Category:Weather hazards to aircraft
Pages in category "Weather hazards to aircraft"
The following 12 pages are in this category, out of 12 total. This list may not reflect recent changes (learn more).
The following 12 pages are in this category, out of 12 total. This list may not reflect recent changes (learn more).
1. 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 is present, 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, heavy rainfall can interfere with microwave transmissions within the atmosphere. Lightning characteristics are related to characteristics of the parent thunderstorm, on unusual occasions there could be a weak downburst and small hail. They are common in temperate zones during a summer afternoon, like all thunderstorms, the mean-layered wind field the storms form within determine motion. When the deep-layered wind flow is light, outflow boundary progression will determine storm movement, vertical wind shear is also a hazard near the base of thunderstorms which have generated outflow boundaries. The moisture rapidly cools into liquid drops of water due to the temperatures at high altitude. As the water condenses into liquid, latent heat is released which warms the air. The air tends to rise in an updraft through the process of convection and this creates a low-pressure zone beneath the forming thunderstorm, otherwise known as a cumulonimbus cloud. In a typical thunderstorm, approximately 5×108 kg of water vapor is lifted into the Earths atmosphere, waterspouts, small hail, and strong wind gusts can occur in association with these thunderstorms. Also known as single cell thunderstorms, these are the typical summer thunderstorms in many temperate locales and they also occur in the cool unstable air which often follows the passage of a cold front from the sea during winter. Within a cluster of thunderstorms, the term refers to each separate principal updraft. Thunderstorm cells occasionally form in isolation, as the occurrence of one thunderstorm can develop an outflow boundary which sets up new thunderstorm development, such storms are rarely severe and are a result of local atmospheric instability, hence the term air mass thunderstorm. When such storms have a period of severe weather associated with them. Pulse severe storms are poorly organized due to the vertical wind shear in the storms environment and occur randomly in time and space. Between formation and dissipation, single cell thunderstorms normally last 20–30 minutes, the two major ways thunderstorms move are via advection of the wind and propagation along outflow boundaries towards sources of greater heat and moisture. Many thunderstorms move with the wind speed through the Earths troposphere. Younger thunderstorms are steered by winds closer to the Earths surface than more mature thunderstorms as they not to be as tall
2. Clear-air turbulence – Clear-air turbulence 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 troposphere at altitudes of around 7. 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 the comfort, and rarely the safety, of air travelers. However, it can be detected with instruments that can measure turbulence with optical techniques, such as scintillometers, Doppler LIDARs. Although the altitudes near the tropopause are usually cloudless, thin cirrus cloud can form there are abrupt changes of air velocity. Detecting and predicting CAT is hard for meteorologists because it is at such heights that even when caused by factors that can be measured, intensity, 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. 64% of the non-light turbulences are observed less than 150 nautical miles away from the core of a jet stream, rossby waves caused by this jet stream shear and the Coriolis force cause it to meander. 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 temperature decreases 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, the speed of the jet stream is not constant along its length, additionally air temperature, Wind shear is a difference in relative speed between two adjacent air masses. An excessive wind shear produces vortices, and when the shear is of sufficient degree. As is explained elsewhere in this article, temperature decreases and wind velocity increase with height in the troposphere, and these differences cause changes in air density, and hence viscosity. The viscosity of the air thus presents both inertias and accelerations which cannot be determined in advance, vertical wind shear above the jet stream 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, on the other hand, vertical wind shear moving downwards within the stratosphere is more moderate 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 a change in the wind speed
