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.
48-399: The atmospheric region most susceptible to CAT is the high troposphere at altitudes of around 7,000–12,000 m (23,000–39,000 ft ) as it meets the tropopause . Here CAT is most frequently encountered in the regions of jet streams . 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
96-451: A current of air caused by vertical displacement, for example orographic lift when the wind blows over a mountain or mountain range . 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. They always occur in groups on
144-460: A change in the wind speed. A stream of wind can change its direction by differences of pressure. CAT appears more frequently when the wind is surrounding a low pressure region, especially with sharp troughs that change the wind direction more than 100°. Extreme CAT has been reported without any other factor than this. Mountain waves are formed when four requirements are met. When these factors coincide with jet streams, CAT can occur: The tropopause
192-598: A hazard for large aircraft; the phenomenon is believed responsible for many aviation accidents and incidents , including the in-flight breakup of BOAC Flight 911 , a Boeing 707 , near Mount Fuji , Japan in 1966, and the in-flight separation of an engine on an Evergreen International Airlines Boeing 747 cargo jet near Anchorage, Alaska in 1993. The rising air of the wave, which allows gliders to climb to great heights, can also result in high-altitude upset in jet aircraft trying to maintain level cruising flight in lee waves . Rising, descending or turbulent air, in or above
240-709: A moon of Saturn, and Triton , a moon of Neptune, have atmospheres mainly of nitrogen . When in the part of its orbit closest to the Sun, Pluto has an atmosphere of nitrogen and methane similar to Triton's, but these gases are frozen when it is farther from the Sun. Other bodies within the Solar System have extremely thin atmospheres not in equilibrium. These include the Moon ( sodium gas), Mercury (sodium gas), Europa (oxygen), Io ( sulfur ), and Enceladus ( water vapor ). The first exoplanet whose atmospheric composition
288-508: A planet from atmospheric escape and that for some magnetizations the presence of a magnetic field works to increase the escape rate. Other mechanisms that can cause atmosphere depletion are solar wind -induced sputtering, impact erosion, weathering , and sequestration—sometimes referred to as "freezing out"—into the regolith and polar caps . Atmospheres have dramatic effects on the surfaces of rocky bodies. Objects that have no atmosphere, or that have only an exosphere, have terrain that
336-451: A variable amount of water vapor is also present, on average about 1% at sea level. The low temperatures and higher gravity of the Solar System's giant planets — Jupiter , Saturn , Uranus and Neptune —allow them more readily to retain gases with low molecular masses . These planets have hydrogen–helium atmospheres, with trace amounts of more complex compounds. Two satellites of the outer planets possess significant atmospheres. Titan ,
384-484: A vertical oscillation of the perturbed air parcels at the Brunt-Väisäla frequency , which for the atmosphere is: N = g θ 0 d θ 0 d z {\displaystyle N={\sqrt {{g \over \theta _{0}}{d\theta _{0} \over dz}}}} , where θ 0 ( z ) {\displaystyle \theta _{0}(z)}
432-464: A wide range of velocities, there will always be some fast enough to produce a slow leakage of gas into space. Lighter molecules move faster than heavier ones with the same thermal kinetic energy , and so gases of low molecular weight are lost more rapidly than those of high molecular weight. It is thought that Venus and Mars may have lost much of their water when, after being photodissociated into hydrogen and oxygen by solar ultraviolet radiation,
480-439: Is a layer of gases that envelop an astronomical object , held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere is the outer region of a star, which includes the layers above the opaque photosphere ; stars of low temperature might have outer atmospheres containing compound molecules . The atmosphere of Earth
528-524: Is a layer which separates two very different types of air. Beneath it, the air gets colder and the wind gets faster with height. Above it, the air warms and wind velocity decreases with height. These changes in temperature and velocity can produce fluctuation in the altitude of the tropopause, called gravity waves . When a pilot experiences CAT, a number of rules should be applied: Because aircraft move so quickly, they can experience sudden unexpected accelerations or 'bumps' from turbulence, including CAT – as
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#1732797937175576-414: Is composed of nitrogen (78%), oxygen (21%), argon (0.9%), carbon dioxide (0.04%) and trace gases. Most organisms use oxygen for respiration ; lightning and bacteria perform nitrogen fixation which produces ammonia that is used to make nucleotides and amino acids ; plants , algae , and cyanobacteria use carbon dioxide for photosynthesis . The layered composition of the atmosphere minimises
