Fronts: 1. Describe the typical characteristics of: (a) a warm front (b) a cold front (c) an occluded front: 1.) Warm front - warm, moist subtropical (mT) air replaces cold maritime polar air from the North Atlantic. The direction of frontal movement is given by half circles, which point into the cold air. Warm fronts often move in a series of rapid jumps. Overrunning (rising of air over the cold air) produces clouds and precipitation in advance of the front. 2.) Cold front - represents a zone where cold, dry, stable polar air is replacing warm, moist, conditionally unstable subtropical air. The front is drawn as a solid blue line with triangles along the front showing its direction of movement. As you move towards the cold front, pressure drops and as you move away from the front, the pressure rises. Most cold fronts move south, southeast, or east. Cold fronts that move for the east or northeast are called backdoor fronts. 3.) Occluded front - if the cold front catches up to and overrides a warm front. It is represented by a purple line with alternating cold-front triangles and warm-front semicircles. Cold-type occluded fronts occur when the air behind the occluded front is colder than the air ahead of it. Warm-type occluded fronts occur when the cold front catches up and overtakes the warm front. The cold front rides ŇpiggybackÓ along the sloping warm front. Cyclogenesis: 1. Describe the stages of a developing wave cyclone using the polar front theory. The development of a wave cyclone begins along the polar front. The polar front is a stationary front. It represents a trough of lower pressure with higher pressure on both sides. Cold air to the north and warm air to the south flow parallel to the front, but in opposite directions. This type of flow sets up a cyclonic wind shear. Under the right conditions, a wavelike kink forms on the front. The wave that forms is known as a frontal wave. The newly formed wave with has a cold front pushing southward and a warm front moving northward. The region of lowest pressure is at the junction of the two fronts. As the cold air displaces the warm air upward along the cold front, and as overrunning occurs ahead of the warm front, a narrow band of precipitation forms. Steered by the winds aloft, the system typically moves east or northeastward and gradually becomes a fully developed open wave in 12 to 24 hours. The central pressure is now much lower. These more tightly packed isobars create a stronger cyclonic flow, as the winds swirl counterclockwise and inward toward the low's center. Precipitation forms in a wide band ahead of the warm front and along a narrow band of the cold front. The region of warm air between the cold and warm fronts is known as the warm sector. Here, the weather tends to be partly cloudy, although scattered showers may develop if the air is conditionally unstable. Condensation supplies energy to the system in the form of latent heat. And, as the surface air converges toward the low center, wind speeds may increase, producing an increase in kinetic energy. As the open wave moves eastward, central pressures continue to decrease, and the winds blow more vigorously. The faster-moving cold front constantly inches closer to the warm front, squeezing the warm sector into a smaller area. The cold front eventually overtakes the warm front and the system becomes occluded. At this point, the storm is usually most intense, with clouds and precipitation covering a large area. The intense storm system gradually dissipates, because cold air now lies on both sides of the occluded front. Without the supply of energy provided by the rising warm, moist air, the old storm system dies out and gradually disappears. The entire life cycle of a wave cyclone can last from a few days to over a week. 2. Why do cyclones tend to develop along the polar front? Cyclones tend to develop along the polar front because the polar front is a semi continuous boundary separating cold air from warm subtropical air. This cold air to the north and war air in the south flow parallel to the front, but in opposite directions. This flows sets up a cyclonic wind shear. 3. List four regions in North America where wave cyclones tend to develop. Eastern Slope of the Rockies Great Basin Gulf of Mexico Atlantic Ocean (east of the Carolinas) 4. Why is it important that for a surface low to develop or intensify, its upper-level counterpart must be to the left (or west) of the surface storm? Regions of converging air usually form on the west side of the trough and a region of diverging air forms on the east side. Aloft the area of diverging air is above the surface low, the area of converging air is directly above the surface high. For a surface storm to intensify, the upper level trough of low pressure must be located to the west of the surface low. The atmosphere is able to redistribute its mass. Thunderstorm: 1. What is a thunderstorm? Merely a storm containing lightning and thunder. 