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It became the basis of a modern international system that divides clouds into five physical forms which can be further divided or classified into altitude levels to derive ten basic genera. The main representative cloud types for each of these forms are stratiform, cumuliform, stratocumuliform, cumulonimbiform, and cirriform. | https://en.wikipedia.org/wiki/Low_cloud |
Low-level clouds do not have any altitude-related prefixes. However mid-level stratiform and stratocumuliform types are given the prefix alto- while high-level variants of these same two forms carry the prefix cirro-. In both cases, strato- is dropped from the latter form to avoid double-prefixing. | https://en.wikipedia.org/wiki/Low_cloud |
Genus types with sufficient vertical extent to occupy more than one level do not carry any altitude-related prefixes. They are classified formally as low- or mid-level depending on the altitude at which each initially forms, and are also more informally characterized as multi-level or vertical. Most of the ten genera derived by this method of classification can be subdivided into species and further subdivided into varieties. | https://en.wikipedia.org/wiki/Low_cloud |
Very low stratiform clouds that extend down to the Earth's surface are given the common names fog and mist, but have no Latin names. In the stratosphere and mesosphere, clouds have common names for their main types. They may have the appearance of stratiform veils or sheets, cirriform wisps, or stratocumuliform bands or ripples. | https://en.wikipedia.org/wiki/Low_cloud |
They are seen infrequently, mostly in the polar regions of Earth. Clouds have been observed in the atmospheres of other planets and moons in the Solar System and beyond. However, due to their different temperature characteristics, they are often composed of other substances such as methane, ammonia, and sulfuric acid, as well as water. | https://en.wikipedia.org/wiki/Low_cloud |
Tropospheric clouds can have a direct effect on climate change on Earth. They may reflect incoming rays from the sun which can contribute to a cooling effect where and when these clouds occur, or trap longer wave radiation that reflects back up from the Earth's surface which can cause a warming effect. The altitude, form, and thickness of the clouds are the main factors that affect the local heating or cooling of the Earth and the atmosphere. Clouds that form above the troposphere are too scarce and too thin to have any influence on climate change. Clouds are the main uncertainty in climate sensitivity. | https://en.wikipedia.org/wiki/Low_cloud |
In meteorology, a corona (plural coronae) is an optical phenomenon produced by the diffraction of sunlight or moonlight (or, occasionally, bright starlight or planetlight) by individual small water droplets and sometimes tiny ice crystals of a cloud or on a foggy glass surface. In its full form, a corona consists of several concentric, pastel-colored rings around the celestial object and a central bright area called aureole. The aureole is often (especially in case of the Moon) the only visible part of the corona and has the appearance of a bluish-white disk which fades to reddish-brown towards the edge. | https://en.wikipedia.org/wiki/Corona_(optical_phenomenon) |
The angular diameter of a corona depends on the sizes of the water droplets involved; smaller droplets produce larger coronae. For the same reason, the corona is the most pronounced when the size of the droplets is most uniform. Coronae differ from halos in that the latter are formed by refraction (rather than diffraction) from comparatively large rather than small ice crystals. The diffraction pattern is called an Airy disk. | https://en.wikipedia.org/wiki/Corona_(optical_phenomenon) |
In meteorology, a cyclone () is a large air mass that rotates around a strong center of low atmospheric pressure, counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere as viewed from above (opposite to an anticyclone). Cyclones are characterized by inward-spiraling winds that rotate about a zone of low pressure. The largest low-pressure systems are polar vortices and extratropical cyclones of the largest scale (the synoptic scale). Warm-core cyclones such as tropical cyclones and subtropical cyclones also lie within the synoptic scale. | https://en.wikipedia.org/wiki/Cyclone |
Mesocyclones, tornadoes, and dust devils lie within the smaller mesoscale. Upper level cyclones can exist without the presence of a surface low, and can pinch off from the base of the tropical upper tropospheric trough during the summer months in the Northern Hemisphere. | https://en.wikipedia.org/wiki/Cyclone |
Cyclones have also been seen on extraterrestrial planets, such as Mars, Jupiter, and Neptune. Cyclogenesis is the process of cyclone formation and intensification. Extratropical cyclones begin as waves in large regions of enhanced mid-latitude temperature contrasts called baroclinic zones. | https://en.wikipedia.org/wiki/Cyclone |
These zones contract and form weather fronts as the cyclonic circulation closes and intensifies. Later in their life cycle, extratropical cyclones occlude as cold air masses undercut the warmer air and become cold core systems. A cyclone's track is guided over the course of its 2 to 6 day life cycle by the steering flow of the subtropical jet stream. | https://en.wikipedia.org/wiki/Cyclone |
Weather fronts mark the boundary between two masses of air of different temperature, humidity, and densities, and are associated with the most prominent meteorological phenomena. Strong cold fronts typically feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Such fronts form west of the circulation center and generally move from west to east; warm fronts form east of the cyclone center and are usually preceded by stratiform precipitation and fog. | https://en.wikipedia.org/wiki/Cyclone |
Warm fronts move poleward ahead of the cyclone path. Occluded fronts form late in the cyclone life cycle near the center of the cyclone and often wrap around the storm center. Tropical cyclogenesis describes the process of development of tropical cyclones. | https://en.wikipedia.org/wiki/Cyclone |
Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm core. Cyclones can transition between extratropical, subtropical, and tropical phases. Mesocyclones form as warm core cyclones over land, and can lead to tornado formation. | https://en.wikipedia.org/wiki/Cyclone |
Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear. In the Atlantic and the northeastern Pacific oceans, a tropical cyclone is generally referred to as a hurricane (from the name of the ancient Central American deity of wind, Huracan), in the Indian and south Pacific oceans it is called a cyclone, and in the northwestern Pacific it is called a typhoon. The growth of instability in the vortices is not universal. For example, the size, intensity, moist-convection, surface evaporation, the value of potential temperature at each potential height can affect the nonlinear evolution of a vortex. | https://en.wikipedia.org/wiki/Cyclone |
In meteorology, a downburst is a strong downward and outward gushing wind system that emanates from a point source above and blows radially, that is, in straight lines in all directions from the area of impact at surface level. Capable of producing damaging winds, it may sometimes be confused with a tornado, where high-velocity winds circle a central area, and air moves inward and upward. These usually last for seconds to minutes. Downbursts are particularly strong downdrafts within thunderstorms (or deep, moist convection as sometimes downbursts emanate from cumulonimbus or even cumulus congestus clouds that are not producing lightning). | https://en.wikipedia.org/wiki/Straight-line_wind |
Downbursts are most often created by an area of significantly precipitation-cooled air that, after reaching the surface (subsiding), spreads out in all directions producing strong winds. Dry downbursts are associated with thunderstorms that exhibit very little rain, while wet downbursts are created by thunderstorms with significant amounts of precipitation. Microbursts and macrobursts are downbursts at very small and larger scales, respectively. | https://en.wikipedia.org/wiki/Straight-line_wind |
A rare variety of dry downburst, the heat burst, is created by vertical currents on the backside of old outflow boundaries and squall lines where rainfall is lacking. Heat bursts generate significantly higher temperatures due to the lack of rain-cooled air in their formation and compressional heating during descent. Downbursts create vertical wind shear, which is dangerous to aviation, especially during landing (or takeoff). | https://en.wikipedia.org/wiki/Straight-line_wind |
Several fatal and historic crashes in past decades are attributed to the phenomenon and flight crew training goes to great lengths on how to properly recognize and recover from a downburst/wind shear event; wind shear recovery, among other adverse weather events, are standard topics across the world in flight simulator training that flight crews receive and must successfully complete. Detection and nowcasting technology was also implemented in much of the world and particularly around major airports, which in many cases actually have wind shear detection equipment on the field. This detection equipment helps air traffic controllers and pilots make decisions on the safety and feasibility of operating on or in the vicinity of the airport during storms. | https://en.wikipedia.org/wiki/Straight-line_wind |
In meteorology, a heat burst is a rare atmospheric phenomenon characterized by a sudden, localized increase in air temperature near the Earth's surface. Heat bursts typically occur during night-time and are associated with decaying thunderstorms. They are also characterized by extremely dry air and are sometimes associated with very strong, even damaging, winds. Although the phenomenon is not fully understood, the event is thought to occur when rain evaporates (virga) into a parcel of cold, dry air high in the atmosphere, making the air denser than its surroundings. The parcel descends rapidly, warming due to compression, overshoots its equilibrium level, and reaches the surface, similar to a downburst.Recorded temperatures during heat bursts have reached well above 40 °C (104 °F), sometimes rising by 10 °C (18 °F) or more within only a few minutes. | https://en.wikipedia.org/wiki/Heat_burst |
In meteorology, a low-pressure area, low area or low is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure areas are commonly associated with inclement weather (such as cloudy, windy, with possible rain or storms), while high-pressure areas are associated with lighter winds and clear skies. Winds circle anti-clockwise around lows in the northern hemisphere, and clockwise in the southern hemisphere, due to opposing Coriolis forces. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the atmosphere (aloft). | https://en.wikipedia.org/wiki/Low_pressure_system |
The formation process of a low-pressure area is known as cyclogenesis. In meteorology, atmospheric divergence aloft occurs in two kinds of places: The first is in the area 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 is an area where wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength.Diverging winds aloft, ahead of these troughs, cause atmospheric lift within the troposphere below as air flows upwards away from the surface, which lowers surface pressures as this upward motion partially counteracts the force of gravity packing the air close to the ground. | https://en.wikipedia.org/wiki/Low_pressure_system |
Thermal lows form due to localized heating caused by greater solar incidence 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. | https://en.wikipedia.org/wiki/Low_pressure_system |
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. | https://en.wikipedia.org/wiki/Low_pressure_system |
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. | https://en.wikipedia.org/wiki/Low_pressure_system |
Atmospheric lift will also generally produce cloud cover through adiabatic cooling once the air temperature drops below the dew point as it rises, the cloudy skies typical of low-pressure areas act to dampen diurnal temperature extremes. Since clouds reflect sunlight, incoming shortwave solar radiation decreases, which causes lower temperatures during the day. At night the absorptive effect of clouds on outgoing longwave radiation, such as heat energy from the surface, allows for warmer night-time minimums in all seasons. | https://en.wikipedia.org/wiki/Low_pressure_system |
