responsible for most day-to-day weather changes. – They bring precipitation to much of the world’s populated regions. Rev. 20 April 2006 Weather Systems of Middle Latitudes 29 Midlatitude cyclones, part 2 • Characteristics – Typical mature midlatitude cyclone is 1,600 kilometers (1,000 miles) in diameter; it has an oval shape. – Patterns of isobars, fronts, and wind flow in the Southern Hemisphere are mirror images of those in the Northern Hemisphere. – Northern Hemisphere patterns: • Circulation pattern converges counterclockwise; • Wind-flow pattern attracts cool air from north and warm air from south; creates two fronts. • These two fronts divide the cyclone into a cool sector north and west of center and a warm sector south and east. Rev. 20 April 2006 Weather Systems of Middle Latitudes 30 Midlatitude cyclones, part 3 • Characteristics (continued) – Northern Hemisphere patterns (continued) • Circulation pattern converges counterclockwise; • Wind-flow pattern attracts cool air from north and warm air from south; creates two fronts. • These two fronts divide the cyclone into a cool sector north and west of center and a warm sector south and east. • Size of sectors varies with location: on ground, cool sector is larger, but in atmosphere, warm sector is more extensive. • Warm air rises along both fronts, causing cloudiness and precipitation, which follows patterns of cold and warm fronts. Rev. 20 April 2006 Weather Systems of Middle Latitudes 31 Midlatitude cyclones, part 4 • Characteristics (continued) – Northern Hemisphere patterns (continued) • Much of cool sector is typified by clear, cold, stable air, while air of warm sector is often moist and tending toward instability, so may have sporadic thunderstorms. May have squall fronts of intense thunderstorms. • Movements – Midlatitude cyclones move throughout their existence. • Movement is typically from West to East. • Cold front moves faster than warm front. Rev. 20 April 2006 Weather Systems of Middle Latitudes 32 Rev. 20 April 2006 Weather Systems of Middle Latitudes 33 Rev. 20 April 2006 Weather Systems of Middle Latitudes 34 Midlatitude cyclones, part 5 • Lifecycle – Origin to maturity typically takes 3 to 6 days, then another 3 to 6 days to dissipate. – Cyclogenesis is the birth of cyclones. – Most common cause believed to be upper-air conditions in the vicinity of the polar-front jet stream. – Most begin as waves along the polar front. – Cyclogenesis can also occur on the leeward side of mountains. – Often bring heavy rain or snowstorms to the northeastern United States and southeastern Canada. Rev. 20 April 2006 Weather Systems of Middle Latitudes 35 Midlatitude cyclones, part 6 • Lifecycle (continued) – A comma cloud may be apparent at the peak of the storm. – After cyclonic circulation is well developed, occlusion begins. – After occluded front is fully developed, cyclone dissipates. • Occurrence and Distribution – Occur at scattered but irregular intervals throughout the zone of the westerlies. – Route of cyclone is likely to be undulating and erratic, but it generally moves west to east. Rev. 20 April 2006 Weather Systems of Middle Latitudes 36 Rev. 20 April 2006 Weather Systems of Middle Latitudes 37 Rev. 20 April 2006 Weather Systems of Middle Latitudes 38 Rev. 20 April 2006 Weather Systems of Middle Latitudes 39 Midlatitude cyclones, part 7 • Bomb cyclogenesis is the rapid development of a cyclone. – Bombs are defined as storms whose central pressure drops by 24 mb in 24 hours. – Most form in winter over warm water, such as over the Kuroshio Current off Japan and the Gulf Stream off the United States. • The “Perfect Storm” of 1991 is a classic example of a bomb. Rev. 20 April 2006 Weather Systems of Middle Latitudes 40 Rev. 20 April 2006 Weather Systems of Middle Latitudes 41 Rev. 20 April 2006 Weather Systems of Middle Latitudes 42 Rev. 20 April 2006 Weather Systems of Middle Latitudes 43 Rev. 20 April 2006 Weather Systems of Middle Latitudes 44 Midlatitude cyclones, part 8 • Cyclone models – In the Norwegian cyclone model, cyclones can form and intensify with adequate upper-air support. • Cyclones form along the polar front under an area of strong horizontal divergence in the upper troposphere. • If divergence aloft removes more air than is replaced by convergence at the surface, surface air pressure drops, a horizontal pressure gradient develops, and cyclonic circulation begins. Rev. 