2. Describe the stages of development of an ordinary (air-mass) thunderstorm. The first stage is known as the cumulus stage, or growth stage. As a parcel of warm, humid air rises, it cools and condenses into a single cumulus cloud or a cluster of clouds. As the cloud builds, the transformation of water vapor into liquid or solid cloud particles releases large quantities of latent heat, a process that keeps the rising air inside the cloud warmer than the air surrounding it. The cloud continues to grow. 3. How do downdrafts form in thunderstorms? As the cloud builds well above the freezing level, the cloud particles grow larger. They also become heavier. Eventually, the rising air is no longer able to keep them suspended, and they begin to fall. While this phenomenon is taking place, drier air from around the cloud is being drawn into it in a process called entrainment. The entrainment of drier air causes some of the raindrops to evaporate, which chills the air. The air, now colder and heavier than the air around it, begins to descend as a downdraft. The downdraft may be enhanced as falling precipitation drags some of the air along with it. 4. Why do ordinary thunderstorms most frequently form in the afternoon? Because the trigger or 'forcing mechanism' needed to start a thurderstorm (air moving upward) is usually unequal heating at the surface. Afternoons, of course, have the most frequent unequal surface heatage. 5. What atmospheric conditions are necessary for the development of an ordinary thunderstorm? The ingredients for an ordinary thunderstorm are humid surface air, plenty of sunlight to heat the ground, and a conditionally unstable atmosphere. 6. a) What are gust fronts and how do they form? (b) If a gust front passes, what kind of weather will you experience? A gust front is the leading edge of the cold downdraft originating inside a thunderstorm. They form when the cold downdraft reaches the surface and the air spreads out horizontally in all directions. B) if a gust front passes, you can look forward to a thunderstorm. 7. a) Describe how a microburst forms. (b) Why is the term wind shear often used in conjunction with a microburst? a. A microburst forms when a downdraft becomes localized so that it hits the ground and spreads horizontally in a radial burst of wind, much like water pouring from a tap and striking the sink below. When this wind extends only 4 km or less, it is termed a microburst. b.Wind shear is often used in conjunction with microburst because of the abrupt change in wind speed and direction common during a microburst. 8. How do severe thunderstorms differ from ordinary thunderstorms? By definition, a severe thunderstorm is any thunderstorm that produces hail at least three-quarters of an inch in diameter and/or surface wind gusts of 50 knots or greater and/or produces a tornado. 9. Explain why ordinary thunderstorms tend to dissipate much sooner than supercell thunderstorms. Ordinary thunderstorms tend to form in regions of low wind shear. Because of this fact, the storm's precipitation and downdraft usually fall into the updraft. The downdraft then cuts off the storm's fuel supply, which causes the storm to dissipate in a relatively short time. In a supercell thunderstorm, however, the winds aloft are much more powerful and end up pushing the precipitation downwind so that id does not fall into the updraft. Hence, the updraft is not suppressed and the storm actually perpetuates itself. 10. Why are severe thunderstorms not very common in polar latitudes? Because atmospheric conditions in the polar latitudes are not favorable for creating severe thunderstorms. 11. Give a possible explanation for the generation of prefrontal squall-line thunderstorms. A prefrontal squall-line thunderstorm may form, initially, when convection begins along the cold front then reforms further away. Moreover, the surging nature of the main cold front itself, or developing cumulus clouds along the front, may cause the air aloft to develop into waves (called gravity waves). Out ahead of the cold front, the rising motion of the wave may be the trigger that initiates the development of cumulus clouds and a pre-frontal squall line. 12. In what region of the United States do dryline thunderstorms most frequently form? In the western half of Texas, Oklahoma, and Kansas, especially during spring and early summer. 13. Where does the highest frequency of thunderstorms occur in the United States? Why there? The Gulf Coast where there is plenty of humid surface air, lots of sunlight to heat the ground, and a very conditionally unstable atmosphere. Lightening: 1. Explain how a cloud-to-ground lightning stroke develops Because unlike charges attract one another, the negative charge at the bottom of the cloud causes a region of the ground beneath it to become positively charged. As the thunderstorm moves along, this region of positive charge follows the cloud like a shadow. The positive charge is most dense on protruding objects, such as trees, poles, and buildings. The difference in charges causes an electric potential between the cloud and ground. In dry air, however, a flow of current does not occur because the air is a good electrical insulator. Gradually, the electrical potential gradient builds, and when it becomes sufficiently large (on the order of one million volts per meter), the insulating properties of the air break down, a current flows, and lightning occurs. 2. How is thunder produced? The sound of thunder is produced from the wave that lightning makes when it heats the air to temperatures around 30,000C, rapidly expanding then contracting. 