The stronger the area of low pressure, the stronger the winds experienced in its vicinity. Globally, low-pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky mountains. In Europe (particularly in the British Isles and Netherlands), recurring low-pressure weather systems are typically known as "low levels". | https://en.wikipedia.org/wiki/Low_pressure_system |
In meteorology, a precipitationshed is the upwind ocean and land surface that contributes evaporation to a given, downwind location's precipitation. The concept has been described as an "atmospheric watershed". The concept itself rests on a broad foundation of scholarly work examining the evaporative sources of rainfall. Since its formal definition, the precipitationshed has become an element in water security studies, examinations of sustainability, and mentioned as a potentially useful tool for examining vulnerability of rainfall dependent ecosystems. | https://en.wikipedia.org/wiki/Precipitationshed |
In meteorology, a pseudo-cold front is a boundary between a supercell's inflow region and the rear flank downdraft. It usually extends outward from a mesocyclone center, usually toward the south or southwest, and is characterized by advancing of the downdraft air toward the inflow region. It is a particular form of gust front. | https://en.wikipedia.org/wiki/Pseudo-cold_front |
In meteorology, absolute angular momentum is the angular momentum in an 'absolute' coordinate system (absolute time and space). | https://en.wikipedia.org/wiki/Absolute_angular_momentum |
In meteorology, air currents are concentrated areas of winds. They are mainly due to differences in atmospheric pressure or temperature. They are divided into horizontal and vertical currents; both are present at mesoscale while horizontal ones dominate at synoptic scale. Air currents are not only found in the troposphere, but extend to the stratosphere and mesosphere. | https://en.wikipedia.org/wiki/Air_current |
In meteorology, an air mass is a volume of air defined by its temperature and humidity. Air masses cover many hundreds or thousands of square miles, and adapt to the characteristics of the surface below them. They are classified according to latitude and their continental or maritime source regions. Colder air masses are termed polar or arctic, while warmer air masses are deemed tropical. | https://en.wikipedia.org/wiki/Air_stream |
Continental and superior air masses are dry, while maritime and monsoon air masses are moist. Weather fronts separate air masses with different density (temperature or moisture) characteristics. Once an air mass moves away from its source region, underlying vegetation and water bodies can quickly modify its character. Classification schemes tackle an air mass's characteristics, as well as modification. | https://en.wikipedia.org/wiki/Air_stream |
In meteorology, an anemometer (from Ancient Greek άνεμος (ánemos) 'wind', and μέτρον (métron) 'measure') is a device that measures wind speed and direction. It is a common instrument used in weather stations. The earliest known description of an anemometer was by Italian architect and author Leon Battista Alberti (1404–1472) in 1450. | https://en.wikipedia.org/wiki/Hot-wire_anemometry |
In meteorology, an inversion is a deviation from the normal change of an atmospheric property with altitude. It almost always refers to an inversion of the air temperature lapse rate, in which case it is called a temperature inversion. Normally, air temperature decreases with an increase in altitude, but during an inversion warmer air is held above cooler air.An inversion traps air pollution, such as smog, close to the ground. | https://en.wikipedia.org/wiki/Atmospheric_inversion |
An inversion can also suppress convection by acting as a "cap". If this cap is broken for any of several reasons, convection of any moisture present can then erupt into violent thunderstorms. Temperature inversion can notoriously result in freezing rain in cold climates. | https://en.wikipedia.org/wiki/Atmospheric_inversion |
In meteorology, an occluded front is a type of weather front formed during cyclogenesis. The classical and usual view of an occluded front is that it starts when a cold front overtakes a warm front near a cyclone, such that the warm air is separated (occluded) from the cyclone center at the surface. The point where the warm front becomes the occluded front is the triple point; a new area of low-pressure that develops at this point is called a triple-point low. A more modern view of the formation process suggests that occluded fronts form directly without the influence of other fronts during the wrap-up of the baroclinic zone during cyclogenesis, and then lengthen due to flow deformation and rotation around the cyclone as the cyclone forms. | https://en.wikipedia.org/wiki/Occluded_front |
In meteorology, an okta is a unit of measurement used to describe the amount of cloud cover at any given location such as a weather station. Sky conditions are estimated in terms of how many eighths of the sky are covered in cloud, ranging from 0 oktas (completely clear sky) through to 8 oktas (completely overcast). In addition, in the SYNOP code there is an extra cloud cover indicator '9' indicating that the sky is totally obscured (i.e. hidden from view), usually due to dense fog or heavy snow.When used in weather charts, okta measurements are shown by means of graphic symbols (rather than numerals) contained within weather circles, to which are attached further symbols indicating other measured data such as wind speed and wind direction.Although relatively straightforward to measure (visually, for instance, by using a mirror), oktas only estimate cloud cover in terms of the area of the sky covered by clouds. They do not account for cloud type or thickness, and this limits their use for estimating cloud albedo or surface solar radiation receipt. | https://en.wikipedia.org/wiki/Okta |
