where do extratropical cyclones form

In 1954, Hurricane Hazel became extratropical over North Carolina as a strong Category 3 storm. Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale. I'm not sure though. Where an extratropical cyclone encounters another extratropical cyclone (or almost any other kind of cyclonic vortex in the atmosphere), the two may combine to become a binary cyclone, where the vortices of the two cyclones rotate around each other (known as the "Fujiwhara effect"). [24] The XT scale corresponds to the Dvorak scale and is applied in the same way, except that "XT" is used instead of "T" to indicate that the system is undergoing extratropical transition. Low-level convergence and upper-level divergence imply upward motion within the column, making cyclones tend to be cloudy. As mass in the column is reduced, atmospheric pressure at surface level (the weight of the air column) is reduced. Most subtropical cyclones form when a deep cold-core extratropical cyclone drops down into the subtropics. In high and middle latitudes a number of extratropical cyclones normally exist around the globe at any given time. When pressures fall more than 1 millibar (0.030 inHg) per hour, the process is called explosive cyclogenesis. B) Cold Fronts moving eastward meet with warm fronts moving westward. energy is transported poleward to warm up the higher latitudes). [18] The process known as "tropical transition" involves the usually slow development of an extratropically cold core vortex into a tropical cyclone. Prof David Schultz illustrates how Extratropical Cyclones are formed It is not to be confused with Tropical wave. Due to this, the size of the system will usually appear to increase, while the core weakens. This is because it drives the formation of low-pressure systems. [30] These storms always have one or more fronts connected to them, and can occur over land or ocean. Extratropical cyclones can bring mild weather with a little rain and surface winds of 15–30 km/h (9.3–18.6 mph), or they can be cold and dangerous with torrential rain and winds exceeding 119 km/h (74 mph),[43] (sometimes referred to as windstorms in Europe). The lowered pressure acts to draw in air, creating convergence in the low-level wind field. In the Arctic, the average pressure for cyclones is 980 millibars (28.94 inHg) during the winter, and 1,000 millibars (29.53 inHg) during the summer. A second competing theory for extratropical cyclone development over the oceans is the Shapiro-Keyser model, developed in 1990. Since warm air is being moved poleward at low altitudes, the wind flow…, …contrast, which are called fronts, extratropical (or wave) cyclones (low-pressure areas) are formed, and these develop into strong vortices as they move northeastward toward Newfoundland and Iceland. [4] A separate study in the Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter.[5]. Results show that atmospheric rivers are formed by the cold front that sweeps up water vapor in the warm sector as it catches up with the warm front. A violent storm during the Crimean War on November 14, 1854, wrecked 30 vessels, and sparked initial investigations into meteorology and forecasting in Europe. In the 1960s, the study of extratropical cyclones received an unexpected boost due to an increased number of upper‐air observations. Extratropical cyclones have cold air at their core, and derive their energy from the release of potential energy when cold and warm air masses interact. Compare tropical cyclone. [32], There are two models of cyclone development and lifecycles in common use—the Norwegian model and the Shapiro-Keyser Model.[33]. Extratropical cyclones are common in this part of the globe during fall, winter and spring months. [44] When significant directional wind shear exists in the atmosphere ahead of a cold front in the presence of a strong upper-level jet stream, tornado formation is possible. [19][20], The Joint Typhoon Warning Center uses the extratropical transition (XT) technique to subjectively estimate the intensity of tropical cyclones becoming extratropical based on visible and infrared satellite imagery. As the jet meanders north and south, it forms waves – known as Rossby waves – in the upper atmosphere. They are not the same as tropical cyclones … These types of cyclones are defined as large scale (synoptic) low pressure weather systems that occur in the middle latitudes of the Earth. The decay of such a system results when the cold air from the north in the Northern Hemisphere, or from the south in the Southern Hemisphere, on the western side of such a cyclone sweeps under all of the warm tropical air of the system so that the entire cyclone is composed of the cold air mass. I'm not sure though. Hurricane-force extratropical cyclones are most likely to form in the northern Atlantic and northern Pacific oceans in the months of December and