20 April 2006 Weather Systems of Middle Latitudes 45 Midlatitude cyclones, part 9 • Cyclone models (continued): – In the conveyor belt model, cyclogenesis is driven by the three intersecting air streams, or conveyor belts. • The first two conveyors form in the rising air to the east of the cyclone, helping form the characteristic comma clouds. – A warm conveyor belt originates in the cyclones warm sector and follows the warm side of the cold front near the surface, to the south and east of the storm center, riding as it blows toward the west. – A cold conveyor forms in the easterly and northeasterly winds flowing ahead of the warm front to the east of the storm center, rising as if blows toward the west. Rev. 20 April 2006 Weather Systems of Middle Latitudes 46 Midlatitude cyclones, part 10 • Cyclone models (continued): – Conveyor belt model (continued): • A dry airstream is the third conveyor belt, which forms aloft and descends along the west side of the cyclone. – One branch flows toward the southward behind the cold front, bringing clear skies. – The other branch moves northward and toward the storm center, which ascends east of the upper-level trough and forms the cry slot that separates the head and tail of the comma. Rev. 20 April 2006 Weather Systems of Middle Latitudes 47 Rev. 20 April 2006 Weather Systems of Middle Latitudes 48 Midlatitude cyclones, part 11 • Cold- and warm-core systems – A cold-core system is one in which the lowest temperatures occur throughout the column of air above the low-pressure center. • In an occluded cold-core system, isobars dip downward in the center of the cyclone. • In a non-occluded cold-core system, the lowest air temperatures are northwest of the storm center and the highest air temperatures are southeast of the center, such that , the column of cold air is tilted toward the cold air. – A warm-core system is stationary, without fronts, and is caused by intense heating of the ground. Rev. 20 April 2006 Weather Systems of Middle Latitudes 49 Rev. 20 April 2006 Weather Systems of Middle Latitudes 50 Rev. 20 April 2006 Weather Systems of Middle Latitudes 51 Midlatitude cyclones, part 12 • Cold- and warm-core systems (continued): – A warm-core system, caused by intense solar heating of the ground, forms over arid or semiarid landscapes. • The heating of the land warms the air above, which in turn lowers the density of the air and leads to formation of a synoptic-scale low. • Warm-core systems are stationary, without fronts. • They are shallow, with their circulation weakening rapidly with altitude. Rev. 20 April 2006 Weather Systems of Middle Latitudes 52 Midlatitude anticyclones, part 1 • A midlatitude anticyclone is an extensive migratory high-pressure cell of the midlatitudes that moves generally with the westerlies. – They are typically larger than a midlatitude cyclone, but also moves west to east. – They travel at the same rate, or little slower, than midlatitude cyclone. • Anticyclones are prone to stagnate or remain over same region (while cyclones do not). • They can cause concentration of air pollutants. Rev. 20 April 2006 Weather Systems of Middle Latitudes 53 Midlatitude anticyclones, part 2 • Cold- and warm-core systems – A cold-core anticyclone is a dome of cold air. • In the Northern Hemisphere, continental polar (cP) or Arctic (A) air masses are cold-core systems. – cP air masses form from the polar high. – A air masses form from the arctic high. • Cold-core anticyclones are the most intense (with highest pressures) in the winter. – Powerful systems can bring freezing weather to South Florida. Rev. 20 April 2006 Weather Systems of Middle Latitudes 54 Midlatitude anticyclones, part 3 • Cold- and warm-core systems (continued): – A warm-core anticyclone forms south of the polar front and consists of subsiding warm, dry air. • Warm-core anticyclones bring intense heat waves and droughts. • The subtropical highs, such as the Bermuda High, are warm-core anticyclones, but others may form over the interior of continents. • Cyclones and anticyclones alternate with one another in an irregular sequence. – There is often a functional relationship between the two. • An anticyclone can be visualized as a polar air mass with the cold front of cyclone as its leading edge. Rev. 20 April 2006 Weather Systems of Middle Latitudes 55 Localized wind systems, part 1 • Lesser winds have a considerable effect on