624-423: Is covered in craters . Without an atmosphere, the planet has no protection from meteoroids , and all of them collide with the surface as meteorites and create craters. For planets with a significant atmosphere, most meteoroids burn up as meteors before hitting a planet's surface. When meteoroids do impact, the effects are often erased by the action of wind. Wind erosion is a significant factor in shaping
672-401: Is the lowest layer of the atmosphere. This extends from the planetary surface to the bottom of the stratosphere . The troposphere contains 75–80% of the mass of the atmosphere, and is the atmospheric layer wherein the weather occurs; the height of the troposphere varies between 17 km at the equator and 7.0 km at the poles. The stratosphere extends from the top of the troposphere to
720-445: Is the vertical profile of potential temperature . Oscillations tilted off the vertical axis at an angle of ϕ {\displaystyle \phi } will occur at a lower frequency of N cos ϕ {\displaystyle N\cos {\phi }} . These air parcel oscillations occur in concert, parallel to the wave fronts (lines of constant phase ). These wave fronts represent extrema in
768-462: The Coriolis force cause it to meander. Although the altitudes near the tropopause are usually cloudless, thin cirrus cloud can form where there are abrupt changes of air velocity, for example associated with jet streams. Lines of cirrus perpendicular to the jet stream indicate possible CAT, especially if the ends of the cirrus are dispersed, in which case the direction of dispersal can indicate if
816-542: 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 miles per hour (160 km/h)) can be created in
864-812: The tropopause in an unpowered glider using lee waves, making the transition into stratospheric standing waves. They did this for the first time on August 30, 2006 in Argentina , climbing to an altitude of 15,460 metres (50,720 ft). The Mountain Wave Project of the Organisation Scientifique et Technique du Vol à Voile focusses on analysis and classification of lee waves and associated rotors. The conditions favoring strong lee waves suitable for soaring are: The rotor turbulence may be harmful for other small aircraft such as balloons , hang gliders and paragliders . It can even be
912-669: The CAT is stronger at the left or at the right of the jet stream. A temperature gradient is the change of temperature over a distance in some given direction. Where the temperature of a gas changes, so does its density and where the density changes CAT can appear. From the ground upwards through the troposphere temperature decreases with height; from the tropopause upwards through the stratosphere temperature increases with height. Such variations are examples of temperature gradients. A horizontal temperature gradient may occur, and hence air density variations, where air velocity changes. An example:
960-406: The air thus presents both inertias and accelerations which cannot be determined in advance. Vertical wind shear above the jet stream (i.e., in the stratosphere) is sharper when it is moving upwards, because wind speed decreases with height in the stratosphere. This is the reason CAT can be generated above the tropopause, despite the stratosphere otherwise being a region which is vertically stable. On
1008-587: The aircraft rapidly cross invisible bodies of air which are moving vertically at many different speeds. Although the vast majority of cases of turbulence are harmless, in rare cases cabin crew and passengers on aircraft have been injured when tossed around inside an aircraft cabin during extreme turbulence. In a small number of cases, people have been killed and at least one aircraft disintegrated mid-air . Atmospheric An atmosphere (from Ancient Greek ἀτμός ( atmós ) 'vapour, steam' and σφαῖρα ( sphaîra ) 'sphere')
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#17327979371751056-516: The appearance of life and its evolution . Mountain wave 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 Giant Mountains . They are periodic changes of atmospheric pressure , temperature and orthometric height in
1104-438: The atmosphere can transport thermal energy from the higher temperature interior up to the surface. From the perspective of a planetary geologist , the atmosphere acts to shape a planetary surface. Wind picks up dust and other particles which, when they collide with the terrain, erode the relief and leave deposits ( eolian processes). Frost and precipitations , which depend on the atmospheric composition, also influence
1152-411: The base of the exosphere at 690 km and contains the ionosphere , where solar radiation ionizes the atmosphere. The density of the ionosphere is greater at short distances from the planetary surface in the daytime and decreases as the ionosphere rises at night-time, thereby allowing a greater range of radio frequencies to travel greater distances. The exosphere begins at 690 to 1,000 km from