3. If you see lightning and ten seconds later you hear thunder, how far away is the lightning stroke? Remember the rule of five. It will take sound 5 seconds to travel one mile. Therefore the lightning was 2 miles away. 4. Why is it unwise to seek shelter under a tree during a thunderstorm? Lightning could strike the tree and send energy through the ground and then up into the body or nearby objects. The tree could also be struck and explode sending debris outward. Tornados: 1. What is a tornado? A tornado is a column of rapidly, spiral moving air associated with a supercell thunderstorm. 2. Give some average statistics about tornadoes, including their size, winds, and direction of movement. Most tornados are weak with stronger tornados less frequent. Winds can range from 100 mph or less to as much as 300mph in F-5 tornados. The size can range from a few yards in width for weak tornadoes and widths as large as a half mile or more for F-5's. Most tornados move from the southwest to northeast. 3. How does a tornado watch differ from a tornado warning? A tornado watch indicates that atmospheric conditions are favorable for their development, while a warning means that a tornado has been spotted on radar or by trained spotters in the field. 4. Why is it suggested that one not open windows when a tornado is approaching? It was commonly thought that pressure differences between the inside of the house and tornado caused homes to explode, but research has proved otherwise. One should not waste time opening windows, but rather seek shelter in a basement or interior room. 5. Explain why the central part of the United States is more susceptible to tornadoes than any other region of the world. The central part of the united states borders the warm tropical gulf region and the cool, dry air from canada. The clash of these airmasses results in storm development favorable for tornados. 6. Explain how a non-supercell tornado, such as a landspout, might form. Landspouts occur in a different formation. Landspouts are smaller in nature and take a rope shape form. They result from cumulus clouds which are developing into thunderstorms. Rotations in the atmosphere are first brought into the cloud via convergence and updraft. The landspout formed is generally weak and shortlived. 7. How has Doppler radar helped in the prediction of severe weather? Doppler radar measures radial velocities and it can help in tracking how fast a storm is coming. This also helps predictions for when the storm will hit a certain area and gives advanced warnings 8. Explain both how and why there is a shift in tornado activity from winter to summer within the continental United States. During the end of February, southward-moving cold air reaches the southern limit of its expansion and encroaches on the Gulf Coast. As spring progresses, the days grow longer and more solar energy is intercepted. Land temperatures rise, and warm, moist air from the Gulf of Mexico progressively drives back the cold air. The centre of activity then moves eastward to the southeastern Atlantic states, with tornado frequency peaking there in April. Late summer through early fall is usually a relatively quiet time because the temperature and moisture contrasts across the boundary between the two air masses are weak. 9. What atmospheric conditions lead to the formation of 'fair weather' waterspouts? The term fair weather comes from the fact that this type of waterspout forms during fair and relatively calm weather, often during the early to mid morning and sometimes during the late afternoon. Fair weather waterspouts usually form along dark flat bases of a line of developing cumulus clouds. This type of waterspout is generally not associated with thunderstorms and usually form in light wind situations. Mesoscale Convective Complex: 1. What is a Mesoscale Convective Complex (MCC)? An ensemble of thunderstorms that form from convection and have an extensive width 2. Describe the atmospheric conditions at the surface and aloft that are necessary for the development of the majority of supercell tornadic thunderstorms. Be sure to include how wind shear plays a role in the storms formation. Shearing winds, such as westerly winds aloft combined with southeasterly winds at the surface provide an initial form of rotation to the storm. Heavy rains can pull down strong downdrafts in the rear flank of the storm. The downdrafts combined with the mesoscale rotation can then spawn a tornado on the rear southwest side of the supercell. Other important factors include the CAPE (convective available potential energy) The higher the CAPE, the greater energy a parcel of air has when lifted. Waves Aloft: 1. Describe some of the necessary ingredients (upper-air support) for a wave cyclone to develop into a huge mid-latitude cyclonic storm system. For a storm to intensify, there must be an upper-level counterpart - a trough of low pressure - that lies to the west of the surface low. At the same time, the polar jet stream must form into waves and swing slightly south of the developing storm. When these conditions exist, zones of converging and diverging air, along with rising and sinking air, provide energy conversions for the stormŐs growth. When these conditions do not exist, we say that the surface storm does not have the proper upper-air support for its development. 