Cloud oktas can also be measured using satellite imagery from geostationary satellites equipped with high-resolution image sensors such as Himawari-8. Similar to traditional approaches, satellite images do not account for cloud composition. Oktas are often referenced in aviation weather forecasts and low level forecasts: SKC = Sky clear (0 oktas); FEW = Few (1 to 2 oktas); SCT = Scattered (3 to 4 oktas); BKN = Broken (5 to 7 oktas); OVC = Overcast (8 oktas); NSC = nil significant cloud; CAVOK = ceiling and visibility okay. | https://en.wikipedia.org/wiki/Okta |
In meteorology, an undular bore is a wave disturbance in the Earth's atmosphere and can be seen through unique cloud formations. They normally occur within an area of the atmosphere which is stable in the low levels after an outflow boundary or a cold front moves through. In hydraulics, an undular bore is a gentle bore with an undular hydraulic jump pattern at the downstream (subcritical) side. | https://en.wikipedia.org/wiki/Undular_bore |
In meteorology, an updraft is a small-scale current of rising air, often within a cloud. | https://en.wikipedia.org/wiki/Vertical_draft |
In meteorology, 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. The atmospheric region most susceptible to CAT is the high troposphere at altitudes of around 7,000–12,000 metres (23,000–39,000 ft) as it meets the tropopause. Here CAT is most frequently encountered in the regions of jet streams. | https://en.wikipedia.org/wiki/Clear-air_turbulence |
At lower altitudes it may also occur near mountain ranges. Thin cirrus clouds can also indicate high probability of CAT. CAT can be hazardous to the comfort, and occasionally the safety, of air travelers. | https://en.wikipedia.org/wiki/Clear-air_turbulence |
In meteorology, 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.In aviation, CAT is defined as "the detection by aircraft of high-altitude inflight bumps in patchy regions devoid of significant cloudiness or nearby thunderstorm activity". It was first noted in the 1940s. | https://en.wikipedia.org/wiki/Clear_air_turbulence |
In meteorology, cloud forcing, cloud radiative forcing (CRF) or cloud radiative effect (CRE) is the difference between the radiation budget components for average cloud conditions and cloud-free conditions. Much of the interest in cloud forcing relates to its role as a feedback process in the present period of global warming. | https://en.wikipedia.org/wiki/Cloud_forcing |
In meteorology, convective available potential energy (commonly abbreviated as CAPE), is the integrated amount of work that the upward (positive) buoyancy force would perform on a given mass of air (called an air parcel) if it rose vertically through the entire atmosphere. Positive CAPE will cause the air parcel to rise, while negative CAPE will cause the air parcel to sink. Nonzero CAPE is an indicator of atmospheric instability in any given atmospheric sounding, a necessary condition for the development of cumulus and cumulonimbus clouds with attendant severe weather hazards. | https://en.wikipedia.org/wiki/Available_potential_energy |
In meteorology, convective instability or stability of an air mass refers to its ability to resist vertical motion. A stable atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear. In an unstable atmosphere, vertical air movements (such as in orographic lifting, where an air mass is displaced upwards as it is blown by wind up the rising slope of a mountain range) tend to become larger, resulting in turbulent airflow and convective activity. Instability can lead to significant turbulence, extensive vertical clouds, and severe weather such as thunderstorms. | https://en.wikipedia.org/wiki/Potential_instability |
In meteorology, diurnal temperature variation is the variation between a high air temperature and a low temperature that occurs during the same day. | https://en.wikipedia.org/wiki/Diurnal_temperature_variation |
In meteorology, eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones, generally with winds greater than 185 km/h (115 mph), or major hurricanes (Category 3 or above). When tropical cyclones reach this intensity, and the eyewall contracts or is already small, some of the outer rainbands may strengthen and organize into a ring of thunderstorms—a new, outer eyewall—that slowly moves inward and robs the original, inner eyewall of its needed moisture and angular momentum. Since the strongest winds are in a tropical cyclone's eyewall, the storm usually weakens during this phase, as the inner wall is "choked" by the outer wall. Eventually the outer eyewall replaces the inner one completely, and the storm may re-intensify.The discovery of this process was partially responsible for the end of the U.S. | https://en.wikipedia.org/wiki/Eyewall_replacement_cycle |
government's hurricane modification experiment Project Stormfury. This project set out to seed clouds outside the eyewall, apparently causing a new eyewall to form and weakening the storm. | https://en.wikipedia.org/wiki/Eyewall_replacement_cycle |
When it was discovered that this was a natural process due to hurricane dynamics, the project was quickly abandoned.Almost every intense hurricane undergoes at least one of these cycles during its existence. Recent studies have shown that nearly half of all tropical cyclones, and nearly all cyclones with sustained winds over 204 kilometres per hour (127 mph; 110 kn), undergo eyewall replacement cycles. Hurricane Allen in 1980 went through repeated eyewall replacement cycles, fluctuating between Category 5 and Category 4 status on the Saffir-Simpson Hurricane Scale several times. Typhoon June (1975) was the first reported case of triple eyewalls, and Hurricane Juliette and Iris (2001) were documented cases of such. | https://en.wikipedia.org/wiki/Eyewall_replacement_cycle |
In meteorology, geopotential heights are used when creating forecasts and analyzing pressure systems. Geopotential heights represent the estimate of the real height of a pressure system above the average sea level. Geopotential heights for weather are divided up into several levels. The lowest geopotential height level is 850 hPa (25.10 inHg), which represents the lowest 1,500 m (5,000 ft) of the atmosphere. | https://en.wikipedia.org/wiki/Cold_wake |