1200-428: The bottom of the mesosphere , and contains the ozone layer , at an altitude between 15 km and 35 km. It is the atmospheric layer that absorbs most of the ultraviolet radiation that Earth receives from the Sun. The mesosphere ranges from 50 km to 85 km and is the layer wherein most meteors are incinerated before reaching the surface. The thermosphere extends from an altitude of 85 km to
1248-408: The comfort, and occasionally the safety, of air travelers , as the aircraft pilots often cannot see and anticipate such turbulences, and a sudden encounter can impart significant stress to the airframe . CAT in the jet stream is expected to become stronger and more frequent because of climate change , with transatlantic wintertime CAT increasing by 60% (light), 95% (moderate), and 150% (severe) by
1296-443: The core of a jet stream . Jet stream produces horizontal wind shear at its edges, caused by the different relative air speeds of the stream and the surrounding air. Wind shear, a difference in relative speed between two adjacent air masses, can produce vortices, and when of sufficient degree, the air will tend to move chaotically. A strong anticyclone vortex can also lead to CAT. Rossby waves caused by this jet stream shear and
1344-426: The diminishing mass of the gas above the point of barometric measurement. The units of air pressure are based upon the standard atmosphere (atm), which is 101,325 Pa (equivalent to 760 Torr or 14.696 psi ). The height at which the atmospheric pressure declines by a factor of e (an irrational number equal to 2.71828) is called the scale height ( H ). For an atmosphere of uniform temperature,
1392-431: The distance from the Sun determines the energy available to heat atmospheric gas to the point where some fraction of its molecules' thermal motion exceed the planet's escape velocity , allowing those to escape a planet's gravitational grasp. Thus, distant and cold Titan , Triton , and Pluto are able to retain their atmospheres despite their relatively low gravities. Since a collection of gas molecules may be moving at
1440-672: 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). Lee waves are a form of internal gravity waves produced when a stable, stratified flow is forced over an obstacle. This disturbance elevates air parcels above their level of neutral buoyancy . Buoyancy restoring forces therefore act to excite
1488-560: The harmful effects of sunlight , ultraviolet radiation, solar wind , and cosmic rays and thus protects the organisms from genetic damage. The current composition of the atmosphere of the Earth is the product of billions of years of biochemical modification of the paleoatmosphere by living organisms. Atmospheres are clouds of gas bound to and engulfing an astronomical focal point of sufficiently dominating mass , adding to its mass, possibly escaping from it or collapsing into it. Because of
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1536-442: The hydrogen escaped. Earth's magnetic field helps to prevent this, as, normally, the solar wind would greatly enhance the escape of hydrogen. However, over the past 3 billion years Earth may have lost gases through the magnetic polar regions due to auroral activity, including a net 2% of its atmospheric oxygen. The net effect, taking the most important escape processes into account, is that an intrinsic magnetic field does not protect
1584-418: The intensity and location cannot be determined precisely. However, because this turbulence affects long range aircraft that fly near the tropopause, CAT has been intensely studied. Several factors affect the likelihood of CAT. Often more than one factor is present. As of 1965 it had been noted that 64% of the non-light turbulences (not only CAT) were observed less than 150 nautical miles (280 km) away from
1632-431: The latter, such planetary nucleus can develop from interstellar molecular clouds or protoplanetary disks into rocky astronomical objects with varyingly thick atmospheres, gas giants or fusors . Composition and thickness is originally determined by the stellar nebula's chemistry and temperature, but can also by a product processes within the astronomical body outgasing a different atmosphere. The atmospheres of
1680-465: The obstruction that forms them. Lee waves provide a possibility for gliders to gain altitude or fly long distances when soaring . World record wave flight performances for speed, distance or altitude have been made in the lee of the Sierra Nevada , Alps , Patagonic Andes , and Southern Alps mountain ranges. The Perlan Project is working to demonstrate the viability of climbing above
1728-424: The other hand, vertical wind shear moving downwards within the stratosphere is more moderate (i.e., because downwards wind shear within the stratosphere is effectively moving against the manner in which wind speed changes within the stratosphere) and CAT is never produced in the stratosphere. Similar considerations apply to the troposphere but in reverse. When strong wind deviates, the change of wind direction implies
