Wind shear

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Cirrus uncinus ice crystal plumes showing high level wind shear, with changes in wind speed and direction. Cirrus clouds2.jpg
Cirrus uncinus ice crystal plumes showing high level wind shear, with changes in wind speed and direction.

Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizontal wind shear. Vertical wind shear is a change in wind speed or direction with change in altitude. Horizontal wind shear is a change in wind speed with change in lateral position for a given altitude. [1]

In common usage, wind gradient, more specifically wind speed gradient or wind velocity gradient, or alternatively shear wind, is the vertical gradient of the mean horizontal wind speed in the lower atmosphere. It is the rate of increase of wind strength with unit increase in height above ground level. In metric units, it is often measured in units of meters per second of speed, per kilometer of height (m/s/km), which reduces to the standard unit of shear rate, inverse seconds (s−1).

Wind speed

Wind speed, or wind flow velocity, is a fundamental atmospheric quantity caused by air moving from high to low pressure, usually due to changes in temperature. Note that wind direction is usually almost parallel to isobars, due to Earth's rotation.

Wind direction

Wind direction is reported by the direction from which it originates. For example, a northerly wind blows from the north to the south. Wind direction is usually reported in cardinal directions or in azimuth degrees. Wind direction is measured in degrees clockwise from due north. Consequently, a wind blowing from the north has a wind direction of 180°; a wind blowing from the east has a wind direction of 270°; a wind blowing from the south has a wind direction of 360°; and a wind blowing from the west has a wind direction of 090°. In general, wind directions are measured in units from 0° to 360°, but can alternatively be expressed from -180° to 180°.

Contents

Wind shear is a microscale meteorological phenomenon occurring over a very small distance, but it can be associated with mesoscale or synoptic scale weather features such as squall lines and cold fronts. It is commonly observed near microbursts and downbursts caused by thunderstorms, fronts, areas of locally higher low-level winds referred to as low level jets, near mountains, radiation inversions that occur due to clear skies and calm winds, buildings, wind turbines, and sailboats. Wind shear has significant effects on control of an aircraft, and it has been a sole or contributing cause of many aircraft accidents.

Microscale meteorology is the study of short-lived atmospheric phenomena smaller than mesoscale, about 1 km or less. These two branches of meteorology are sometimes grouped together as "mesoscale and microscale meteorology" (MMM) and together study all phenomena smaller than synoptic scale; that is they study features generally too small to be depicted on a weather map. These include small and generally fleeting cloud "puffs" and other small cloud features. Microscale meteorology controls the most important mixing and dilution processes in the atmosphere. Important topics in microscale meteorology include heat transfer and gas exchange between soil, vegetation, and/or surface water and the atmosphere caused by near-ground turbulence. Measuring these transport processes involves use of micrometeorological towers. Variables often measured or derived include net radiation, sensible heat flux, latent heat flux, ground heat storage, and fluxes of trace gases important to the atmosphere, biosphere, and hydrosphere.

Mesoscale meteorology

Mesoscale meteorology is the study of weather systems smaller than synoptic scale systems but larger than microscale and storm-scale cumulus systems. Horizontal dimensions generally range from around 5 kilometers to several hundred kilometers. Examples of mesoscale weather systems are sea breezes, squall lines, and mesoscale convective complexes.

Microburst

A microburst is an intense small-scale downdraft produced by a thunderstorm or rain shower. There are two types of microbursts: wet microbursts and dry microbursts. They go through three stages in their cycle, the downburst, outburst, and cushion stages also called "Suriano's Stroke". A microburst can be particularly dangerous to aircraft, especially during landing, due to the wind shear caused by its gust front. Several fatal and historic crashes have been attributed to the phenomenon over the past several decades, and flight crew training goes to great lengths on how to properly recover from a microburst/wind shear event.

Wind shear is sometimes experienced by pedestrians at ground level when walking across a plaza towards a tower block and suddenly encountering a strong wind stream that is flowing around the base of the tower.

