A high-pressure area, high, or anticyclone, is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment. Winds within high-pressure areas flow outward from the higher pressure areas near their centers towards the lower pressure areas further from their centers. Gravity adds to the forces causing this general movement, because the higher pressure compresses the column of air near the center of the area into greater density – and so greater weight compared to lower pressure, lower density, and lower weight of the air outside the center. However, because the planet is rotating, the air flow from center to periphery is not direct, but is twisted due to the Coriolis effect.
Introduction
High-pressure area
High-pressure area | Introduction
Winds within high-pressure areas flow outward from the higher pressure areas near their centers towards the lower pressure areas further from their centers. Gravity adds to the forces. The winds are further twisted due to Coriolis effect. High-pressure areas form due to downward motion through the troposphere, the atmospheric layer where weather occurs. By the downward motion through the troposphere, the atmospheric layer where weather occurs. High-pressure areas form due to downward motion through the troposphere, the atmospheric layer where weather occurs.
the general surface wind circulation associated with the high-pressure center (h) is most likely
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A high-pressure area, high ,or anticyclone.
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how is it formed?
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High-pressure areas form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as convergence, near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere, and about half the atmospheric pressure at the surface. High-pressure systems are alternatively referred to as anticyclones. On English-language weather maps, high-pressure centers are identified by the letter H in English, within the isobar with the highest pressure value.
High-pressure area | Formation
High-pressure systems form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere. On weather maps, high-pressure centers are associated with the letter H placed within the isobar with the highest pressure value. On constant pressure upper level charts, it is located within the highest height line contour.
Anticyclogenesis | Process
In terms of climatology, high pressure forms at the horse latitudes, or torrid zone, between the latitudes of 20 and 40 degrees from the equator, as a result of air that has been uplifted at the equator. As the hot air rises it cools, losing moisture; it is then transported poleward where it descends, creating the high-pressure area. This is part of the Hadley cell circulation and is known as the subtropical ridge or subtropical high, and is strongest in the summer. The subtropical ridge is a warm core high-pressure system, meaning it strengthens with height. Many of the world's deserts are caused by these climatological high-pressure systems.
High-pressure area | In climatology
A cyclone forms around a low-pressure area. Anticyclone, the term for the kind of weather around a high-pressure area, was coined in 1877 by Francis Galton to indicate an area whose winds revolved in the opposite direction of a cyclone. In British English, the opposite direction of clockwise is referred to as anticlockwise, making the label "anticyclones" a logical extension.
High-pressure area | Wind circulation in the northern and southern hemispheres
A high-pressure area, high, or anticyclone, is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment. Winds within high-pressure areas flow outward from the higher pressure areas near their centers towards the lower pressure areas further from their centers. Gravity adds to the forces causing this general movement, because the higher pressure compresses the column of air near the center of the area into greater density – and so greater weight compared to lower pressure, lower density, and lower weight of the air outside the center. However, because the planet is rotating, the air flow from center to periphery is not direct, but is twisted due to the Coriolis effect.
High-pressure area | Introduction
Mesocyclones form when strong changes of wind speed and/or direction with height ("wind shear") sets parts of the lower part of the atmosphere spinning in invisible tube-like rolls. The convective updraft of a thunderstorm then draw up this spinning air, tilting the rolls' orientation upward (from parallel to the ground to perpendicular) and causing the entire updraft to rotate as a vertical column. As the updraft rotates and ingests cooler moister air from the forward flank downdraft (FFD), it may form a wall cloud, a spinning layer of clouds lowered from ambient storm cloud base under the mid-level mesocyclone. The wall cloud tends to form closer to the center of the mesocyclone.
Mesocyclone | Formation
Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation. Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place. Mesocyclones form as warm core cyclones over land, and can lead to tornado formation. Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear. In deserts, lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense, rapid solar heating of the lower layers of air. The hot air is less dense than surrounding cooler air.
Low-pressure area | Formation
Wind is initially accelerated from areas of high pressure to areas of low pressure. This is due to density (or temperature and moisture) differences between two air masses. Since stronger high-pressure systems contain cooler or drier air, the air mass is denser and flows towards areas that are warm or moist, which are in the vicinity of low-pressure areas in advance of their associated cold fronts. The stronger the pressure difference, or pressure gradient, between a high-pressure system and a low-pressure system, the stronger the wind. Thus, stronger areas of low pressure are associated with stronger winds. The Coriolis force caused by the Earth's rotation is what gives winds around low-pressure areas (such as in hurricanes, cyclones, and typhoons) their counter-clockwise (anticlockwise) circulation in the northern hemisphere (as the wind moves inward and is deflected right from the center of high pressure) and clockwise circulation in the southern hemisphere (as the wind moves inward and is deflected left from the center of high pressure).
Low-pressure area | Associated weather
Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear. Cyclolysis is the opposite of cyclogenesis, and is the high-pressure system equivalent, which deals with the formation of high-pressure areas—Anticyclogenesis.
Cyclone | Formation
Wind flows from areas of high pressure to areas of low pressure. This is due to density differences between the two air masses. Since stronger high-pressure systems contain cooler or drier air, the air mass is more dense and flows towards areas that are warm or moist, which are in the vicinity of low pressure areas in advance of their associated cold fronts. The stronger the pressure difference, or pressure gradient, between a high-pressure system and a low-pressure system, the stronger the wind. The coriolis force caused by the Earth's rotation is what gives winds within high-pressure systems their clockwise circulation in the northern hemisphere (as the wind moves outward and is deflected right from the center of high pressure) and counterclockwise circulation in the southern hemisphere (as the wind moves outward and is deflected left from the center of high pressure).
High-pressure area | Connection to wind
Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm-core with well-defined circulations. Certain criteria need to be met for their formation. In most situations, water temperatures of at least are needed down to a depth of at least ; waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms. Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone. High humidity is needed, especially in the lower-to-mid troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop.