The moisture content, gained by using either the relative humidity or the precipitable water value, is used in creating forecasts for precipitation. The next level, 700 hPa (20.67 inHg), is at a height of 2,300–3,200 m (7,700–10,500 ft); 700 hPa is regarded as the highest point in the lower atmosphere. At this layer, both vertical movement and moisture levels are used to locate and create forecasts for precipitation. | https://en.wikipedia.org/wiki/Cold_wake |
The middle level of the atmosphere is at 500 hPa (14.76 inHg) or a height of 4,900–6,100 m (16,000–20,000 ft). The 500 hPa level is used for measuring atmospheric vorticity, commonly known as the spin of air. The relative humidity is also analyzed at this height in order to establish where precipitation is likely to materialize. | https://en.wikipedia.org/wiki/Cold_wake |
The next level occurs at 300 hPa (8.859 inHg) or a height of 8,200–9,800 m (27,000–32,000 ft). The top-most level is located at 200 hPa (5.906 inHg), which corresponds to a height of 11,000–12,000 m (35,000–41,000 ft). Both the 200 and 300 hPa levels are mainly used to locate the jet stream. | https://en.wikipedia.org/wiki/Cold_wake |
In meteorology, haar or sea fret is a cold sea fog. It occurs most often on the east coast of Great Britain between April and September, when warm air passes over the cold North Sea. The term is also known as harr, hare, harl, har and hoar. | https://en.wikipedia.org/wiki/Haar_(fog) |
In meteorology, helicity corresponds to the transfer of vorticity from the environment to an air parcel in convective motion. Here the definition of helicity is simplified to only use the horizontal component of wind and vorticity, and to only integrate in the vertical direction, replacing the volume integral with a one-dimensional definite integral or line integral: H = ∫ Z 1 Z 2 V → h ⋅ ζ → h d Z = ∫ Z 1 Z 2 V → h ⋅ ∇ × V → h d Z , {\displaystyle H=\int _{Z_{1}}^{Z_{2}}{{\vec {V}}_{h}}\cdot {\vec {\zeta }}_{h}\,d{Z}=\int _{Z_{1}}^{Z_{2}}{{\vec {V}}_{h}}\cdot \nabla \times {\vec {V}}_{h}\,d{Z},} where Z {\displaystyle Z} is the altitude, V → h {\displaystyle {\vec {V}}_{h}} is the horizontal velocity, ζ → h {\displaystyle {\vec {\zeta }}_{h}} is the horizontal vorticity.According to this formula, if the horizontal wind does not change direction with altitude, H will be zero as V h {\displaystyle V_{h}} and ∇ × V h {\displaystyle \nabla \times V_{h}} are perpendicular, making their scalar product nil. H is then positive if the wind veers (turns clockwise) with altitude and negative if it backs (turns counterclockwise). This helicity used in meteorology has energy units per units of mass and thus is interpreted as a measure of energy transfer by the wind shear with altitude, including directional. | https://en.wikipedia.org/wiki/Helicity_(fluid_mechanics) |
This notion is used to predict the possibility of tornadic development in a thundercloud. In this case, the vertical integration will be limited below cloud tops (generally 3 km or 10,000 feet) and the horizontal wind will be calculated to wind relative to the storm in subtracting its motion: S R H = ∫ Z 1 Z 2 ( V → h − C → ) ⋅ ∇ × V → h d Z {\displaystyle \mathrm {SRH} =\int _{Z_{1}}^{Z_{2}}{\left({\vec {V}}_{h}-{\vec {C}}\right)}\cdot \nabla \times {\vec {V}}_{h}\,d{Z}} where C → {\displaystyle {\vec {C}}} is the cloud motion relative to the ground. Critical values of SRH (Storm Relative Helicity) for tornadic development, as researched in North America, are: SRH = 150-299 ... supercells possible with weak tornadoes according to Fujita scale SRH = 300-499 ... very favourable to supercells development and strong tornadoes SRH > 450 ... violent tornadoes When calculated only below 1 km (4,000 feet), the cut-off value is 100.Helicity in itself is not the only component of severe thunderstorms, and these values are to be taken with caution. | https://en.wikipedia.org/wiki/Helicity_(fluid_mechanics) |
That is why the Energy Helicity Index (EHI) has been created. It is the result of SRH multiplied by the CAPE (Convective Available Potential Energy) and then divided by a threshold CAPE: E H I = C A P E × S R H 160,000 {\displaystyle \mathrm {EHI} ={\frac {\mathrm {CAPE} \times \mathrm {SRH} }{\text{160,000}}}} This incorporates not only the helicity but the energy of the air parcel and thus tries to eliminate weak potential for thunderstorms even in strong SRH regions. The critical values of EHI: EHI = 1 ... possible tornadoes EHI = 1-2 ... moderate to strong tornadoes EHI > 2 ... strong tornadoes | https://en.wikipedia.org/wiki/Helicity_(fluid_mechanics) |
In meteorology, hodographs are used to plot winds from soundings of the Earth's atmosphere. It is a polar diagram where wind direction is indicated by the angle from the center axis and its strength by the distance from the center. In the figure to the right, at the bottom one finds values of wind at 4 heights above ground. They are plotted by the vectors V → 0 {\displaystyle {\vec {V}}_{0}} to V → 4 {\displaystyle {\vec {V}}_{4}} . | https://en.wikipedia.org/wiki/Hodograph |
One has to notice that direction are plotted as mentioned in the upper right corner. With the hodograph and thermodynamic diagrams like the tephigram, meteorologists can calculate: Wind shear: The lines uniting the extremities of successive vectors represent the variation in direction and value of the wind in a layer of the atmosphere. Wind shear is important information in the development of thunderstorms and future evolution of wind at these levels. | https://en.wikipedia.org/wiki/Hodograph |
Turbulence: wind shear indicate the possible turbulence that would cause a hazard to aviation. Temperature advection: change of temperature in a layer of air can be calculated by the direction of the wind at that level and the direction of the wind shear with the next level. In the northern hemisphere, warm air is to the right of a wind shear between levels in the atmosphere. The opposite is true in the southern one (see thermal wind). So in the example hodograph, the wind V → 3 {\displaystyle {\vec {V}}_{3}} from southwest meet the right side of the wind shear which means a warm advection and thus warming of the air at that level. | https://en.wikipedia.org/wiki/Hodograph |