2. Explain the role that upper-level divergence plays in the development of a wave cyclone. For mid-latitude cyclones to maintain themselves and intensify, the winds aloft must blow in such a way that zones of converging and diverging air form. Upper-level divergence of air must be greater than surface convergence if air. When this happens, surface air pressure decreases (forms low at the surface). 3. How does the polar jet stream influence the formation of a wave cyclone? Jet streams play an additional part in the formation of surface mid-latitude cyclones and anticyclones. When the polar jet stream flows in a wavy west-to-east, deep troughs and ridges exist in the flow aloft. The curving of the jet stream coupled with the changing wind speeds around the jet streak produce regions of strong convergence and divergence. The region of diverging air above the storm draws warm surface air upward to the jet stream, which quickly sweeps the air downstream. Since the air above the storm is being removed more quickly than converging surface winds can supply air to the storm's center, the central pressure of the storm drops rapidly. As surface pressure gradients increase, the wind speed increases. Above the high-pressure area, a region of converging air feeds cold air downward into the anticyclone to replace the diverging surface air. Hence, we find the jet stream removing air above the surface cyclone and supplying air to the surface anticyclone. Additionally, the sinking of cold air and the rising of warm air provide energy for the developing cyclone as potential energy is transformed into kinetic energy As the jet stream steers the storm along the surface storm occludes, and cold air surrounds the surface low. Since the surface low has moved out from under the pocket of diverging air aloft, the occluded storm gradually fills as the surface air flows into the system. Since the polar jet stream is strongest and moves farther south in winter, we can see why mid-latitude cyclonic storms are better developed and move more quickly during the coldest months. During the summer when the polar jet shifts northward, developing midlatitude storm activity shifts northward and occurs principally over the Canadian provinces of Alberta and the Northwest Territories. 4. Explain why, in the eastern half of the United States, a mid-latitude cyclonic storm often moves eastward or northeastward. Winds aloft steer the movement of the surface pressure systems. Since the winds above the storm are blowing from the southwest, the surface low should move northeastward. Airmass: 1. (a) What is an air mass? (b) If an area is described as a 'good air-mass source region,' what information can you give about it? (a)air mass: large body of air whose properties of temperature and humidity are fairly similar in any horizontal direction at any given altitude; (b)source region: areas dominated by surface high pressure, ice and snow-covered arctic plains in winter and subtropical oceans in summer 2. How does a cA air mass differ from a cP air mass? cT is a air mass with hot dry air and generally unstable while cP is cold, dry stable air 3. Why is cP air not welcome to the Central Plains in winter and yet very welcome in summer? cP air is very cold and dry so it brings unwanted conditions to the area, but in the summer, the air is cold so it cools down the hot desolate climate 4. Explain why the central United States is not a good airmass source region. The central U.S. is not a good airmass source because of the Rocky Mountains making it not a flat, uniform surface. 5. What are lake-effect snows and how do they form? On which side of a lake do they typically occur? Lake-effect snow is when a the air above picks up moisture from a underlying body of water, then when the air hits a cold land mass, the moisture falls as snow. Typically in the U.S. this happens on the east side of lakes because the winds in the U.S. move from west to east. 6. List the temperature and moisture characteristics of each of the major air mass types. mT-maritime Tropical: warm temperatures and moist. mP-maritime Polar: cold temperatures and moist. cT-continental Tropical: warm temperatures and dry. cP-continental Polar: cold temperatures and dry. 7. Which air mass only forms in summer over the United States? cT-continental Tropical 8. Why are mP air masses along the east coast of the United States usually colder than those along the nation's west coast? Why are they also less prevalent? mP air masses are colder on the east coast because they come off the cold upper Atlantic ocean instead of the warmer mP masses on the west coast originating from the Pacific Ocean. They are less prevalent because the winds move from west to east over the U.S. but for east coast mP air masses, the wind must move from the northeast to the west. 9. Explain how the airflow aloft regulates the movement of air masses. The airflow aloft dictates the movement of air masses and storms on the surface, so air masses move based on the direction of the winds aloft 10. The boundaries between neighboring air masses tend to be more distinct during the winter than during the summer. Explain why. During the winter months, the temperature differences across the U.S. are much greater than in the summer, so this makes more distinct boundaries between polar and tropical air masses as the temperatures differ more. 7
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