Sound movement through the atmosphere is affected by wind shear, which can bend the wave front, causing sounds to be heard where they normally would not, or vice versa. Strong vertical wind shear within the troposphere also inhibits tropical cyclone development, but helps to organize individual thunderstorms into longer life cycles which can then produce severe weather. The thermal wind concept explains how differences in wind speed at different heights are dependent on horizontal temperature differences, and explains the existence of the jet stream. [2]

Troposphere The lowest layer of the atmosphere

The troposphere is the lowest layer of Earth's atmosphere, and is also where nearly all weather conditions take place. It contains approximately 75% of the atmosphere's mass and 99% of the total mass of water vapor and aerosols. The average height of the troposphere is 18 km in the tropics, 17 km in the middle latitudes, and 6 km in the polar regions in winter. The total average height of the troposphere is 13 km.

Tropical cyclone Is a rotating storm system

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms".

Severe weather

Severe weather refers to any dangerous meteorological phenomena with the potential to cause damage, serious social disruption, or loss of human life. Types of severe weather phenomena vary, depending on the latitude, altitude, topography, and atmospheric conditions. High winds, hail, excessive precipitation, and wildfires are forms and effects of severe weather, as are thunderstorms, downbursts, tornadoes, waterspouts, tropical cyclones, and extratropical cyclones. Regional and seasonal severe weather phenomena include blizzards (snowstorms), ice storms, and duststorms.

Down draft winds with associated virga allow these clouds in the eastern sky at civil twilight to mimic aurora borealis in the Mojave desert Downdraft Wind shear-2.jpg
Down draft winds with associated virga allow these clouds in the eastern sky at civil twilight to mimic aurora borealis in the Mojave desert

Definition

Wind shear refers to the variation of wind over either horizontal or vertical distances. Airplane pilots generally regard significant wind shear to be a horizontal change in airspeed of 30 knots (15 m/s) for light aircraft, and near 45 knots (23 m/s) for airliners at flight altitude. [3] Vertical speed changes greater than 4.9 knots (2.5 m/s) also qualify as significant wind shear for aircraft. Low level wind shear can affect aircraft airspeed during take off and landing in disastrous ways, and airliner pilots are trained to avoid all microburst wind shear (headwind loss in excess of 30 knots [15 m/s]). [4] The rationale for this additional caution includes:

Knot (unit) unit of speed

The knot is a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h. The ISO standard symbol for the knot is kn. The same symbol is preferred by the Institute of Electrical and Electronics Engineers (IEEE); kt is also common, especially in aviation where it is the form recommended by the International Civil Aviation Organization (ICAO). The knot is a non-SI unit. Worldwide, the knot is used in meteorology, and in maritime and air navigation—for example, a vessel travelling at 1 knot along a meridian travels approximately one minute of geographic latitude in one hour.

Wind shear is also a key factor in the creation of severe thunderstorms. The additional hazard of turbulence is often associated with wind shear.

In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between those layers.

Where and when it is strongly observed

Microburst schematic from NASA. Note the downward motion of the air until it hits ground level, then spreads outward in all directions. The wind regime in a microburst is completely opposite to a tornado. Microburstnasa.JPG
Microburst schematic from NASA. Note the downward motion of the air until it hits ground level, then spreads outward in all directions. The wind regime in a microburst is completely opposite to a tornado.

Weather situations where shear is observed include:

The tropopause is the boundary in the Earth's atmosphere between the troposphere and the stratosphere. It is a thermodynamic gradient stratification layer, marking the end of troposphere. It lies, on average, at 17 kilometres (11 mi) above equatorial regions, and above 9 kilometres (5.6 mi) over the polar regions.

Windward and leeward

Windward is the direction upwind from the point of reference, alternatively the direction from which the wind is coming. Leeward is the direction downwind from the point of reference. The leeward region of mountains generally remains dry as compared to the windward. The side of a ship that is towards the leeward is its lee side. If the vessel is heeling under the pressure of the wind, this will be the "lower side". During the age of sail, the term weather was used as a synonym for windward in some contexts, as in the weather gage.

Eddy (fluid dynamics) The swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime

In fluid dynamics, an eddy is the swirling of a fluid and the reverse current created when the fluid is in a turbulent flow regime. The moving fluid creates a space devoid of downstream-flowing fluid on the downstream side of the object. Fluid behind the obstacle flows into the void creating a swirl of fluid on each edge of the obstacle, followed by a short reverse flow of fluid behind the obstacle flowing upstream, toward the back of the obstacle. This phenomenon is naturally observed behind large emergent rocks in swift-flowing rivers.