Low-pressure area | Formation
In the absence of rotation, the wind tends to blow from areas of high pressure to areas of low pressure. The stronger the pressure difference (pressure gradient) between a high-pressure system and a low-pressure system, the stronger the wind. The coriolis force caused by Earth's rotation gives winds within high-pressure systems their clockwise circulation in the northern hemisphere (as the wind moves outward and is deflected right from the center of high pressure) and anticlockwise circulation in the southern hemisphere (as the wind moves outward and is deflected left from the center of high pressure). Friction with land slows down the wind flowing out of high-pressure systems and causes wind to flow more outward (more ageostrophically) from the center.
Anticyclone | Structure
Tropical cyclones typically form from large, disorganized areas of disturbed weather in tropical regions. As more thunderstorms form and gather, the storm develops rainbands which start rotating around a common center. As the storm gains strength, a ring of stronger convection forms at a certain distance from the rotational center of the developing storm. Since stronger thunderstorms and heavier rain mark areas of stronger updrafts, the barometric pressure at the surface begins to drop, and air begins to build up in the upper levels of the cyclone. This results in the formation of an upper level anticyclone, or an area of high atmospheric pressure above the central dense overcast.
Eye (cyclone) | Formation and detection
Consequently, most of this built up air flows outward anticyclonically above the tropical cyclone. Outside the forming eye, the anticyclone at the upper levels of the atmosphere enhances the flow towards the center of the cyclone, pushing air towards the eyewall and causing a positive feedback loop. However, a small portion of the built-up air, instead of flowing outward, flows inward towards the center of the storm. This causes air pressure to build even further, to the point where the weight of the air counteracts the strength of the updrafts in the center of the storm. Air begins to descend in the center of the storm, creating a mostly rain-free area—a newly formed eye.
Eye (cyclone) | Formation and detection
This is an apparent, but fictitious, force that arises when an observer is in a rotating reference frame, due to the conservation of angular momentum of the air as it moves towards or away from the Earth's axis of rotation. Viewed from above this twist in wind direction is in the opposite direction as the rotation of the planet.
High-pressure area | Introduction
Sir Francis Galton first discovered anticyclones in the 1860s. Preferred areas within a synoptic flow pattern in higher levels of the hydrosphere are beneath the western side of troughs, or dips in the Rossby wave pattern. High-pressure systems are alternatively referred to as anticyclones. Their circulation is sometimes referred to as cum sole. Subtropical high pressure zones form under the descending portion of the Hadley cell circulation. Upper-level high-pressure areas lie over tropical cyclones due to their warm core nature. Surface anticyclones form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs.
Anticyclone | History
In the region called the Horse latitudes, between 30 and 35 degrees of latitude North or South, there is a series of semi-permanent anticyclones on the downward side of the Hadley cell of the general atmospheric circulation. The thermal contrasts over oceanic waters leads to the formation of these centers of action as a direct convective circulation is established between the equator and higher latitudes in a very weak Coriolis force zone. The Azores/Bermuda High is found in this zone. Similarly, low pressure areas are formed in the oceanic circulation near the Polar regions were sea water is much warmer than the land.
Centers of action | Causes
Organized areas of thunderstorm activity reinforce pre-existing frontal zones, and can outrun cold fronts. The resultant mesoscale convective system (MCS) often forms at the point of the strongest divergence of the upper-level flow in the area of greatest low-level inflow and convergence. The convection tends to move east or toward the equator, roughly parallel to low-level thickness lines and usually somewhat to the right of the mean tropospheric flow. When the convection is strongly linear or slightly curved, the MCS is called a squall line, with the strongest winds typically occurring just behind the leading edge of the significant wind shift and pressure rise.
Derecho | Development
In North and Central America, during the Northern Hemisphere winter, high-pressure systems are created between the equator and 35th parallels north via atmospheric circulation. Air near the equator is warmed by the sun. This heated air is more buoyant than colder air so it rises and is then pushed poleward by more air rising from below. Once the air reaches northern latitudes it begins to cool and as a result it falls back towards to the Earth's surface. As the air is falling it exerts more pressure downward on the surface, creating a high-pressure systems. This cold, high-pressure air mass then travels equatorward.
Papagayo Jet | Formation
There are many aspects of this process which remain a mystery. Scientists do not know why a ring of convection forms around the center of circulation instead of on top of it, or why the upper-level anticyclone only ejects a portion of the excess air above the storm. Many theories exist as to the exact process by which the eye forms: all that is known for sure is that the eye is necessary for tropical cyclones to achieve high wind speeds.
Eye (cyclone) | Formation and detection
Many of the ocean's largest currents circulate around warm, high-pressure areas called gyres. Though the circulation is not as significant as that in the air, the deflection caused by the Coriolis effect is what creates the spiralling pattern in these gyres. The spiralling wind pattern helps the hurricane form. The stronger the force from the Coriolis effect, the faster the wind spins and picks up additional energy, increasing the strength of the hurricane. Air within high-pressure systems rotates in a direction such that the Coriolis force is directed radially inwards, and nearly balanced by the outwardly radial pressure gradient. As a result, air travels clockwise around high pressure in the Northern Hemisphere and anticlockwise in the Southern Hemisphere.
Coriolis force | Meteorology
On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up the troposphere. Because they weaken with height, these high-pressure systems are cold.
Anticyclone | History
Cyclogenesis is the development and strengthening of cyclonic circulations, or low-pressure areas, within the atmosphere. Cyclogenesis is the opposite of , and has an anticyclonic (high-pressure system) equivalent which deals with the formation of high-pressure areas—anticyclogenesis. Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone. Meteorologists use the term "cyclone" where circular pressure systems flow in the direction of the Earth's rotation, which normally coincides with areas of low pressure. The largest low-pressure systems are cold-core polar cyclones and extratropical cyclones which lie on the synoptic scale. Warm-core cyclones such as tropical cyclones, mesocyclones, and polar lows lie within the smaller mesoscale.
Low-pressure area | Formation
In response to relaxing shear aloft, convection redeveloped around the newly formed center of circulation. It subsequently acquired a sufficiently organized structure and post-analysis found that a tropical depression formed at around 1200 UTC on October 8. Upon developing, the depression was designated as "Sixteen-E" about 120 mi (195 km) south-southwest of San Salvador, El Salvador, and operationally, advisories were initiated at 2100 UTC — 9 hours after post-analytic estimates of formation. Despite being in an area with favorable conditions, lack of inner core organization proved difficult for rapid deepening to occur, and initially, model guidance did not forecast any significant strengthening.