In meteorology, hypotheses are often formulated using weather data up to the present and tested against future weather data, which ensures that, even subconsciously, future data could not influence the formulation of the hypothesis. Of course, such a discipline necessitates waiting for new data to come in, to show the formulated theory's predictive power versus the null hypothesis. This process ensures that no one can accuse the researcher of hand-tailoring the predictive model to the data on hand, since the upcoming weather is not yet available. | https://en.wikipedia.org/wiki/Data-snooping_bias |
As another example, suppose that observers note that a particular town appears to have a cancer cluster, but lack a firm hypothesis of why this is so. However, they have access to a large amount of demographic data about the town and surrounding area, containing measurements for the area of hundreds or thousands of different variables, mostly uncorrelated. Even if all these variables are independent of the cancer incidence rate, it is highly likely that at least one variable correlates significantly with the cancer rate across the area. While this may suggest a hypothesis, further testing using the same variables but with data from a different location is needed to confirm. Note that a p-value of 0.01 suggests that 1% of the time a result at least that extreme would be obtained by chance; if hundreds or thousands of hypotheses (with mutually relatively uncorrelated independent variables) are tested, then one is likely to obtain a p-value less than 0.01 for many null hypotheses. | https://en.wikipedia.org/wiki/Data-snooping_bias |
In meteorology, isopycnals are used to display different layers of gases in the atmosphere. In the atmosphere, varying degrees of humidity, temperature, and pressure change the density of air. Isopycnals are not used in meteorology as frequently as they are in oceanography, since the density gradients observed in the atmosphere are typically gradual, unlike in stratified bodies of water. In these cases, isopycnals are less relevant, as they do not display any substantial features. | https://en.wikipedia.org/wiki/Diapycnal_mixing |
In meteorology, latent heat flux is the flux of energy from the Earth's surface to the atmosphere that is associated with evaporation or transpiration of water at the surface and subsequent condensation of water vapor in the troposphere. It is an important component of Earth's surface energy budget. Latent heat flux has been commonly measured with the Bowen ratio technique, or more recently since the mid-1900s by the eddy covariance method. | https://en.wikipedia.org/wiki/Latent_heat |
In meteorology, lee waves are atmospheric stationary waves. The most common form is mountain waves, which are atmospheric internal gravity waves. These were discovered in 1933 by two German glider pilots, Hans Deutschmann and Wolf Hirth, above the Krkonoše. They are periodic changes of atmospheric pressure, temperature and orthometric height in a current of air caused by vertical displacement, for example orographic lift when the wind blows over a mountain or mountain range. | https://en.wikipedia.org/wiki/Lee_wave |
They can also be caused by the surface wind blowing over an escarpment or plateau, or even by upper winds deflected over a thermal updraft or cloud street. The vertical motion forces periodic changes in speed and direction of the air within this air current. | https://en.wikipedia.org/wiki/Lee_wave |
They always occur in groups on the lee side of the terrain that triggers them. Sometimes, mountain waves can help to enhance precipitation amounts downwind of mountain ranges. Usually a turbulent vortex, with its axis of rotation parallel to the mountain range, is generated around the first trough; this is called a rotor. The strongest lee waves are produced when the lapse rate shows a stable layer above the obstruction, with an unstable layer above and below.Strong winds (with wind gusts over 100 mph ) can be created in the foothills of large mountain ranges by mountain waves. These strong winds can contribute to unexpected wildfire growth and spread (including the 2016 Great Smoky Mountains wildfires when sparks from a wildfire in the Smoky Mountains were blown into the Gatlinburg and Pigeon Forge areas). | https://en.wikipedia.org/wiki/Lee_wave |
In meteorology, mixing ratio usually refers to the mass ratio of water ζ {\displaystyle \zeta } , which is defined as the mass of water m H 2 O {\displaystyle m_{\mathrm {H2O} }} divided by the mass of dry air ( m a i r − m H 2 O {\displaystyle m_{\mathrm {air} }-m_{\mathrm {H2O} }} ) in a given air parcel: ζ = m H 2 O m a i r − m H 2 O {\displaystyle \zeta ={\frac {m_{\mathrm {H2O} }}{m_{\mathrm {air} }-m_{\mathrm {H2O} }}}} The unit is typically given in g k g − 1 {\displaystyle \mathrm {g} \,\mathrm {kg} ^{-1}} . The definition is similar to that of specific humidity. | https://en.wikipedia.org/wiki/Mixing_ratio |
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor (reaching 100% relative humidity), so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation but colloids, because the water vapor does not condense sufficiently to precipitate. | https://en.wikipedia.org/wiki/Convectional_Precipitation |
Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called showers.Moisture that is lifted or otherwise forced to rise over a layer of sub-freezing air at the surface may be condensed into clouds and rain. | https://en.wikipedia.org/wiki/Convectional_Precipitation |
This process is typically active when freezing rain occurs. A stationary front is often present near the area of freezing rain and serves as the focus for forcing and rising air. Provided there is necessary and sufficient atmospheric moisture content, the moisture within the rising air will condense into clouds, namely nimbostratus and cumulonimbus if significant precipitation is involved. | https://en.wikipedia.org/wiki/Convectional_Precipitation |