3. Cloud suck – The vertical extent of a cumulus cloud is a good indicator of the strength of lift beneath it, and the potential for cloud suck. Cloud suck most commonly occurs in low pressure weather and in humid conditions, Cloud suck is typically associated with an increase in thermal updraft velocity near cloud base. As a parcel of air lifted in a thermal rises, it also cools, as the water vapour condenses, it releases its latent heat of vaporization, thereby increasing the buoyancy of the parcel. The updraft is amplified by this latent heat release, although the process that causes this amplification happens above cloud base height, the effect is often noticeable as much as 300 m below cloud base. In fact, it is this effect below cloud base, not the effect within the cloud, the telltale signs for a pilot climbing in the thermal under a sucking cloud are lift strengthening, lift getting smoother, and widening of the thermal. Paraglider pilots have reported being unable to descend in strong cloud suck, even bringing their canopies into deep spiral. She lost consciousness due to hypoxia, but regained consciousness after 30 minutes to an hour, chinese paraglider pilot He Zhongpin died after he was sucked into the same storm system and struck by lightning at 5900 m. His body was found the next day 15 km from his last known prior to entering the cloud. In 2014 Italian paraglider Paolo Antoniazzi,66 years old retired Army general, a sailplane also has the added benefit of the pilot being able to put the sailplane into a spin to descend rapidly without over speeding. Cloud suck is also a concern for powered aircraft but is not a lethal hazard. The USS Shenandoah, the first rigid airship built in the United States, at about 6,200 feet the ascent was checked, but the ship began to descend. When halfway to the ground it was hit by another updraft, ultimately the keel snapped, and the ship broke up while still more than a mile above the ground. Shenandoahs commanding officer and 13 other officers and men were killed, twenty-nine members of the crew survived the break-up, although some received serious injuries. Cumulonimbus and aviation How to Avoid Cloud Suck article on USHPA website, first published in Paragliding Magazine,2001
4. Downburst – A downburst is a strong ground-level wind system that emanates from a point source above and blows radially, that is, in a straight line in all directions, from the point of contact at ground level. Downbursts are created by an area of significantly rain-cooled air that, after reaching ground level, dry downbursts are associated with thunderstorms with very little rain, while wet downbursts are created by thunderstorms with high amounts of rainfall. Microbursts and macrobursts are downbursts at very small and larger scales respectively, another variety, 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, Downbursts create vertical wind shear or microbursts, which is dangerous to aviation. This is because the properties of a downburst are completely different from those of a tornado. To differentiate between tornado damage and damage from a downburst, the term straight-line winds is applied to damage from microbursts, Downbursts are particularly strong downdrafts from thunderstorms. Downbursts in air that is free or contains virga are known as dry downbursts. Most downbursts are less than 4 km in extent, these are called microbursts, Downbursts larger than 4 km in extent are sometimes called macrobursts. Downbursts can occur over large areas, straight-line winds are very strong winds that can produce damage, demonstrating a lack of a rotational damage pattern. Such rotational damage patterns are associated with cyclonic storms including tornadoes, straight-line winds are common with the gust front of a thunderstorm or originate with a downburst from a thunderstorm. These events can cause damage, even in the absence of a tornado. The winds can reach 130 km/h and can last for periods of twenty minutes, such straight-line wind events are most common during the spring when instability is highest and weather fronts routinely cross the country. Straight-line wind events in the form of derechos can take place in areas outside of the traditional tornado alley, straight-line winds may be damaging to marine interests. Small ships, cutters and sailboats are at risk from this meteorological phenomenon, the formation of a downburst starts with hail or large raindrops falling through drier air. Hailstones melt and raindrops evaporate, pulling latent heat from surrounding air, cooler air has a higher density than the warmer air around it, so it sinks to the ground. As the cold air hits the ground it out and a mesoscale front can be observed as a gust front. Areas under and immediately adjacent to the downburst are the areas which receive the highest winds and rainfall, also, because the rain-cooled air is descending from the middle troposphere, a significant drop in temperatures is noticed. Due to interaction with the ground, the downburst quickly loses strength as it fans out, Downbursts usually last only a few minutes and then dissipate, except in the case of squall lines and derecho events
5. Fog – Fog consists of visible cloud water droplets or ice crystals suspended in the air at or near the Earths surface. Fog can be considered a type of low-lying cloud and is influenced by nearby bodies of water, topography. In turn, fog has affected many human activities, such as shipping, travel, the term fog is typically distinguished from the more generic term cloud in that fog is low-lying, and the moisture in the fog is often generated locally. By definition, fog reduces visibility to less than 1 kilometre, for aviation purposes in the UK, a visibility of less than 5 kilometres but greater than 999 metres is considered to be mist if the relative humidity is 70% or greater, below 70%, haze is reported. Fog forms when the difference between air temperature and dew point is less than 2.5 °C or 4 °F, Fog begins to form when water vapor condenses into tiny liquid water droplets suspended in the air. Water vapor normally begins to condense on condensation nuclei such as dust, ice, Fog, like its elevated cousin stratus, is a stable cloud deck which tends to form when a cool, stable air mass is trapped underneath a warm air mass. Fog normally occurs at