Tropical Storm Odile (2008) | Meteorological history
Passive-flow folds are extremely dependent on the rock composition of the stratum and can typically occur in areas with high temperatures.
Anticline | Formation processes
On constant pressure upper level charts, it is located within the highest height line contour.
High-pressure area | Formation
The existence of upper-level (altitude) high pressure allows upper level divergence which leads to surface convergence. If a capping mid-level ridge does not exist, this leads to free convection and the development of showers and thunderstorms if the lower atmosphere is humid. Because a positive feedback loop develops between the convective tropical cyclone and the upper level high, the two system are strengthened. This loop stops once ocean temperatures cool to below , reducing the thunderstorm activity, which then weakens the upper level high pressure system.
Anticyclone | Upper tropospheric systems
The growth of helictites is still quite enigmatic. To this day, there has been no satisfactory explanation for how they are formed. Currently, formation by capillary forces is the most likely theory, but another theory based on wind formation is also viable.
Helictite | Formation
Usually, an anticyclone in the upper layers of the troposphere above the storm must be present as well—for extremely low surface pressures to develop. This is because air must be converging towards the low pressure at the surface which then forces the air to rise very rapidly in the eyewall of the storm, and due to conservation of mass requires a divergence of wind at the top of the troposphere. This process is aided by an upper-level anticyclone which helps channel this air away from the cyclone efficiently. Hot towers have been implicated in tropical cyclone rapid intensification, though they have diagnostically seen varied impacts across basins.
Rapid intensification | Internal
A mesohigh forms underneath the downdraft in a squall line and is associated with the cold pool of a thunderstorm. It is largely formed by hydrostatic phenomenon, specifically the evaporation of falling precipitation. As precipitation, primarily rain, falls in the downdraft, it evaporates in the unsaturated air, leading to cooling in the downdraft due to an absorption of latent heat. The cooling of the air leads to an increase in the pressure as the air becomes denser. While not the primary mechanism behind the mesohigh, melting or sensible cooling of hail can also lead to increased pressure in the mesohigh.
Mesohigh | Formation
Mesocyclones range in size from mesoscale beta to microscale. The term mesocyclone is usually reserved for mid-level rotations within severe thunderstorms, and are warm core cyclones driven by latent heat of its associated thunderstorm activity. Tornadoes form in the warm sector of extratropical cyclones where a strong upper level jet stream exists. Mesocyclones are believed to form when strong changes of wind speed and/or direction with height ("wind shear") sets parts of the lower part of the atmosphere spinning in invisible tube-like rolls. The convective updraft of a thunderstorm is then thought to draw up this spinning air, tilting the rolls' orientation upward (from parallel to the ground to perpendicular) and causing the entire updraft to rotate as a vertical column.
Cyclogenesis | Mesocyclones
Low-pressure areas can also form due to organized thunderstorm activity over warm water. When this occurs over the tropics in concert with the Intertropical Convergence Zone, it is known as a monsoon trough. Monsoon troughs reach their northerly extent in August and their southerly extent in February. When a convective low acquires a well-hot circulation in the tropics it is termed a tropical cyclone. Tropical cyclones can form during any month of the year globally, but can occur in either the northern or southern hemisphere during December.
Low-pressure area | Introduction
A low-pressure area, or "low", is a region where the atmospheric pressure at sea level is below that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in upper levels of the troposphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of atmospheric dynamics, areas of wind divergence aloft occur in two areas: Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity. Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses.
Pressure system | Low-pressure system
High-pressure systems, also known as anticyclones, rotate outward at the surface and clockwise in the northern hemisphere as opposed to outward and counterclockwise in the southern hemisphere. Under surface highs, sinking of the atmosphere slightly warms the air by compression, leading to clearer skies, winds that are lighter, and a reduced chance of precipitation. The descending air is dry, hence less energy is required to raise its temperature. If high pressure persists, air pollution will build up due to pollutants trapped near the surface caused by the subsiding motion associated with the high.
Surface weather analysis | High pressure
This, combined with the rising of the hot air, results in a low-pressure area called a thermal low. Monsoon circulations are caused by thermal lows which form over large areas of land and their strength is driven by how land heats more quickly than the surrounding nearby ocean. This generates a steady wind blowing toward the land, bringing the moist near-surface air over the oceans with it. Similar rainfall is caused by the moist ocean-air being lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface. However the lifting occurs, the air cools due to expansion in lower pressure, which in turn produces condensation.
Low-pressure area | Formation
Along its southerly periphery, the clockwise circulation often impels easterly waves, and tropical cyclones that develop from them, across the ocean towards landmasses in the western portion of ocean basins during the hurricane season. The highest barometric pressure ever recorded on Earth was measured in Tosontsengel,_Zavkhan, Mongolia on 19 December 2001.
High-pressure area | In climatology
A "mesocyclone" is a type of mesovortex, approximately 2 to 10 km in diameter (the mesoscale of meteorology), within a convective storm. Mesocyclones are air that rises and rotates around a vertical axis, usually in the same direction as low pressure systems in a given hemisphere. They are most often associated with a localized low-pressure region within a severe thunderstorm. Mesocyclones are believed to form when strong changes of wind speed and/or direction with height ("wind shear") sets parts of the lower part of the atmosphere spinning in invisible tube-like rolls. The convective updraft of a thunderstorm is then thought to draw up this spinning air, tilting the air's axis of rotation upward (from parallel to the ground to perpendicular) and causing the entire updraft to rotate as a vertical column.
Mesovortices | Mesocyclone
High pressures that build to the north and extend southwards will often bring clear weather. This is due to being cooled at the base (as opposed to warmed) which helps prevent clouds from forming. On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere. High-pressure systems are alternatively referred to as anticyclones. On weather maps, high-pressure centers are associated with the letter H in English, or A in Spanish, because alta is the Spanish word for high, within the isobar with the highest pressure value.
Synoptic scale meteorology | Surface high-pressure systems
The development of anticyclones aloft occurs in warm core cyclones such as tropical cyclones when latent heat caused by the formation of clouds is released aloft increasing the air temperature; the resultant thickness of the atmospheric layer increases high pressure aloft which evacuates their outflow.