Horizontal component

Weather fronts

Weather fronts are boundaries between two masses of air of different densities, or different temperature and moisture properties, which normally are convergence zones in the wind field and are the principal cause of significant weather. Within surface weather analyses, they are depicted using various colored lines and symbols. The air masses usually differ in temperature and may also differ in humidity. Wind shear in the horizontal occurs near these boundaries. Cold fronts feature narrow bands of thunderstorms and severe weather, and may be preceded by squall lines and dry lines. Cold fronts are sharper surface boundaries with more significant horizontal wind shear than warm fronts. When a front becomes stationary, it can degenerate into a line which separates regions of differing wind speed, known as a shear line, though the wind direction across the front normally remains constant. In the tropics, tropical waves move from east to west across the Atlantic and eastern Pacific basins. Directional and speed shear can occur across the axis of stronger tropical waves, as northerly winds precede the wave axis and southeast winds are seen behind the wave axis. Horizontal wind shear can also occur along local land breeze and sea breeze boundaries. [10]

Near coastlines

Wind shear along the coast with low level clouds moving towards the east and higher level clouds moving towards the south-west

The magnitude of winds offshore are nearly double the wind speed observed onshore. This is attributed to the differences in friction between land masses and offshore waters. Sometimes, there are even directional differences, particularly if local sea breezes change the wind on shore during daylight hours. [11]

Vertical component

Thermal wind

Thermal wind is a meteorological term not referring to an actual wind, but a difference in the geostrophic wind between two pressure levels p1 and p0, with p1 < p0; in essence, wind shear. It is only present in an atmosphere with horizontal changes in temperature (or in an ocean with horizontal gradients of density), i.e. baroclinicity. In a barotropic atmosphere, where temperature is uniform, the geostrophic wind is independent of height. The name stems from the fact that this wind flows around areas of low (and high) temperature in the same manner as the geostrophic wind flows around areas of low (and high) pressure. [12]

The thermal wind equation is

where the φ are geopotential height fields with φ1 > φ0, f is the Coriolis parameter, and k is the upward-pointing unit vector in the vertical direction. The thermal wind equation does not determine the wind in the tropics. Since f is small or zero, such as near the equator, the equation reduces to stating that ∇(φ1φ0) is small. [12]

This equation basically describes the existence of the jet stream, a westerly current of air with maximum wind speeds close to the tropopause which is (even though other factors are also important) the result of the temperature contrast between equator and pole.

Effects on tropical cyclones

Strong wind shear in the high troposphere forms the anvil-shaped top of this mature cumulonimbus cloud, or thunderstorm. Thunderhead.anvil.jpg
Strong wind shear in the high troposphere forms the anvil-shaped top of this mature cumulonimbus cloud, or thunderstorm.

Tropical cyclones are basically heat engines that are fueled by the temperature gradient between the warm tropical ocean surface and the colder upper atmosphere. Tropical cyclone development requires relatively low values of vertical wind shear so that their warm core can remain above their surface circulation center, thereby promoting intensification. Vertical wind shear tears up the "machinery" of the heat engine causing it to break down. Strongly sheared tropical cyclones weaken as the upper circulation is blown away from the low level center.

The vertical wind shear in a tropical cyclone's environment is very important. When the wind shear is weak, the storms that are part of the cyclone grow vertically, and the latent heat from condensation is released into the air directly above the storm, aiding in development. When there is stronger wind shear, this means that the storms become more slanted and the latent heat release is dispersed over a much larger area [14] [15]

Effects on thunderstorms and severe weather

Severe thunderstorms, which can spawn tornadoes and hailstorms, require wind shear to organize the storm in such a way as to maintain the thunderstorm for a longer period of time. This occurs as the storm's inflow becomes separated from its rain-cooled outflow. An increasing nocturnal, or overnight, low level jet can increase the severe weather potential by increasing the vertical wind shear through the troposphere. Thunderstorms in an atmosphere with virtually no vertical wind shear weaken as soon as they send out an outflow boundary in all directions, which then quickly cuts off its inflow of relatively warm, moist air and kills the thunderstorm. [16]

Planetary boundary layer

Depiction of where the planetary boundary layer lies on a sunny day PBLimage.jpg
Depiction of where the planetary boundary layer lies on a sunny day

The atmospheric effect of surface friction with winds aloft force surface winds to slow and back counterclockwise near the surface of the Earth blowing inward across isobars (lines of equal pressure), when compared to the winds in frictionless flow well above the Earth's surface. [17] This layer where friction slows and changes the wind is known as the planetary boundary layer, sometimes the Ekman layer, and it is thickest during the day and thinnest at night. Daytime heating thickens the boundary layer as winds at the surface become increasingly mixed with winds aloft due to insolation, or solar heating. Radiative cooling overnight further enhances wind decoupling between the winds at the surface and the winds above the boundary layer by calming the surface wind which increases wind shear. These wind changes force wind shear between the boundary layer and the wind aloft, and is most emphasized at night.