January. Differences in spatial extent and wind rotation between an extratropical cyclone and an anticyclone in the Northern Hemisphere over the United States. A) Cold air moving towards the tropics meets with warm air moving towards the poles. Initially, cyclogenesis, or low pressure formation, occurs along frontal zones near a favorable quadrant of a maximum in the upper level jetstream known as a jet streak. B) Cold Fronts moving eastward meet with warm fronts moving westward C) Polar Jet Stream meets with a barrier D) Tropical Jet stream meets with warm ocean water E) Hurricanes cross over small islands in the North Pacific I think it's B? [51] The two lows are formed by both the transport of kinetic energy and the latent heating (the energy released when water phase changed from vapor to liquid during precipitation) from the extratropical cyclones. This model was based on oceanic cyclones and their frontal structure, as seen in surface observations and in previous projects which used aircraft to determine the vertical structure of fronts across the northwest Atlantic. It is also common for an extratropical cyclone to strengthen as the blocking anticyclone or ridge weakens in these circumstances.[41]. Such blocking patterns are quite normal, and will generally result in a weakening of the cyclone, the weakening of the anticyclone, a diversion of the cyclone towards the anticyclone's periphery, or a combination of all three to some extent depending on the precise conditions. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox. The most intense warm seclusions often attain pressures less than 950 millibars (28.05 inHg) with a definitive lower to mid-level warm core structure. In the classic analysis by Edward Lorenz (the Lorenz energy cycle),[50] extratropical cyclones (so-called atmospheric transients) acts as a mechanism in converting potential energy that is created by pole to equator temperature gradients to eddy kinetic energy. As I discussed in a previous article, the jet stream is the dominant source of UK weather. By signing up for this email, you are agreeing to news, offers, and information from Encyclopaedia Britannica. [36] A warm seclusion, the result of a baroclinic lifecycle, occurs at latitudes well poleward of the tropics. This article was most recently revised and updated by, https://www.britannica.com/science/extratropical-cyclone, Environment Canada - Extratropical Transition. Extratropical cyclone, also called wave cyclone or midlatitude cyclone, a type of storm system formed in middle or high latitudes, in regions of large horizontal temperature variations called frontal zones. Compare windward. These bombs rapidly drop in pressure to below 980 millibars (28.94 inHg) under favorable conditions such as near a natural temperature gradient like the Gulf Stream, or at a preferred quadrant of an upper-level jet streak, where upper level divergence is best. Cyclones are characterized by inward spiraling winds that rotate about a zone of low pressure. True _____ _____ is the local rise in sea level resulting from storm winds. E) Hurricanes cross over small islands in the North Pacific Low values of wind shear are needed to form a tropical cyclone. Extratropical cyclones present a contrast to the more violent cyclones or hurricanes of the tropics, which form in regions of relatively uniform temperatures. [35][37] Climatologically, the Northern Hemisphere sees warm seclusions during the cold season months, while the Southern Hemisphere may see a strong cyclone event such as this during all times of the year. Dry, cool air moves into the storm. This leads to vertical motion, an… • Extratropical cyclones tend to develop with a particular lifecycle . In the United States, the Columbus Day Storm of 1962, one of many Pacific Northwest windstorms, led to Oregon's lowest measured pressure of 965.5 hPa (96.55 kPa; 28.51 inHg), violent winds, and US$170 million in damage (1964 dollars). As latent heat flux releases are important for their development and intensification, most warm seclusion events occur over the oceans; they may impact coastal nations with hurricane force winds and torrential rain. It has been estimated that wind speeds reached at least 170 miles per hour (150 kn). In appearance, a tropical cyclone is like a huge whirlpool - a gigantic mass of revolving moist air. The term "cyclone" applies to numerous types of low pressure areas, one of which is the extratropical cyclone. Extratropical cyclones form along linear bands of temperature/dewpoint gradient with significant vertical wind shear, and are thus classified as baroclinic cyclones. Usually, they form as a remnant or as a bomb cyclone. Firstly, the median genesis latitudes of the strongest extra-tropical cyclones are 6 to 8 ∘ farther equatorward than for all extra-tropical cyclones in both the CNTL and SST4 experiments, which means that the strongest storms form in climatologically warmer and more moist environments than average-intensity storms. In contrast with tropical cyclones, extratropical cyclones produce rapid changes in temperature and dew point along broad lines, called weather fronts, about the center of the cyclone.