Anticyclone | Formation aloft
The direction of wind flow around an atmospheric high-pressure area and a low-pressure area, as seen from above, depends on the hemisphere. High-pressure systems rotate clockwise in the northern Hemisphere; low-pressure systems rotate clockwise in the southern hemisphere. The scientific terms in English used to describe the weather systems generated by highs and lows were introduced in the mid-1800s, mostly by the British. The scientific theories which explain the general phenomena originated about two centuries earlier. The term cyclone was coined by Henry Piddington of the British East India Company to describe the devastating storm of December 1789 in Coringa, India.
High-pressure area | Wind circulation in the northern and southern hemispheres
As the mesocyclone lowers below the cloud base, it begins to take in cool, moist air from the downdraft region of the storm. The convergence of warm air in the updraft and cool air causes a rotating wall cloud to form. The RFD also focuses the mesocyclone's base, causing it to draw air from a smaller and smaller area on the ground. As the updraft intensifies, it creates an area of low pressure at the surface. This pulls the focused mesocyclone down, in the form of a visible condensation funnel. As the funnel descends, the RFD also reaches the ground, fanning outward and creating a gust front that can cause severe damage a considerable distance from the tornado.
Tornado | Formation
The systems begin in the afternoon as scattered thunderstorms which organize overnight in the presence of wind shear (wind speed and direction changes with height). The probability for severe weather is highest in the early stages of formation, during the afternoon. The MCC persists at its mature and strongest stage overnight and into the early morning in which the rainfall is characterized as stratiform rainfall (rather than convective rainfall which occurs with thunderstorms). Dissipation of the MCC commonly occurs during the morning hours. After dissipation, a remnant mid-level circulation known as a mesoscale convective vortex can initiate another round of thunderstorms later in the day.
Mesoscale convective complex | Development
The atmospheric circulation pattern that George Hadley described was an attempt to explain the trade winds. The Hadley cell is a closed circulation loop which begins at the equator. There, moist air is warmed by the Earth's surface, decreases in density and rises. A similar air mass rising on the other side of the equator forces those rising air masses to move poleward. The rising air creates a low pressure zone near the equator. As the air moves poleward, it cools, becomes denser, and descends at about the 30th parallel, creating a high-pressure area. The descended air then travels toward the equator along the surface, replacing the air that rose from the equatorial zone, closing the loop of the Hadley cell.
Atmospheric circulation | Hadley cell
A brinicle has now been formed, resembling an inverted "chimney" of ice enclosing a downward flow of this supercooled, supersaline water.
Brinicle | Formation
High-pressure systems are frequently associated with light winds at the surface and subsidence of air from higher portions of the troposphere. Subsidence will generally warm an air mass by adiabatic (compressional) heating. Thus, high pressure typically brings clear skies. Because no clouds are present to reflect sunlight during the day, there is more incoming solar radiation and temperatures rise rapidly near the surface. At night, the absence of clouds means that outgoing longwave radiation (i.e. heat energy from the surface) is not blocked, giving cooler diurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to a buildup of particulates in urban areas under the high pressure, leading to widespread haze.
Anticyclone | Surface-based systems
A broad low-pressure area formed on October 21, after convection intensified near the wave's axis. However, the upper-level winds were only described as "marginally favorable" and relatively dry air in the region was expected to act as a hindrance to development. Throughout the day on October 22, atmospheric pressures decreased and the circulation continuously became better defined, while convection increased significantly on the western side of the system and near the center. Around 06:00 UTC on October 23, the system strengthened into Tropical Depression Eighteen, while situated about north of Providencia Island. The depression initially moved northward into a weakness in a ridge, caused by a broad mid-level trough situated over the Southeastern United States.
Hurricane Rina | Meteorological history
Heat waves form when high pressure aloft (from ) strengthens and remains over a region for several days up to several weeks. This is common in summer (in both Northern and Southern Hemispheres) as the jet stream 'follows the sun'. On the equator side of the jet stream, in the upper layers of the atmosphere, is the high pressure area. Summertime weather patterns are generally slower to change than in winter. As a result, this upper level high pressure also moves slowly. Under high pressure, the air subsides (sinks) toward the surface, warming and drying adiabatically, inhibiting convection and preventing the formation of clouds.
Heat wave | Formation
However, deep convection associated with the system remained disorganized, despite surface and air data indicating an organizing surface circulation. A low pressure area formed on September 22 near Curaçao in the south-central Caribbean Sea; at that point, the NHC assessed a high chance of tropical cyclone development. The thunderstorm activity remained disorganized, but on September 23 the convection increased and became better organized, and curved bands developed throughout the day. Late that day, a Hurricane Hunters flight into the system confirmed the development of a well-defined circulation, and the NHC initiated advisories on Tropical Depression Fifteen about 485 mi (755 km) east of Puerto Cabezas, Nicaragua.
Tropical Storm Matthew (2010) | Meteorological history
On weather maps, high-pressure centers are associated with the letter H in English, within the isobar with the highest pressure value. On constant-pressure upper-level charts, anticyclones are located within the highest height line contour.
Anticyclone | Depiction on weather maps
In the Horse latitudes, generally in the region between 30 and 35 degrees of north and south latitude, there are more or less permanent highs. This is the subsiding part of Hadley cells. Indeed, near the equator, where the Coriolis force is quite weak, a direct circulation of air is established. In the lower levels of the atmosphere, the difference in temperature between the equator and the northerly warmer regions gives rise to the intertropical convergence zone where the warmer air rises because of convergence and relatively lower density. Subsequently, this air spreads north and south and cools at higher altitude and descends further north and south.
South Atlantic High | Formation
In winter, the land cools off quickly, but the ocean keeps the heat longer due to its higher specific heat. The hot air over the ocean rises, creating a low-pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling. Monsoons resemble sea and land breezes, terms usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale - also stronger and seasonal.
Low-pressure area | Formation
For instance, when the ITCZ is situated north of the Equator, the southeast trade wind changes to a southwest wind as it crosses the Equator. The ITCZ is formed by vertical motion largely appearing as convective activity of thunderstorms driven by solar heating, which effectively draw air in; these are the trade winds. The ITCZ is effectively a tracer of the ascending branch of the Hadley cell and is wet. The dry descending branch is the horse latitudes. The location of the ITCZ gradually varies with the seasons, roughly corresponding with the location of the thermal equator. As the heat capacity of the oceans is greater than air over land, migration is more prominent over land.