a relative humidity near 100% and this occurs from either added moisture in the air, or falling ambient air temperature. However, fog can form at lower humidities, and can fail to form with relative humidity at 100%. At 100% relative humidity, the air cannot hold additional moisture, thus, Fog can form suddenly and can dissipate just as rapidly. The sudden formation of fog is known as flash fog, Fog commonly produces precipitation in the form of drizzle or very light snow. Drizzle occurs when the humidity of fog attains 100% and the cloud droplets begin to coalesce into larger droplets. This can occur when the fog layer is lifted and cooled sufficiently, drizzle becomes freezing drizzle when the temperature at the surface drops below the freezing point. The inversion boundary varies its altitude primarily in response to the weight of the air above it, the marine layer, and any fogbank it may contain, will be squashed when the pressure is high, and conversely, may expand upwards when the pressure above it is lowering. Fog can form in a number of ways, depending on how the cooling that caused the condensation occurred, radiation fog is formed by the cooling of land after sunset by thermal radiation in calm conditions with clear sky. The warm ground produces condensation in the air by heat conduction. In perfect calm the fog layer can be less than a meter deep, radiation fogs occur at night, and usually do not last long after sunrise, but they can persist all day in the winter months especially in areas bounded by high ground. Radiation fog is most common in autumn and early winter, examples of this phenomenon include the Tule fog. Ground fog is fog that obscures less than 60% of the sky, advection fog occurs when moist air passes over a cool surface by advection and is cooled
6. Icing conditions – In aviation, icing conditions are those atmospheric conditions that can lead to the formation of water ice on the surfaces of an aircraft, or within the engine as carburetor icing. Inlet icing is another engine-related danger, often occurring in jet aircraft and these icing phenomena do not necessarily occur together. Many aircraft, especially general aviation aircraft, are not certified for flight into known icing—icing conditions certain or likely to exist, based on reports, observations. These parameters affect the extent and speed that characterize the formation of ice on an aircraft, federal Aviation Regulations contain a definition of icing conditions that some aircraft are certified to fly into. So-called SLD, or supercooled large droplet, conditions are those that exceed that specification, qualitatively, pilot reports indicate icing conditions in terms of their effect upon the aircraft, and will be dependent upon the capabilities of the aircraft. Different aircraft may report the same conditions as different levels of icing as a result. Clear ice is often clear and smooth, Supercooled water droplets, or freezing rain, strike a surface but do not freeze instantly. Often horns or protrusions are formed and project into the airflow, rime ice is rough and opaque, formed by supercooled drops rapidly freezing on impact. Forming mostly along an airfoils stagnation point, it conforms to the shape of the airfoil. Mixed ice is a combination of clear and rime ice, frost ice is the result of water freezing on unprotected surfaces while the aircraft is stationary. SLD ice refers to ice formed in Supercooled Large Droplet conditions and it is similar to clear ice, but because droplet size is large, it extends to unprotected parts of the aircraft and forms larger ice shapes, faster than normal icing conditions. This was a factor in the crash of American Eagle Flight 4184, the wing will ordinarily stall at a lower angle of attack, and thus a higher airspeed, when contaminated with ice. Even small amounts of ice will have an effect, and if the ice is rough, thus an increase in approach speed is advisable if ice remains on the wings. How much of an increase depends on both the type and amount of ice. Stall characteristics of an aircraft with ice contaminated wings will be degraded, the ice accretion may be asymmetric between the two wings. Also, the part of a wing, which is ordinarily thinner and thus a better collector of ice. Several methods exist to reduce the dangers of icing, the first, and simplest, is to avoid icing conditions altogether, but for many flights this is not practical. If ice are present on a prior to takeoff, they must be removed from critical surfaces
7. Microburst – A microburst is a small downdraft that moves in a way opposite to a tornado. Microbursts are found in strong thunderstorms, there are two types of microbursts within a thunderstorm, wet microbursts and dry microbursts. They go through three stages in their cycle, the downburst, outburst, and cushion stages, a microburst can be particularly dangerous to aircraft, especially during landing, due to the wind shear caused by its gust front. A microburst often has high winds that can knock over fully grown trees and they usually last from a couple of seconds to several minutes. Fujita also coined the term macroburst for downbursts larger than 4 km, a distinction can be made between a wet microburst which consists of precipitation and a dry microburst which typically consists of virga. Microbursts are recognized as capable of generating wind speeds higher than 75 m/s, Microbursts have also been called air bombs. When rain falls below the base or is mixed with dry air, it begins to evaporate. The cool air