Effects on flight

Gliding
Glider ground launch affected by wind shear. FAA-8083-13 Fig 7-20.PNG
Glider ground launch affected by wind shear.

In gliding, wind gradients just above the surface affect the takeoff and landing phases of flight of a glider. Wind gradient can have a noticeable effect on ground launches, also known as winch launches or wire launches. If the wind gradient is significant or sudden, or both, and the pilot maintains the same pitch attitude, the indicated airspeed will increase, possibly exceeding the maximum ground launch tow speed. The pilot must adjust the airspeed to deal with the effect of the gradient. [18]

When landing, wind shear is also a hazard, particularly when the winds are strong. As the glider descends through the wind gradient on final approach to landing, airspeed decreases while sink rate increases, and there is insufficient time to accelerate prior to ground contact. The pilot must anticipate the wind gradient and use a higher approach speed to compensate for it. [19]

Wind shear is also a hazard for aircraft making steep turns near the ground. It is a particular problem for gliders which have a relatively long wingspan, which exposes them to a greater wind speed difference for a given bank angle. The different airspeed experienced by each wing tip can result in an aerodynamic stall on one wing, causing a loss of control accident. [19] [20]

Parachuting

Wind shear or wind gradients are a threat to parachutists, particularly to BASE jumping and wingsuit flying. Skydivers have been pushed off of their course by sudden shifts in wind direction and speed, and have collided with bridges, cliffsides, trees, other skydivers, the ground, and other obstacles.[ citation needed ] Skydivers routinely make adjustments to the position of their open canopies to compensate for changes in direction while making landings to prevent accidents such as canopy collisions and canopy inversion.

Soaring

Soaring related to wind shear, also called dynamic soaring, is a technique used by soaring birds like albatrosses, who can maintain flight without wing flapping. If the wind shear is of sufficient magnitude, a bird can climb into the wind gradient, trading ground speed for height, while maintaining airspeed. [21] By then turning downwind, and diving through the wind gradient, they can also gain energy. [22] It has also been used by glider pilots on rare occasions.

Wind shear can also create wave. This occurs when an atmospheric inversion separates two layers with a marked difference in wind direction. If the wind encounters distortions in the inversion layer caused by thermals coming up from below, it will create significant shear waves that can be used for soaring. [23]

Impact on passenger aircraft
Effect of wind shear on aircraft trajectory. Note how merely correcting for the initial gust front can have dire consequences. Windshearaircraftnasa.gif
Effect of wind shear on aircraft trajectory. Note how merely correcting for the initial gust front can have dire consequences.

Strong outflow from thunderstorms causes rapid changes in the three-dimensional wind velocity just above ground level. Initially, this outflow causes a headwind that increases airspeed, which normally causes a pilot to reduce engine power if they are unaware of the wind shear. As the aircraft passes into the region of the downdraft, the localized headwind diminishes, reducing the aircraft's airspeed and increasing its sink rate. Then, when the aircraft passes through the other side of the downdraft, the headwind becomes a tailwind, reducing lift generated by the wings, and leaving the aircraft in a low-power, low-speed descent. This can lead to an accident if the aircraft is too low to effect a recovery before ground contact.

Wreckage of Delta Air Lines Flight 191 tail section after a microburst slammed the aircraft into the ground. Another aircraft can be seen flying in the background past the crash scene. Delta 191 wreckage.jpg
Wreckage of Delta Air Lines Flight 191 tail section after a microburst slammed the aircraft into the ground. Another aircraft can be seen flying in the background past the crash scene.