Intertropical Convergence Zone | Meteorology
An anticyclone (that is, opposite to a cyclone) is a weather phenomenon defined as a large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Effects of surface-based anticyclones include clearing skies as well as cooler, drier air. Fog can also form overnight within a region of higher pressure. Mid-tropospheric systems, such as the subtropical ridge, deflect tropical cyclones around their periphery and cause a temperature inversion inhibiting free convection near their center, building up surface-based haze under their base. Anticyclones aloft can form within warm core lows such as tropical cyclones, due to descending cool air from the backside of upper troughs such as polar highs, or from large scale sinking such as the subtropical ridge.
Anticyclone | Introduction
Rotation in these synoptic scale systems stems partly from the Coriolis effect, but thunderstorms and tornadoes are too small to be significantly affected. The common property here is an area of lower pressure, thus surrounding air flows into the area of less dense air forming cyclonic rotation. The rotation of the thunderstorm itself is induced mostly by vertical wind shear, specifically clockwise turning as altitude increases (called a veered vertical profile, although backed profiles can occur with anticyclonic supercells]]. Various processes can produce an anticyclonic tornado. Most often they are satellite tornadoes of larger tornadoes which are directly associated with the tornadocyclone and mesocyclone.
Anticyclonic tornado | Formation
This results in an inclined axial plane, creating an asymmetric fold. This is unexpected because the structure is folding "towards" the Hajar Mountains in the east. If the structure is related to the formation of the Hajar Mountains, the anticline would typically be west verging. This indicates that the Jabal Hafit anticline formed due to a backthrust (antithetic fault), and is a fault-propagation fold. Within the eroded core of the anticline, at the Green Mubazzarah Park, are hot springs with temperatures of around . There is no volcanic activity in the area, therefore the water is being heated geothermally. Meteoric water is percolating from the surface to about depth and then returning to the surface.
Jebel Hafeet | Geology
In anticyclones (air circulation around pressure highs), the Coriolis force is always inward (and positive), and the pressure force outward (and negative) irrespective of the hemisphere. The cross-trajectory momentum equation is formula_26. Dividing both sides by |"f"|"V", we obtain
Balanced flow | Pressure highs and anticyclones
Friction with land slows down the wind flowing out of high-pressure systems and causes wind to flow more outward than would be the case in the absence of friction. This is known as a geostrophic wind.
High-pressure area | Connection to wind
Heating of the earth near the equator forces upward motion and convection along the monsoon trough or intertropical convergence zone. The divergence over the near-equatorial trough leads to air rising and moving away from the equator aloft. As air moves towards the mid-latitudes, it cools and sinks leading to subsidence near the 30° parallel of both hemispheres. This circulation known as the Hadley cell forms the subtropical ridge. Many of the world's deserts are caused by these climatological high-pressure areas. Because these anticyclones strengthen with height, they are known as warm core ridges.
Anticyclone | Subtropical ridge
An additional source of increased pressure is hydrometeor loading, the weight of precipitation increasing the speed of the downdraft, leading to increased pressure as the air converges at the surface. While hydrometeor loading is not a main contributor of increased pressure to the mesohigh, and it is a non-hydrostatic process, it can increase the pressure as much as 2 mb.
Mesohigh | Formation
Given the direction of the winds around an anticyclonic circulation and the fact that weather systems move from west to east: Surface ridges, just like highs, generate fair weather because they develop under wind convergence in the negative vorticity advection zone ahead of the upper level ridge. The vertical downward air motion then gives a divergence of the winds near the surface. The subsidence of the air causes a warming in the column compared to the previous environment and therefore a drying of it because its relative humidity decreases, which has the effect of clearing the sky.
Ridge (meteorology) | Related weather
The subsiding air is generally dry and as it subsides the air warms adiabatically and can form a clearing in the cloud cover called a clear slot. A clear slot can be observed to wrap around a tornado or form away from a tornado in the shape of a horseshoe. This clearing is most likely the formation of the hook echo region associated with tornado formation. An RFD originating in dry air warming adiabatically can produce warmer observations out of the RFD at the surface.
Rear flank downdraft | Formation
In meteorology, an occluded front is a weather front formed during the process of cyclogenesis, when a cold front overtakes a warm front. When this occurs, the warm air is separated (occluded) from the cyclone center at the Earth's surface. The point where the warm front and the occluded front meet (and consequently the nearest location of warm air to the center of the cyclone) is called the "triple point". The trowal (short for TROugh of Warm air ALoft) is the projection on the Earth's surface of the trough of warm air aloft formed during the occlusion process of the depression.
Occluded front | Introduction
If the low level relative humidity rises towards 100 percent overnight, fog can form. Strong, vertically shallow high-pressure systems moving from higher latitudes to lower latitudes in the northern hemisphere are associated with continental arctic air masses. The low, sharp inversion can lead to areas of persistent stratocumulus or stratus cloud, colloquially known as anticyclonic gloom. The type of weather brought about by an anticyclone depends on its origin. For example, extensions of the Azores high pressure may bring about anticyclonic gloom during the winter, as they are warmed at the base and will trap moisture as they move over the warmer oceans.
Synoptic scale meteorology | Surface high-pressure systems
Most strong tornadoes form in the inflow and updraft area bordering the updraft-downdraft interface (which is also near the mesoscale "triple point") zone of supercell thunderstorms. The thunderstorm itself is rotating, with a rotating updraft known as a mesocyclone, and then a smaller area of rotation at lower altitude the tornadocyclone (or low-level mesocyclone) which produces or enables the smaller rotation that is a tornado. All of these may be quasi-vertically aligned continuing from the ground to the mid-upper levels of the storm. All of these cyclones and scaling all the way up to large extratropical (low-pressure systems) and tropical cyclones rotate cyclonically.