weather and climate on a localized scale. • Sea and land breezes are a common local wind system along coastlines. – They are essentially a convectional circulation caused by differential heating of land and water surfaces. – A land breeze is a local wind blowing from land to water, usually at night (and normally considerably weaker flow than that of sea breeze). – A sea breeze is a local wind blowing from sea toward the land, usually during the day. Rev. 20 April 2006 Weather Systems of Middle Latitudes 56 Rev. 20 April 2006 Weather Systems of Middle Latitudes 57 Localized wind systems, part 2 • Valley and mountain breezes are a daily cycle of airflow occurs with valley and mountain breezes. – The are convectional circulation caused by differential heating of higher versus lower elevations. – Mountain air cools quickly at night, allowing cooler air to drain down the slope in the evening. Conversely, valley air heats more rapidly during the day, allowing warm air to move upslope during the day. • A valley breeze is upslope flow during the day. • A mountain breeze is downslope flow during the night. Rev. 20 April 2006 Weather Systems of Middle Latitudes 58 Rev. 20 April 2006 Weather Systems of Middle Latitudes 59 Localized wind systems, part 3 • Valley and mountain breezes (continued) – Air drainage is the sliding of cold air downslope to collect in the lowest spots, usually at night; it is a modified form of mountain breeze common in winter. • Katabatic winds originate in cold upland areas and cascades toward lower elevations under the influence of gravity. – The air is cold and dense, and usually colder than the air it displaces in its downslope flow. Rev. 20 April 2006 Weather Systems of Middle Latitudes 60 Localized wind systems, part 4 • Katabatic winds (continued): – A mistral is a cold, high-velocity wind that sometimes surges down France’s Rhone Valley, from the Alps to the Mediterranean Sea. The mistral has considerable destructive power. – Similar winds are called bora in Adriatic region and taku in Alaska. Rev. 20 April 2006 Weather Systems of Middle Latitudes 61 Localized wind systems, part 5 • Chinook winds are localized downslope winds of relatively dry and warm air, which is further warmed adiabatically as it moves down the leeward slope of the Rocky Mountains. – Similar winds are called foehns in Europe. – Santa Anas are similar to chinooks/foehns. They have high speed, high temperature, and extreme dryness. They may prompt wildfires. Rev. 20 April 2006 Weather Systems of Middle Latitudes 62 Rev. 20 April 2006 Weather Systems of Middle Latitudes 63 Rev. 20 April 2006 Weather Systems of Middle Latitudes 64 Localized wind systems, part 5 • Desert winds form because of the intense solar heating of the ground. – Most of the absorbed radiation goes into sensible heating because of the lack of water. – In some cases, surface temperatures may exceed 55 °C, which in turn may generate a superadiabatic lapse rate (greater than 9.8 °C). • Superadiabatic lapse rates are associated with great instability, intense convection, and gusty winds at the surface. Rev. 20 April 2006 Weather Systems of Middle Latitudes 65 Localized wind systems, part 6 • Desert winds (continued): – Dust devils are whirling masses of dust-laden air, commonly seen over flat, dry terrain. • They are microscale systems. – Dust devils rarely exceed 300 m in diameter and last more than 20 minutes or more. » Large dust devils may be seen to extend to 900 m in altitude, but the part not seen may extend to 4,500 m. – Most are less than 1 m. across and last less than a minute. • They develop in response to uneven heating of a dry, but heterogeneous landscape. • Air is forced to rise over the hot spot by converging surface winds. • Wind shear sets the column spinning. Rev. 20 April 2006 Weather Systems of Middle Latitudes 66 Localized wind systems, part 7 • Desert winds (continued): – Surface winds generated by thunderstorms or migrating cyclones can trigger dust storms or sandstorms. • The difference between the two is in the size of the particles carried by the winds. – Dust: less than 0.06 mm – Sand: from 0.06 mm to 2.0 mm • Haboobs are spectacular dust storms formed by powerful downdrafts that spread out as the downdraft reaches the ground. – Common throughout northern and central Sudan and the deserts of the American Southwest. Rev. 20 April 2006 Weather Systems of Middle Latitudes 67