As the result of the accidents in the 1970s and 1980s, most notably following the 1985 crash of Delta Air Lines Flight 191, in 1988 the U.S. Federal Aviation Administration mandated that all commercial aircraft have on-board wind shear detection systems by 1993. Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries. [24] Since 1995, the number of major civil aircraft accidents caused by wind shear has dropped to approximately one every ten years, due to the mandated on-board detection as well as the addition of Doppler weather radar units on the ground (NEXRAD).[ citation needed ] The installation of high-resolution Terminal Doppler Weather Radar stations at many U.S. airports that are commonly affected by wind shear has further aided the ability of pilots and ground controllers to avoid wind shear conditions. [25]

Sailing

Wind shear affects sailboats in motion by presenting a different wind speed and direction at different heights along the mast. The effect of low level wind shear can be factored into the selection of sail twist in the sail design, but this can be difficult to predict since wind shear may vary widely in different weather conditions. Sailors may also adjust the trim of the sail to account for low level wind shear, for example using a boom vang. [26]

Sound propagation

Wind shear can have a pronounced effect upon sound propagation in the lower atmosphere, where waves can be "bent" by refraction phenomenon. The audibility of sounds from distant sources, such as thunder or gunshots, is very dependent on the amount of shear. The result of these differing sound levels is key in noise pollution considerations, for example from roadway noise and aircraft noise, and must be considered in the design of noise barriers. [27] This phenomenon was first applied to the field of noise pollution study in the 1960s, contributing to the design of urban highways as well as noise barriers. [28]

Hodograph plot of wind vectors at various heights in the troposphere. Meteorologists can use this plot to evaluate vertical wind shear in weather forecasting. (Source: NOAA) Hodographe NOAA.PNG
Hodograph plot of wind vectors at various heights in the troposphere. Meteorologists can use this plot to evaluate vertical wind shear in weather forecasting. (Source: NOAA)

The speed of sound varies with temperature. Since temperature and sound velocity normally decrease with increasing altitude, sound is refracted upward, away from listeners on the ground, creating an acoustic shadow at some distance from the source. [29] In the 1862, during the American Civil War Battle of Iuka, an acoustic shadow, believed to have been enhanced by a northeast wind, kept two divisions of Union soldiers out of the battle, [30] because they could not hear the sounds of battle only six miles downwind. [31]

Effects on architecture

Wind engineering is a field of engineering devoted to the analysis of wind effects on the natural and built environment. It includes strong winds which may cause discomfort as well as extreme winds such as tornadoes, hurricanes and storms which may cause widespread destruction. Wind engineering draws upon meteorology, aerodynamics and a number of specialist engineering disciplines. The tools used include climate models, atmospheric boundary layer wind tunnels and numerical models. It involves, among other topics, how wind impacting buildings must be accounted for in engineering. [32]

Wind turbines are affected by wind shear. Vertical wind-speed profiles result in different wind speeds at the blades nearest to the ground level compared to those at the top of blade travel, and this in turn affects the turbine operation. [33] This low level wind shear can create a large bending moment in the shaft of a two bladed turbine when the blades are vertical. [34] The reduced wind shear over water means shorter and less expensive wind turbine towers can be used in shallow seas. [35]

See also

Related Research Articles

Inversion (meteorology) deviation from the normal change of an atmospheric property with altitude

In meteorology, an inversion, also known as a temperature inversion, is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to an inversion of the thermal lapse rate. Normally, air temperature decreases with an increase in altitude. During an inversion, warmer air is held above cooler air; the normal temperature profile with altitude is inverted.

Surface weather analysis

Surface weather analysis is a special type of weather map that provides a view of weather elements over a geographical area at a specified time based on information from ground-based weather stations.

Squall sudden, sharp increase in the sustained winds over a short time interval

A squall is a sudden, sharp increase in wind speed lasting minutes, contrary to a wind gust lasting seconds. They are usually associated with active weather, such as rain showers, thunderstorms, or heavy snow. Squalls refer to the increase to the sustained winds over that time interval, as there may be higher gusts during a squall event. They usually occur in a region of strong sinking air or cooling in the mid-atmosphere. These force strong localized upward motions at the leading edge of the region of cooling, which then enhances local downward motions just in its wake.

Dynamic soaring is a flying technique used to gain energy by repeatedly crossing the boundary between air masses of different velocity. Such zones of wind gradient are generally found close to obstacles and close to the surface, so the technique is mainly of use to birds and operators of radio-controlled gliders, but glider pilots are sometimes able to soar dynamically in meteorological wind shears at higher altitudes.