Anticyclonic tornado | Formation
On June 11, 2014, a broad, poorly-defined area of low pressure formed over the South China Sea. Extensive, though disorganized, deep convection accompanied the system. Situated within a region of low to moderate wind shear and weak outflow, slow development ensued. By June 13, the Japan Meteorological Agency (JMA) classified the disturbance as a tropical depression. A monsoonal system, the depression featured an extensive circulation with the strongest winds well away from the center, contrary to most tropical cyclones. A scatterometer pass revealed winds up to 65 km/h (40 mph) within a banding feature to the east of the storm's center by the evening of June 13.
Tropical Storm Hagibis (2014) | Meteorological history
It is commonly formed into a solid by a process called hot pressing, where the powders are pressed together using both heat and pressure. The material has potential for use in hypervelocity reentry vehicles such as ICBM heat shields or aerodynamic leading-edges, due to its strength and thermal properties. Unlike polymer and composite material, HfB can be formed into aerodynamic shapes that will not ablate during reentry.
Hafnium diboride | Introduction
Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity. A low-pressure area is commonly associated with inclement weather, while a high-pressure area is associated with light winds and fair skies. Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses. Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth's surface. Large-scale thermal lows over continents help drive monsoon circulations.
Low-pressure area | Introduction
A low-pressure area, low area or low is a region where the air pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas. The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies (a trough with large wavelength that extends through the troposphere). A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength.
Low-pressure area | Introduction
Supercells derive their rotation through tilting of horizontal vorticity (an invisible horizontal vortex) caused by wind shear. Strong updrafts lift the air turning about a horizontal axis and cause this air to turn about a vertical axis. This forms the "deep rotating updraft", the mesocyclone. A "cap" or capping inversion is usually required to form an updraft of sufficient strength. The moisture-laden air is then cooled enough to precipitate as it is rotated toward the cooler region, represented by the turbulent air of the mammatus clouds where the warm air is spilling over top of the cooler, invading air. The cap is formed where shear winds block further uplift for a time, until a relative weakness allows a breakthrough of the cap (an overshooting top); cooler air to the right in the image may or may not form a shelf cloud, but the precipitation zone will occur where the heat engine of the uplift intermingles with the invading, colder air.
Supercell | Supercell anatomy
By contrast the ocean is heated from above, which tends to suppress convection. Instead ocean deep water is formed in polar regions where cold salty waters sink in fairly restricted areas. This is the beginning of the thermohaline circulation. Oceanic currents are largely driven by the surface wind stress; hence the large-scale atmospheric circulation is important to understanding the ocean circulation. The Hadley circulation leads to Easterly winds in the tropics and Westerlies in mid-latitudes. This leads to slow equatorward flow throughout most of a subtropical ocean basin (the Sverdrup balance). The return flow occurs in an intense, narrow, poleward western boundary current.
Physical oceanography | Circulation
Major whirlwind A major whirlwind (such as a tornado) is formed from supercell thunderstorms (the most powerful type of thunderstorm) or other powerful storms. When the storms start to spin, they react with other high altitude winds, causing a funnel to spin. A cloud forms over the funnel, making it visible. Minor whirlwind
Whirlwind | Formation
It might have formed either through erosion of surrounding sediments by ocean currents or through isostatic subsidence.
Anton Dohrn Seamount | Geography and geomorphology
Because the planetary angular momentum term vanishes at the equator (where formula_6 ), tropical cyclones rarely form within 5° of the equator. As air flows radially inward at low levels, it begins to rotate cyclonically in order to conserve angular momentum. Similarly, as rapidly rotating air flows radially outward near the tropopause, its cyclonic rotation decreases and ultimately changes sign at large enough radius, resulting in an upper-level anti-cyclone. The result is a vertical structure characterized by a strong cyclone at low levels and a strong anti-cyclone near the tropopause; from thermal wind balance, this corresponds to a system that is warmer at its center than in the surrounding environment at all altitudes (i.e.
Tropical cyclone | Primary circulation: rotating winds
A good way to visualize how the multiple layers are manipulated, is to bend a deck of cards and to imagine each card as a layer of rock stratum. The amount of slip on each side of the anticline increases from the hinge to the inflection point. Passive-flow folds form when the rock is so soft that it behaves like weak plastic and slowly flows. In this process different parts of the rock body move at different rates causing shear stress to gradually shift from layer to layer. There is no mechanical contrast between layers in this type of fold.
Anticline | Formation processes
Not until the Late Jurassic to Early Cretaceous did Huangling massif uplift. This period is critical for the tectonic development of Huangling massif since the dome-shaped structure was formed. The dome-shaped structure indicated a compressional environment. The dome has a steeper west flank and a gentle east flank, indicating the compressive stress exerted to the west and to the east were different. Coevally, recumbent folds trending north-south overturning to the west and east on the western and eastern flanks respectively, and layer-parallel slip were formed. In the next session, different models suggesting the kinematics of the dome-shaped structure will be further discussed.
Huangling Complex | Middle Mesozoic formation of dome-shaped anticline
Cold cyclones are stronger aloft than at the Earth's surface, or stronger in areas of the troposphere with lower pressures, per the thermal wind relationship and the hypsometric equation. The hypsometric equation dictates that colder atmospheres have less room between pressure surfaces, which corresponds to the concept of lower atmospheric thickness, and the thermal wind relation indicates that in this situation, the wind increases with height. It also means that an isolated pool of cold air aloft is associated with the feature. Because of its cold center, the center shows an area of thickness minimum. Since the temperature at any given radius is similar and parallel to the orientation of the pressure gradient, and therefore the wind, it is barotropic in nature.
Cold-core low | Characteristics
A weak surface area of low pressure detached from a frontal system on August 8 while 1,150 miles (1,850 km) to the east of Bermuda. It moved southwestward, and on August 9, it generated convection as it passed beneath a cold-core upper-level low. The surface low and the upper-level low turned westward as it revolved around a common center, and by August 11, the surface low developed into a trough while 440 miles (700 km) south of Bermuda. As the system rapidly continued westward, much of the convection remained near the center of the upper-level low, preventing development of a closed surface circulation.
Hurricane Erika (2003) | Meteorological history
A mesohigh (sometimes called a "bubble high") is a mesoscale high-pressure area that forms beneath thunderstorms. While not always the case, it is usually associated with a mesoscale convective system. In the early stages of research on the subject, the mesohigh was often referred to as a "thunderstorm high".