Eventually, the cloud droplets will grow large enough to form raindrops and descend toward the Earth where they will freeze on contact with exposed objects. Where relatively warm water bodies are present, for example due to water evaporation from lakes, lake-effect snowfall becomes a concern downwind of the warm lakes within the cold cyclonic flow around the backside of extratropical cyclones. Lake-effect snowfall can be locally heavy. | https://en.wikipedia.org/wiki/Convectional_Precipitation |
Thundersnow is possible within a cyclone's comma head and within lake effect precipitation bands. In mountainous areas, heavy precipitation is possible where upslope flow is maximized within windward sides of the terrain at elevation. On the leeward side of mountains, desert climates can exist due to the dry air caused by compressional heating. | https://en.wikipedia.org/wiki/Convectional_Precipitation |
Most precipitation occurs within the tropics and is caused by convection. The movement of the monsoon trough, or intertropical convergence zone, brings rainy seasons to savannah regions. Precipitation is a major component of the water cycle, and is responsible for depositing fresh water on the planet. | https://en.wikipedia.org/wiki/Convectional_Precipitation |
Approximately 505,000 cubic kilometres (121,000 cu mi) of water falls as precipitation each year: 398,000 cubic kilometres (95,000 cu mi) over oceans and 107,000 cubic kilometres (26,000 cu mi) over land. Given the Earth's surface area, that means the globally averaged annual precipitation is 990 millimetres (39 in), but over land it is only 715 millimetres (28.1 in). | https://en.wikipedia.org/wiki/Convectional_Precipitation |
Climate classification systems such as the Köppen climate classification system use average annual rainfall to help differentiate between differing climate regimes. Global warming is already causing changes to weather, increasing precipitation in some geographies, and reducing it in others, resulting in additional extreme weather.Precipitation may occur on other celestial bodies. Saturn's largest satellite, Titan, hosts methane precipitation as a slow-falling drizzle, which has been observed as Rain puddles at its equator and polar regions. | https://en.wikipedia.org/wiki/Convectional_Precipitation |
In meteorology, prevailing wind in a region of the Earth's surface is a surface wind that blows predominantly from a particular direction. The dominant winds are the trends in direction of wind with the highest speed over a particular point on the Earth's surface at any given time. A region's prevailing and dominant winds are the result of global patterns of movement in the Earth's atmosphere. In general, winds are predominantly easterly at low latitudes globally. | https://en.wikipedia.org/wiki/Prevailing_wind |
In the mid-latitudes, westerly winds are dominant, and their strength is largely determined by the polar cyclone. In areas where winds tend to be light, the sea breeze/land breeze cycle is the most important cause of the prevailing wind; in areas which have variable terrain, mountain and valley breezes dominate the wind pattern. Highly elevated surfaces can induce a thermal low, which then augments the environmental wind flow. | https://en.wikipedia.org/wiki/Prevailing_wind |
Wind roses are tools used to display the direction of the prevailing wind. Knowledge of the prevailing wind allows the development of prevention strategies for wind erosion of agricultural land, such as across the Great Plains. Sand dunes can orient themselves perpendicular to the prevailing wind direction in coastal and desert locations. | https://en.wikipedia.org/wiki/Prevailing_wind |
Insects drift along with the prevailing wind, but the flight of birds is less dependent on it. Prevailing winds in mountain locations can lead to significant rainfall gradients, ranging from wet across windward-facing slopes to desert-like conditions along their lee slopes. Prevailing winds can vary due to the uneven heating of the Earth. | https://en.wikipedia.org/wiki/Prevailing_wind |
In meteorology, rapid intensification is a situation where a tropical cyclone intensifies dramatically in a short period of time. The United States National Hurricane Center defines rapid intensification as an increase in the maximum sustained winds of a tropical cyclone of at least 30 knots (35 mph; 55 km/h) in a 24-hour period. | https://en.wikipedia.org/wiki/Rapid_intensification |
In meteorology, station models are symbolic illustrations showing the weather occurring at a given reporting station. Meteorologists created the station model to fit a number of weather elements into a small space on weather maps. This allows map users to analyze patterns in atmospheric pressure, temperature, wind speed and direction, cloud cover, precipitation, and other parameters. The most common station plots depict surface weather observations although upper air plots at various mandatory levels are also frequently depicted. Station model plots use an internationally accepted coding convention that has changed little since August 1, 1941. Elements in the plot show the key weather elements, including temperature, dew point, wind, cloud cover, air pressure, pressure tendency, and precipitation. | https://en.wikipedia.org/wiki/Wind_barb |
In meteorology, the 'therefore' sign is used to indicate 'moderate rain' on a station model; the similar typographic symbol asterism (⁂, three asterisks) indicates moderate snow. | https://en.wikipedia.org/wiki/Thus_symbol |
In meteorology, the Coandă effect theory has also been applied to some air streams flowing out of mountain ranges such as the Carpathian Mountains and Transylvanian Alps, where effects on agriculture and vegetation have been noted. It also appears to be an effect in the Rhone Valley in France and near Big Delta in Alaska. | https://en.wikipedia.org/wiki/Coandă_Effect |
In meteorology, the different types of precipitation often include the character, formation, or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is generally more intense, and of shorter duration, than stratiform precipitation. | https://en.wikipedia.org/wiki/Convective_precipitation |