The synoptic scale in meteorology is a horizontal length scale of the order of 1000 kilometers or more. This corresponds to a horizontal scale typical of mid-latitude depressions. Most high and low-pressure areas seen on weather maps such as surface weather analyses are synoptic-scale systems, driven by the location of Rossby waves in their respective hemisphere. Low-pressure areas and their related frontal zones occur on the leading edge of a trough within the Rossby wave pattern, while high-pressure areas form on the back edge of the trough. Most precipitation areas occur near frontal zones. The word synoptic is derived from the Greek word συνοπτικός, meaning seen together.

Downburst

A downburst is a strong ground-level wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the point of contact at ground level. Often producing damaging winds, it may be confused with a tornado, where high-velocity winds circle a central area, and air moves inward and upward; by contrast, in a downburst, winds are directed downward and then outward from the surface landing point.

Planetary boundary layer The lowest part of the atmosphere directly influenced by contact with the planetary surface

In meteorology the planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL) or peplosphere, is the lowest part of the atmosphere and its behaviour is directly influenced by its contact with a planetary surface. On Earth it usually responds to changes in surface radiative forcing in an hour or less. In this layer physical quantities such as flow velocity, temperature, and moisture display rapid fluctuations (turbulence) and vertical mixing is strong. Above the PBL is the "free atmosphere", where the wind is approximately geostrophic, while within the PBL the wind is affected by surface drag and turns across the isobars.

Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere. 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.

Thermal wind

The thermal wind is the variation in strength of wind with height due to, on one hand, a balance between the Coriolis and pressure-gradient forces in the atmosphere and, on the other hand, horizontal temperature gradients. It is the primary physical mechanism for the jet stream and plays an important role in other large-scale atmospheric phenomena. The thermal wind ensures the jet stream is typically strongest in the upper half of the troposphere, which is the atmospheric layer extending from the surface of the planet up to a height of 12 km to 15 km.

Outflow boundary

An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air (outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually a drop in temperature and a related pressure jump. Outflow boundaries can persist for 24 hours or more after the thunderstorms that generated them dissipate, and can travel hundreds of kilometers from their area of origin. New thunderstorms often develop along outflow boundaries, especially near the point of intersection with another boundary. Outflow boundaries can be seen either as fine lines on weather radar imagery or else as arcs of low clouds on weather satellite imagery. From the ground, outflow boundaries can be co-located with the appearance of roll clouds and shelf clouds.

Clear-air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual clues, such as clouds, and is caused when bodies of air moving at widely different speeds meet.

Weather front boundary separating two masses of air of different densities

A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena outside the tropics. In surface weather analyses, fronts are depicted using various colored triangles and half-circles, depending on the type of front. The air masses separated by a front usually differ in temperature and humidity.

Berg wind hot, dry foehn-type wind in South Africa

Berg wind is the South African name for a katabatic wind: a hot dry wind blowing down the Great Escarpment from the high central plateau to the coast.

Air-mass thunderstorm

An air-mass thunderstorm, also called an "ordinary", "single cell", or "garden variety" thunderstorm, is a thunderstorm that is generally weak and usually not severe. These storms form in environments where at least some amount of Convective Available Potential Energy (CAPE) is present, but very low levels of wind shear and helicity. The lifting source, which is a crucial factor in thunderstorm development, is usually the result of uneven heating of the surface, though they can be induced by weather fronts and other low-level boundaries associated with wind convergence. The energy needed for these storms to form comes in the form of insolation, or solar radiation. Air-mass thunderstorms do not move quickly, last no longer than an hour, and have the threats of lightning, as well as showery light, moderate, or heavy rainfall. Heavy rainfall can interfere with microwave transmissions within the atmosphere.

Atmospheric convection

Atmospheric convection is the result of a parcel-environment instability, or temperature difference layer in the atmosphere. Different lapse rates within dry and moist air masses lead to instability. Mixing of air during the day which expands the height of the planetary boundary layer leads to increased winds, cumulus cloud development, and decreased surface dew points. Moist convection leads to thunderstorm development, which is often responsible for severe weather throughout the world. Special threats from thunderstorms include hail, downbursts, and tornadoes.

The following is a glossary of tornado terms. It includes scientific as well as selected informal terminology.

Glossary of meteorology Wikimedia list article

This glossary of meteorology is a list of terms and concepts relevant to meteorology and the atmospheric sciences, their sub-disciplines, and related fields.

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