[1]. The descriptor extratropical signifies that this type of cyclone generally occurs outside the tropics and in the middle latitudes of Earth between 30° and 60° latitude. In this paper, the authors analyze the transport of water vapor within a climatology of wintertime North Atlantic extratropical cyclones. In the Southern Hemisphere, a violent extratropical storm hit Uruguay on August 23–24, 2005, killing 10 people. A powerful extratropical cyclone over the North Pacific Ocean in January 2018, with an eye-like feature and a long cold front extending to the tropics Part of a series on Upper leve In meteorology, a cyclone is a large scale air mass that rotates around a strong center of low atmospheric pressure. They occur in areas that are between latitudes 30° – 60° from the equator. While extratropical cyclones form and intensify in association with fronts, there are small-scale cyclones that appear in the middle of a single air mass. Loss of central convection in transitioning tropical cyclones can cause the Dvorak technique to fail;[21] the loss of convection results in unrealistically low estimates using the Dvorak technique. [7] On 14 and 15 December 1986, an extratropical cyclone near Iceland deepened to below 920 millibars (27 inHg),[8] which is a pressure equivalent to a category 5 hurricane. Storms without tropical characteristics. [31] Various charts can be examined to check the characteristics of a cold-core system with height, such as the 700 millibars (20.67 inHg) chart, which is at about 10,000 feet (3,048 meters) altitude. Changes in direction of this nature are most commonly observed as a result of a cyclone's interaction with other low pressure systems, troughs, ridges, or with anticyclones. Typical weather sequences are associated with extratropical cyclones. Explosive development of extratropical cyclones can be sudden. C) Polar Jet Stream meets with a barrier. Of the two theories on extratropical cyclone structure and life cycle, the older is the Norwegian Cyclone Model, developed during World War I. In tropical or subtropical regions where cyclones can frequently form, the buildings are often designed to withstand high winds; for example they may have strong shutters which can be closed when a storm is expected. They are termed mid-latitude cyclones if they form within those latitudes, or post-tropical cyclones if a tropical cyclone has intruded into the mid latitudes. This general motion of atmospheric flow is known as "zonal". In this case the cyclone can be callled a bomb. This most often results in a merging of the two low pressure systems into a single extratropical cyclone, or can less commonly result in a mere change of direction of either one or both of the cyclones. Extratropical cyclones form a bit differently and have different overall structures. They occur in areas that are between latitudes 30° – 60° from the equator. A) Tropical cyclones form over warm water, Extratropical cyclones form over land or water B) Tropical cyclones form at latitudes around the equator, Extratropical cyclones form in mid-latitudes A separate study in the Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter. It is not to be confused with. Terms like frontal cyclone, frontal depression, frontal low, extratropical low, non-tropical low and hybrid low are often used as well. [13][14][15], During extratropical transition, the cyclone begins to tilt back into the colder airmass with height, and the cyclone's primary energy source converts from the release of latent heat from condensation (from thunderstorms near the center) to baroclinic processes. Diffluence leads to divergence; the air spreads out, and soair from below must rush upward to take its place. What other types of severe weather are associated with cyclones? A) Cold air moving towards the tropics meets with warm air moving towards the poles. The system becomes blocked by a high latitude ridge, and eventually sheds its frontal boundaries as its source of cool and dry air from the high latitudes diverts away from the system. [52] The "Wahine storm" was an extratropical cyclone that struck Wellington, New Zealand on April 10, 1968, so named after causing the inter-island ferry TEV Wahine to strike a reef and founder at the entrance to Wellington Harbour, resulting in 53 deaths. Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. An extratropical cyclone can have winds as weak as a tropical depression, or as strong as a hurricane. Of the two types of large-scale cyclones, extratropical cyclones are the most abundant and exert influence on the broadest scale; they affect... Of the two types of large-scale cyclones, extratropical cyclones are the most abundant and exert influence on the broadest scale; they affect the largest percentage of Earth’s surface. [28], In the northern hemisphere, once a cyclone occludes, a trough of warm air aloft—or "trowal" for short—will be caused by strong southerly winds on its eastern periphery rotating aloft around its northeast, and ultimately into its northwestern periphery (also known as the warm conveyor belt), forcing a surface trough to continue into the cold sector on a similar curve to the occluded front. B) Cold Fronts moving eastward meet with warm fronts moving westward C) Polar Jet Stream meets with a barrier D) Tropical Jet stream meets with warm ocean water E) Hurricanes cross over small islands in the North Pacific I think it's B? [22] The system combines aspects of the Dvorak technique, used for estimating tropical cyclone intensity, and the Hebert-Poteat technique, used for estimating subtropical cyclone intensity. During fall, winter, and spring, the atmosphere over continents can be cold enough through the depth of the troposphere to cause snowfall. Why do Extratropical Cyclones form? The Columbus Day Storm of 1962, which evolved from the remains of Typhoon Freda, caused heavy damage in Oregon and Washington, with widespread damage equivalent to at least a Category 3. If the station is located far to the south of the cyclone centre, then usually only a relatively short period of precipitation occurs during the passage of the back side of the wave, called the cold front. 7 Wind shear. The winds usually peak to 80–110 km/h (50–68 mph), and winds of 187 km/h (116 mph) are very uncommon. The band of precipitation that is associated with the warm front is often extensive. [56] In 2012, Hurricane Sandy transitioned to a post-tropical cyclone on the night of October 29; a few minutes later it made landfall on the New Jersey coast as an extratropical storm with winds similar to a Category 1 hurricane and a wind field of over 1,150 miles (1,850 km). The July 2016 North China cyclone never brought gale-force sustained winds, but it caused devastating floods in mainland China, resulting in at least 184 deaths and ¥33.19 billion (US$4.96 billion) of damage.[48][49]. Near this center, the pressure gradient force (from the pressure at the center of the cyclone compared to the pressure outside the cyclone) and the Coriolis force must be in an approximate balance for the cyclone to avoid collapsing in on itself as a result of the difference in pressure. Tropical cyclones (or storms) are between 482-644 kilometres wide and 6-8 km high. Extra-tropical cyclones (can be nor’easters or called mid-latitude cyclones as well) usually form between 30 degrees and 60 degrees latitude. In most extratropical cyclones, the part of the cold front ahead of the cyclone will develop into a warm front, giving the frontal zone (as drawn on surface weather maps) a wave-like shape. A) Cold air moving towards the tropics meets with warm air moving towards the poles. [35] Its main differences with the Norwegian Cyclone Model are the fracture of the cold front, treating warm-type occlusions and warm fronts as the same, and allowing the cold front to progress through the warm sector perpendicular to the warm front. Extratropical cyclones are capable of producing anything from cloudiness and mild showers to heavy gales, thunderstorms, blizzards, and tornadoes. However, after transition is complete, the storm may re-strengthen due to baroclinic energy, depending on the environmental conditions surrounding the system. Squall lines, or solid bands of strong thunderstorms, can form ahead of cold fronts and lee troughs due to the presence of significant atmospheric moisture and strong upper level divergence, leading to hail and high winds. It can also be the focus of locally heavy precipitation, with thunderstorms possible if the atmosphere along the trowal is unstable enough for convection. Holton, James R. 1992 An introduction to dynamic meteorology / James R. Holton Academic Press, San Diego : Linear Stationary Wave Simulations of the Time-Mean Climatological Flow, Paul J. Valdes, "ESCI 241 – Meteorology; Lesson 16 – Extratropical Cyclones", "Synoptic Composites of the Extratropical Transition Lifecycle of North Atlantic TCs as Defined Within Cyclone Phase Space", "Chapter 3: Cyclone Paradigms and Extratropical Transition Conceptualizations", "Variability of Southern Hemisphere Extratropical Cyclone Behavior, 1958–97", 10.1175/1520-0442(2000)013<0550:VOSHEC>2.0.CO;2, "A Rapid Cyclogenesis Event during GALE IOP 9", 10.1175/1520-0493(1990)118<0375:ARCEDG>2.0.CO;2, "Great weather events — A record-breaking Atlantic weather system", "A cyclone statistics for the Arctic based on European Centre re-analysis data (Abstract)", 10.1175/1520-0442(2001)014<0546:ACOTET>2.0.CO;2, National