Mesohigh | Introduction
Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere (a low-pressure area). Cyclogenesis is an umbrella term for at least three different processes, all of which result in the development of some sort of cyclone, and at any size from the microscale to the synoptic scale. The process in which an extratropical cyclone undergoes a rapid drop in atmospheric pressure (24 millibars or more) in a 24-hour period is referred to as explosive cyclogenesis, and is usually present during the formation of a nor'easter. The anticyclonic equivalent, the process of formation of high pressure areas, is anticyclogenesis.
Cyclogenesis | Introduction
Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth's surface. Large-scale thermal lows over continents help create pressure gradients that drive monsoon circulations. Low-pressure areas can also form due to organized thunderstorm activity over warm water. When this occurs over the tropics in concert with the Intertropical Convergence Zone, it is known as a monsoon trough. Monsoon troughs reach their northerly extent in August and their southerly extent in February. When a convective low acquires a well-defined circulation in the tropics it is termed a tropical cyclone.
Pressure system | Low-pressure system
This structure is formed under a combination of unidirectional and oscillatory flow that is generated by relatively large storm waves in the ocean. Deposition involves fallout from suspension and lateral tractive flow due to wave oscillation. As the large waves drape sand over an irregular scoured surface, this strong storm-wave action erodes the seabed into low hummocks and swales that lack any significant orientation. It is usually formed by redeposition below normal fair weather wave base delivered offshore by flooding rivers and shoals by large waves.
Hummocky cross-stratification | Formation of structure
From the overflow orifice, the coal goes into the overflow chamber, and is discharged to the next stage of the process. Fluid on entry commences in the outer regions of the cyclone body. This combined with rotational motion to which it is constrained creates an outer spiral. The existence of a top central outlet and inability for all the fluid to leave at the cone apex outlet, assist the inward migration of some of the fluid from the external moving mass. The amount of inward migration increases as the apex is neared, i.e. the radius decreased and the fluid which flows in this migratory stream, ultimately reverses its vertical velocity direction and flows upward to the cyclone over flow outlet, i.e.
Coal preparation plant | Dense medium cyclones
A trough is the result of the movements of the air in the atmosphere. In regions where there is upward movement near the ground and divergence at altitude, there is a loss of mass. The pressure becomes lower at this point. At upper levels of the atmosphere, this occurs when there is a meeting of a mass of cold air and another hot one along a thin ribbon called a frontal baroclinic zone. We then have the creation of a jet stream that plunges the cold air towards the equator and hot air towards the poles, creating a ripple in the circulation that is called a Rossby wave.
Trough (meteorology) | Formation
This mid-level circulation is referred to as a Mesoscale Convective Vortex. The upper-levels contain an anti-cyclonic (clockwise in the Northern Hemisphere) rotating high pressure which is a sign of divergence of air. This high pressure is colder relative to its surrounding environment. This divergence at upper-levels and convergence of air at the surface along the cool pool's outflow boundary results in rising motion which aids maintenance of the MCC.
Mesoscale convective complex | Structure
Tornado formation is not completely understood, but often occurs in one of two ways. In the first method, two conditions must be satisfied. One, a horizontal spinning effect must form on the Earth's surface. This usually originates in sudden changes in wind direction or speed, known as wind shear. Two, a thundercloud, or occasionally a cumulus cloud, must be present. During a thunderstorm, updrafts are occasionally powerful enough to lift the horizontal spinning row of air upwards, turning it into a vertical air column. This vertical air column then becomes the basic structure for the tornado. Tornadoes that form in this way are often weak and generally last less than 10 minutes.
Mesocyclone | Tornado formation
The wind belts girdling the planet are organised into three cells in each hemisphere—the Hadley cell, the Ferrel cell, and the polar cell. Those cells exist in both the northern and southern hemispheres. The vast bulk of the atmospheric motion occurs in the Hadley cell. The high pressure systems acting on the Earth's surface are balanced by the low pressure systems elsewhere. As a result, there is a balance of forces acting on the Earth's surface. The horse latitudes are an area of high pressure at about 30° to 35° latitude (north or south) where winds diverge into the adjacent zones of Hadley or Ferrel cells, and which typically have light winds, sunny skies, and little precipitation.
Atmospheric circulation | Latitudinal circulation features
An area of low pressure soon formed within the system just off the coast of South Carolina. Generally favorable environmental conditions fostered further organization of the low, though convection remained minimal for several days. During the latter half of June 30, a United States Air Force weather reconnaissance mission into the system confirmed the presence of a well-organized circulation; however, displacement of showers and thunderstorms from its center delayed its classification. Subsequent organization of convection into a banding feature along the system's southern flank, as depicted by radar imagery, prompted the NHC to designate the low as Tropical Depression One at 00:00 UTC on July 1.
Hurricane Arthur | Meteorological history
The rear flank downdraft can arise owing to negative buoyancy, which can be generated by cold anomalies produced at the rear of the supercell thunderstorm by evaporative cooling of precipitation or hail melting, or injection of dry and cooler air in the cloud, and by vertical perturbation pressure gradients that can arise from vertical gradients of vertical vorticity, "stagnation" of environmental flow at an updraft, and pressure perturbations due to vertical buoyancy variations (which are partially due to hydrostatic effects). Vertical pressure perturbations are generated by the buildup of pressure due to the vertical buoyancy, creating a pressure perturbation gradient.
Rear flank downdraft | Formation
On 3 October, a broad low-pressure area had formed along a stationary front about west of the Azores. The low meandered over the north Atlantic for days. On 6 October, a large wind field had formed associated with the low. The low only developed shallow, weak convection, along with a long, curved cloud band, and a cold-core center—typical characteristics of an extratropical low. The system began to acquire subtropical characteristics on the next day, benefits from warm sea surface temperature of , thus the National Hurricane Center (NHC) noted for a high chance of tropical cyclogenesis. Although the system lost some of its organization due to dry mid-level air, it managed to develop gale-force winds and a well-defined center.