1776-406: The past. The circulation of the atmosphere occurs due to thermal differences when convection becomes a more efficient transporter of heat than thermal radiation . On planets where the primary heat source is solar radiation, excess heat in the tropics is transported to higher latitudes. When a planet generates a significant amount of heat internally, such as is the case for Jupiter , convection in
1824-414: The perturbed pressure field (i.e., lines of lowest and highest pressure), while the areas between wave fronts represent extrema in the perturbed buoyancy field (i.e., areas most rapidly gaining or losing buoyancy). Energy is transmitted along the wave fronts (parallel to air parcel oscillations), which is the direction of the wave group velocity . In contrast, the phase propagation (or phase speed ) of
1872-429: The planets Venus and Mars are principally composed of carbon dioxide and nitrogen , argon and oxygen . The composition of Earth's atmosphere is determined by the by-products of the life that it sustains. Dry air (mixture of gases) from Earth's atmosphere contains 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, and traces of hydrogen, helium, and other "noble" gases (by volume), but generally
1920-416: The relief. Climate changes can influence a planet's geological history. Conversely, studying the surface of the Earth leads to an understanding of the atmosphere and climate of other planets. For a meteorologist , the composition of the Earth's atmosphere is a factor affecting the climate and its variations. For a biologist or paleontologist , the Earth's atmospheric composition is closely dependent on
1968-468: The scale height is proportional to the atmospheric temperature and is inversely proportional to the product of the mean molecular mass of dry air, and the local acceleration of gravity at the point of barometric measurement. Surface gravity differs significantly among the planets. For example, the large gravitational force of the giant planet Jupiter retains light gases such as hydrogen and helium that escape from objects with lower gravity. Secondly,
Clear-air turbulence - Misplaced Pages Continue
2016-438: The speed of the jet stream is not constant along its length; additionally air temperature and hence density will vary between the air within the jet stream and the air outside. As is explained elsewhere in this article, temperature decreases and wind velocity increase with height in the troposphere, and the reverse is true within the stratosphere. These differences cause changes in air density, and hence viscosity. The viscosity of
2064-413: The surface, and extends to roughly 10,000 km, where it interacts with the magnetosphere of Earth. Atmospheric pressure is the force (per unit-area) perpendicular to a unit-area of planetary surface, as determined by the weight of the vertical column of atmospheric gases. In said atmospheric model, the atmospheric pressure , the weight of the mass of the gas, decreases at high altitude because of
2112-417: The terrain of rocky planets with atmospheres, and over time can erase the effects of both craters and volcanoes . In addition, since liquids cannot exist without pressure, an atmosphere allows liquid to be present at the surface, resulting in lakes , rivers and oceans . Earth and Titan are known to have liquids at their surface and terrain on the planet suggests that Mars had liquid on its surface in
2160-427: The time of CO 2 doubling . 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
2208-437: The waves points perpendicular to energy transmission (or group velocity ). Both lee waves and the rotor may be indicated by specific wave cloud formations if there is sufficient moisture in the atmosphere, and sufficient vertical displacement to cool the air to the dew point . Waves may also form in dry air without cloud markers. Wave clouds do not move downwind as clouds usually do, but remain fixed in position relative to
2256-461: Was determined is HD 209458b , a gas giant with a close orbit around a star in the constellation Pegasus . Its atmosphere is heated to temperatures over 1,000 K, and is steadily escaping into space. Hydrogen, oxygen, carbon and sulfur have been detected in the planet's inflated atmosphere. The atmosphere of Earth is composed of layers with different properties, such as specific gaseous composition, temperature, and pressure. The troposphere
2304-463: Was first noted in the 1940s. Clear-air turbulence is usually impossible to detect with the naked eye and very difficult to detect with a conventional radar , with the result that it is difficult for aircraft pilots to detect and avoid it. However, it can be remotely detected with instruments that can measure turbulence with optical techniques, such as scintillometers , Doppler LIDARs , or N-slit interferometers . At typical heights where it occurs,
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