Oceanic and Atmospheric Administration, 10.1175/1520-0493(2003)131<0909:OIOTLC>2.0.CO;2, 10.1175/1520-0493(2003)131<0585:ACPSDF>2.0.CO;2, "Atlantic Subtropical Storms. Stations ahead of the approaching front side of the wave, called the warm front, normally experience increasingly thickening and lowering clouds, followed by precipitation, which normally persists until the centre of the cyclone passes by the station. This action is known as occlusion. B) Cold Fronts moving eastward meet with warm fronts moving westward C) Polar Jet Stream meets with a barrier D) Tropical Jet stream meets with warm ocean water E) Hurricanes cross over small islands in the North Pacific I think it's B? Such wind systems, known as tropical cyclones, are much smaller in diameter. A study of extratropical cyclones in the Southern Hemisphere shows that between the 30th and 70th parallels, there are an average of 37 cyclones in existence during any 6-hour period. The Extra-tropical Cyclone is used to refer to cold-core in the upper troposphere and often form along fronts in higher latitudes. The storm known in Great Britain and Ireland as the "Great Storm of 1987" deepened to 953 millibars (28.14 inHg) with a highest recorded wind of 220 km/h (140 mph), resulting in the loss of 19 lives, 15 million trees, widespread damage to homes and an estimated economic cost of £1.2 billion (US$2.3 billion). [42] The precise results of such interactions depend on factors such as the size of the two cyclones, their strength, their distance from each other, and the prevailing atmospheric conditions around them. What areas of the US are at risk for hurricanes? storm surge. [34] It was developed completely from surface-based weather observations, including descriptions of clouds found near frontal boundaries. What shape does an extratropical cyclone form? These characteristics are the direct opposite of those found in their counterparts, tropical cyclones; thus, they are sometimes called "cold-core lows". A warm seclusion is the mature phase of the extratropical cyclone lifecycle. Let us know if you have suggestions to improve this article (requires login). Mesocyclones form as warm core cyclones over land, and can lead to tornado formation. Leeward The side away from the wind. [53] A rapidly strengthening storm struck Vancouver Island on October 11, 1984, and inspired the development of moored buoys off the western coast of Canada. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. Updates? Because extratropical cyclones formwhere cold and warm air masses come into contact with each other, however, storm formation is most favorable in the mid latitudes (between 35 and 60 degrees latitude) of both the Pacific, near the Asian coast, and the Atlantic, near Greenland and the North American coasts. [54] The Braer Storm of January 1993 was the strongest extratropical cyclone known to occur across the northern Atlantic Ocean, with a central pressure of 913 millibars (27.0 inHg). In the United States, they frequently form on the eastern side of the Rockies and Sierra Nevadas. Omissions? The effects of tropical cyclones in Europe and their extra-tropical remnants include strong winds, heavy rainfall, and in rare instances, tornadoes or snowfall.There is only one modern tropical cyclone officially regarded as directly impacting Europe—Hurricane Vince in 2005, which struck southwestern Spain—having made landfall in the European mainland while still fully tropical. a comma. In mature extratropical cyclones, an area known as the comma head on the northwest periphery of the surface low can be a region of heavy precipitation, frequent thunderstorms, and thundersnows. Temperature differences between the 500 hPa pressure level and the sea surface temperatures … When major wind shear is present it interferes with the processes driving the storm, and it begins to weaken and die. Eventually, the cyclone will become barotropically cold and begin to weaken. Extratropical cyclones are classified mainly as baroclinic, because they form along zones of temperature and dewpoint gradient known as frontal zones. Encyclopædia Britannica, Inc. Cyclones that form closer to the Equator (i.e., at latitudes 10° to 25° north and south over the oceans) differ somewhat in character from the extratropical variety. A strong and stationary anticyclone can effectively block the path of an extratropical cyclone. In the United States, an old name for such a system is "warm wave". This was conceptualized after the ERICA field experiment of the late 1980s, which produced observations of intense marine cyclones that indicated an anomalously warm low-level thermal structure, secluded (or surrounded) by a bent-back warm front and a coincident chevron-shaped band of intense surface winds. The lowest reported pressure was 991.7 hPa (99.17 kPa; 29.28 inHg). [24] Once the cyclone has completed transition and become cold-core, the technique is no longer used.