Hurricane Ophelia (2017) | Meteorological history
Living vegetation covered around 55% of the land area, with litter covering 30% and the remaining 15% of the ground being bare. A braided river flowing to the north periodically deposited the sandy layers found in cores when it flooded its banks. Sedimentary textures which appear to have formed in the hydrothermal vents themselves are preserved with a brecciated texture; "geyserite", a sediment with a botryoidal form reminiscent of modern vent margins, is also found. Spores collected from within surrounding rocks had been heated to different degrees, implying a complex history of local heating by volcanic processes.
Rhynie chert | Conditions of formation
A lee trough, also known as a dynamic trough, is "A pressure trough formed on the lee side of a mountain range in situations where the wind is blowing with a substantial component across the mountain ridge; often seen on United States weather maps east of the Rocky Mountains, and sometimes east of the Appalachians, where it is less pronounced." It can be formed either as a result of the adiabatic compression of sinking air on the lee side of a mountain range, or through cyclogenesis resulting from "the horizontal convergence associated with vertical stretching of air columns passing over the ridge and descending the lee slope."
Trough (meteorology) | Lee trough
Despite the asymmetry, the NHC initiated advisories on a tropical depression around 15:00 UTC that day, after surface and satellite observations revealed a sufficiently defined circulation center west of the deep convection. Post-season reassessments, however, indicated that a tropical storm had, in fact, formed three hours earlier, about 75 miles (120 km) south of Cuba's Isle of Youth, which operationally was not named Nicole until a day later. For most of its duration, Nicole maintained a generally northeastward motion, caught in the steering flow between a large mid- to upper-level trough and an anticyclone to the west. Within hours of the storm's formation, observations from a hurricane hunters flight confirmed a composition similar to the one initially discerned, with the strongest gusts and thunderstorms dislocated 250 mi (400 km) east from the ill-defined center.
Tropical Storm Nicole (2010) | Meteorological history
The center of a subtropical gyre is a high pressure zone. Circulation around the high pressure is clockwise in the northern hemisphere and counterclockwise in the southern hemisphere, due to the Coriolis effect. The high pressure in the center is due to the westerly winds on the northern side of the gyre and easterly trade winds on the southern side. These cause frictional surface currents towards the latitude at the center of the gyre. This build-up of water in the center creates flow towards the equator in the upper of the ocean, through rather complex dynamics. This flow is returned towards the pole in an intensified western boundary current.
Ocean gyre | Subtropical gyres
The low became well defined, interacting with the adjacent upper vortex to produce a wide convective band—a concentrated region in which strong showers and thunderstorms can form—over the northeastern Caribbean. Over the next day, the system continued to acquire a prominent subtropical structure: it developed a markedly larger wind radius than typically seen in pure tropical cyclones. Satellite observations also revealed that although the lower wind circulation was distinct, it had become intertwined with the cyclonic vortex aloft to its southwest, confirming the hybrid nature of the system. The National Hurricane Center (NHC) thus classified the system as a subtropical depression at 06:00 UTC on October 6, when it was 265 mi (425 km) north-northwest of San Juan, Puerto Rico.
Hurricane Otto (2010) | Meteorological history
Although normally it is possible to obtain information about the wind direction from the orientation and form of sand dunes, the complexity of shapes of the Hagal dunes makes it difficult to determine the direction of the forming winds. In the case of the Hagal dunes, it is theorised that a local circular crater, probably formed due to meteorite impact and filled with sand, has decreased the quantity of dune-forming sand; this, in turn, impacted the local topography, causing a change in wind patterns. The linear dunes (dashes) were formed through the action of bidirectional winds, acting perpendicular to the line of the sand dune, causing a funneling effect directing the sand to accumulate along the linear axis of the dune.
Hagal dune field | Formation
On August 11, a low-level circulation center formed in conjunction with a persistent area of convection about southeast of Naha, Okinawa. Northerly upper-level winds initially provided beneficial divergence, though also resulted in detrimental wind shear and lack of inflow. Upper-level conditions gradually became more favorable for development as the disturbance tracked along the eastern periphery of an upper-level anticyclone. With convective rain bands wrapping into the circulation, the Joint Typhoon Warning Center (JTWC) issued a tropical cyclone formation alert on the system late on August 11. Remaining nearly stationary between ridges to its northeast and southwest, the system developed further, and early on August 12 the JTWC classified it as Tropical Depression 09W.
Typhoon Sepat (2007) | Meteorological history
Hamadas are produced by the wind removing the fine products of weathering: an aeolian process known as deflation. The finer-grained products are taken away in suspension, while the sand is removed through saltation and surface creep, leaving behind a landscape of gravel, boulders and bare rock.
Hamada | Formation
An occluded front is formed during the process of cyclogenesis when a cold front overtakes a warm front. The cold and warm fronts curve naturally poleward into the point of occlusion, which is also known as the triple point in meteorology. It lies within a sharp trough, but the air mass behind the boundary can be either warm or cold. In a cold occlusion, the air mass overtaking the warm front is cooler than the cool air ahead of the warm front, and plows under both air masses. In a warm occlusion, the air mass overtaking the warm front is not as cool as the cold air ahead of the warm front, and rides over the colder air mass while lifting the warm air.
Surface weather analysis | Occluded front
The authors of the study believe W75N(B)-VLA2 is forming in a dense, gaseous environment, surrounded by a dusty torus. The star intermittently ejects a hot, ionized wind for several years. Initially the wind can expand in all directions and forms a spherical shell; later it hits the dusty torus, which slows it. There is less resistance along the poles of the torus, so the wind moves more quickly there, giving rise to an elongated shape.
W75N(B)-VLA2 | Introduction
A larger cyclonic circulation associated with the trough and induced a gradual increase in deep convection. The low level convection center was temporarily displaced from the convection, and a smaller embedded circulation developed and was eventually detached from the larger circulation. It is estimated that a tropical depression formed at 1200 UTC 6 September when it was centered about south of Manzanillo, Mexico. As convective banding formed, the depression gradually intensified and was upgraded a Tropical Storm Lowell six hours later. A deep-layer anticyclone over Mexico steered Lowell northwestward until September 8. However, strong upper-level winds on the south side of the anticyclone produced strong northeasterly to easterly wind shear over the tropical cyclone for several days.
Tropical Storm Lowell (2008) | Meteorological history