Orographic precipitation occurs when moist air is forced upwards over rising terrain and condenses on the slope, such as a mountain. Precipitation can fall in either liquid or solid phases, is mixed with both, or transition between them at the freezing level. Liquid forms of precipitation include rain and drizzle and dew. | https://en.wikipedia.org/wiki/Convective_precipitation |
Rain or drizzle which freezes on contact with a surface within a subfreezing air mass gains the preceding adjective "freezing", becoming the known freezing rain or freezing drizzle. Slush is a mixture of both liquid and solid precipitation. Frozen forms of precipitation include snow, ice crystals, ice pellets (sleet), hail, and graupel. Their respective intensities are classified either by rate of precipitation, or by visibility restriction. | https://en.wikipedia.org/wiki/Convective_precipitation |
In meteorology, the equilibrium level (EL), or level of neutral buoyancy (LNB), or limit of convection (LOC), is the height at which a rising parcel of air is at the same temperature as its environment. This means that unstable air is now stable when it reaches the equilibrium level and convection stops. This level is often near the tropopause and can be indicated as near where the anvil of a thunderstorm because it is where the thunderstorm updraft is finally cut off, except in the case of overshooting tops where it continues rising to the maximum parcel level (MPL) due to momentum. More precisely, the cumulonimbus will stop rising around a few kilometres prior to reaching the level of neutral buoyancy and on average anvil glaciation occurs at a higher altitude over land than over sea (despite little difference in LNB from land to sea). | https://en.wikipedia.org/wiki/Equilibrium_level |
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 (parallel to the isobars), while within the PBL the wind is affected by surface drag and turns across the isobars (see Ekman layer for more detail). | https://en.wikipedia.org/wiki/Planetary_boundary_layer |
In meteorology, the polar front is the weather front boundary between the polar cell and the Ferrel cell around the 60° latitude, near the polar regions, in both hemispheres. At this boundary a sharp gradient in temperature occurs between these two air masses, each at very different temperatures.The polar front arises as a result of cold polar air meeting warm tropical air. It is a stationary front as the air masses are not moving against each other and stays stable. | https://en.wikipedia.org/wiki/Polar_front |
Off the coast of eastern North America, especially in winter, there is a sharp temperature gradient between the snow-covered land and the warm offshore currents. The polar front theory says that mid-latitude extratropical cyclones form on boundaries between warm and cold air. In winter, the polar front shifts towards the Equator, whereas high pressure systems dominate more in the summer. | https://en.wikipedia.org/wiki/Polar_front |
In meteorology, the polar highs are areas of high atmospheric pressure, sometimes similar to anticyclones, around the north and south poles; the south polar high being the stronger one because land gains and loses heat more effectively than sea, which the north has much less of. The cold temperatures in the polar regions cause air to descend, creating the high pressure (a process called subsidence), just as the warm temperatures around the equator cause air to rise instead and create the low pressure intertropical convergence zone. Rising air also occurs along bands of low pressure situated just below the polar highs around the 50th parallel of latitude. These extratropical convergence zones are occupied by the polar fronts where air masses of polar origin meet and clash with those of tropical or subtropical origin in a stationary front. | https://en.wikipedia.org/wiki/Polar_High |
This convergence of rising air completes the vertical cycle around the polar cell in each latitudinal hemisphere's polar region. Closely related to this concept is the polar vortex, a rotating low-pressure circle of cold air around the poles. Surface temperatures under the polar highs are one of the coldest on Earth, with no month having an average temperature above freezing. Regions under the polar high also experience very low levels of precipitation, which leads them to be known as "polar deserts". Air flows outwards from the poles to create the polar easterlies in the arctic and antarctic areas. | https://en.wikipedia.org/wiki/Polar_High |
In meteorology, the synoptic scale (also called the large scale or cyclonic scale) is a horizontal length scale of the order of 1,000 km (620 mi) or more. This corresponds to a horizontal scale typical of mid-latitude depressions (e.g. extratropical cyclones). 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. | https://en.wikipedia.org/wiki/Synoptic_scale |
Most precipitation areas occur near frontal zones. The word synoptic is derived from the Ancient Greek word συνοπτικός (sunoptikós), meaning "seen together". | https://en.wikipedia.org/wiki/Synoptic_scale |
The Navier–Stokes equations applied to atmospheric motion can be simplified by scale analysis in the synoptic scale. It can be shown that the main terms in horizontal equations are Coriolis force and pressure gradient terms; therefore, one can use geostrophic approximation. In vertical coordinates, the momentum equation simplifies to the hydrostatic equilibrium equation. | https://en.wikipedia.org/wiki/Synoptic_scale |
In meteorology, the term vapor pressure means the partial pressure of water vapor in the atmosphere, even if it is not in equilibrium. This differs from its meaning in other sciences. According to the American Meteorological Society Glossary of Meteorology, saturation vapor pressure properly refers to the equilibrium vapor pressure of water above a flat surface of liquid water or solid ice, and is a function only of temperature and whether the condensed phase is liquid or solid.Relative humidity is defined relative to saturation vapor pressure.Equilibrium vapor pressure does not require the condensed phase to be a flat surface; it might consist of tiny droplets possibly containing solutes (impurities), such as a cloud. | https://en.wikipedia.org/wiki/Saturation_pressure |
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