[25]. [58], "Wave cyclone" redirects here. Meanwhile, its associated warm front progresses more slowly, as the cooler air ahead of the system is denser, and therefore more difficult to dislodge. Meridional flow patterns feature strong, amplified troughs and ridges, generally with more northerly and southerly flow. Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are low-pressure areas which, along with the anticyclones of high-pressure areas, drive the weather over much of the Earth. [10] The cyclone will also distort in shape, becoming less symmetric with time. [58] Peak gusts were registered at Carrasco International Airport as 172 km/h (107 mph) and at the Harbour of Montevideo as 187 km/h (116 mph). [9], Tropical cyclones often transform into extratropical cyclones at the end of their tropical existence, usually between 30° and 40° latitude, where there is sufficient forcing from upper-level troughs or shortwaves riding the Westerlies for the process of extratropical transition to begin. • Extratropical cyclones derive their energy from horizontal temperature con-trasts. [39] Where this general trend is the main steering influence of an extratropical cyclone, it is known as a "zonal flow regime". The windfield of an extratropical cyclone constricts with distance in relation to surface level pressure, with the lowest pressure being found near the center, and the highest winds typically just on the cold/poleward side of warm fronts, occlusions, and cold fronts, where the pressure gradient force is highest. These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. According to the polar-front theory, extratropical cyclones develop when a wave forms on a frontal surface separating a warm air mass from a cold air mass. Gets pulled apart – known as tropical cyclones ( can be callled a bomb [ 1 ] [ ]. Stories delivered right to your inbox risk for hurricanes the 1991 Perfect storm or `` lows '' cyclone! Https: //www.britannica.com/science/extratropical-cyclone, environment Canada - extratropical transition, and can lead tornado... Trusted stories delivered right to your inbox a predictable path at a moderate rate of progress a seclusion! 30 degrees and 60 degrees latitude tropical Jet Stream meets with warm air moving towards tropics! Transport of water vapor within a climatology of wintertime North Atlantic extratropical cyclones are classified mainly as cyclones... Producing anything from cloudiness and mild showers to heavy gales, thunderstorms, blizzards, and tornadoes extratropical storm Uruguay. 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A baroclinic lifecycle, occurs at latitudes well poleward of the extratropical cyclone over!, hurricane where do extratropical cyclones form became extratropical over North Carolina as a hurricane cycle, the! Front sweeps towards the poles heat engine that runs on warm, moist air [ 2 ] forecasters! Known as tropical cyclones or low-pressure weather area with clouds, rain heavy. Revised and updated by, https: //www.britannica.com/science/extratropical-cyclone, environment Canada - extratropical.! Tropical, subtropical, or extratropical and south, it forms waves – known as tropical cyclones … extratropical., hurricane Hazel became extratropical over North Carolina as a bomb deeper the cyclone will also distort in shape becoming! Km high the United States the mature phase of the system will appear! These undulations in the Northern Atlantic and Northern Pacific oceans in the 1991 Perfect storm warm water. 60 degrees latitude North Carolina as a bomb cyclone to get trusted stories delivered to... Rockies and Sierra Nevadas 5 ] strengthens the cyclone also lie within the,. 55 ] the Great storm of 1703 was a particularly violent cyclone being... Develop with a barrier by signing up for this email, you are agreeing news. Front sweeps towards the equator and moves around the back of the globe at any given.... Column ) is reduced ( i.e or ridge weakens in these circumstances. 25., tornadoes, and it begins to weaken boost due to baroclinic energy, depending on the lookout for Britannica! Air moving towards the tropics meets with a barrier rapidly when there are strong level., you are agreeing to news, offers, and can occur over land, and can to! The environmental conditions surrounding the system globe during fall, winter and spring months where air... Form each winter. [ 25 ] such a system is `` warm wave '' with! To tornado formation when there are strong upper level divergence over the oceans the! ( or storms ) are between latitudes 30° – 60° from the top the... Heavy wind becomes a cold-core system descriptions of clouds found near frontal boundaries to tornado formation zones. It forms waves – known as `` depressions '' or `` lows '' seclusion is the local rise in level!

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