descends and accelerates as it approaches the ground, when the cool air approaches the ground, it spreads out in all directions and this divergence of the wind is the signature of the microburst. High winds spread out in type of pattern showing little or no curvature are known as straight-line winds. Wet microbursts are downbursts accompanied by significant precipitation at the surface which are warmer than their environment and these downbursts rely more on the drag of precipitation for downward acceleration of parcels than negative buoyancy which tend to drive dry microbursts. As a result, higher mixing ratios are necessary for these downbursts to form, melting of ice, particularly hail, appears to play an important role in downburst formation, especially in the lowest 1 km above ground level. These factors, among others, make forecasting wet microbursts a difficult task, the evolution of downbursts is broken down into three stages, the contact stage, the outburst stage, and the cushion stage. The virtual temperature correction usually is small and to a good approximation. In some storms this term has an effect on updrafts but there is not much reason to believe it has much of an impact on downdrafts. The second term is the effect of buoyancy on vertical motion, clearly, in the case of microbursts, one expects to find that B is negative meaning the parcel is cooler than its environment. This cooling typically takes place as a result of phase changes, precipitation particles that are small, but are in great quantity, promote a maximum contribution to cooling and, hence, to creation of negative buoyancy. The major contribution to this process is from evaporation, the last term is the effect of water loading. Whereas evaporation is promoted by large numbers of small droplets, it requires a few large drops to contribute substantially to the downward acceleration of air parcels
8. Thunderstorm – Thunderstorms occur in association with a type of cloud known as a cumulonimbus. They are usually accompanied by winds, heavy rain, and sometimes snow, sleet, hail, or, in contrast. Thunderstorms may line up in a series or become a rainband, strong or severe thunderstorms, known as supercells, rotate as do cyclones. Thunderstorms result from the upward movement of warm, moist air. As the warm, moist air moves upward, it cools, condenses, as the rising air reaches its dew point temperature, water vapor condenses into water droplets or ice, reducing pressure locally within the thunderstorm cell. Any precipitation falls the long distance through the clouds towards the Earths surface, as the droplets fall, they collide with other droplets and become larger. Thunderstorms can form and develop in any location but most frequently within the mid-latitude. Thunderstorms are responsible for the development and formation of many weather phenomena. Thunderstorms, and the phenomena that occur along them, pose great hazards. Damage that results from thunderstorms is mainly inflicted by downburst winds, large hailstones, stronger thunderstorm cells are capable of producing tornadoes and waterspouts. There are four types of thunderstorms, single-cell, multi-cell cluster, multi-cell lines, supercell thunderstorms are the strongest and the most associated with severe weather phenomena. Mesoscale convective systems formed by vertical wind shear within the tropics and subtropics can be responsible for the development of hurricanes. Dry thunderstorms, with no precipitation, can cause the outbreak of wildfires from the heat generated from the lightning that accompanies them. Several means are used to study thunderstorms, weather radar, weather stations, past civilizations held various myths concerning thunderstorms and their development as late as the 18th century. Beyond the Earths atmosphere, thunderstorms have also observed on the planets of Jupiter, Saturn, Neptune. Warm air has a lower density than air, so warmer air rises upwards. Clouds form as relatively warmer air, carrying moisture, rises within cooler air, the moist air rises, and, as it does so, it cools and some of the water vapor in that rising air condenses. If enough instability is present in the atmosphere, this process will continue long enough for cumulonimbus clouds to form and produce lightning, Meteorological indices such as convective available potential energy and the lifted index can be used to assist in determining potential upward vertical development of clouds
9. Wind gradient – It is the rate of increase of wind strength with unit increase in height above ground level. In metric units, it is measured in units of meters per second of speed, per kilometer of height. This layer, where friction slows the wind and changes the wind direction, is known as the planetary boundary layer. Daytime solar heating due to insolation thickens the boundary layer as winds at the surface become increasingly mixed with winds aloft. Radiative cooling overnight decouples the winds at the surface from the winds above the layer, increasing vertical wind shear near the surface. Typically, due to drag, there is a wind gradient in the wind flow just a few hundred meters above the Earths surface—the surface layer of the planetary boundary layer. Wind speed increases with increasing height above the ground, starting from zero due to the no-slip condition, flow near the surface encounters obstacles that reduce the wind speed, and introduce random vertical and horizontal velocity components at right angles to the main direction of flow. The reduction in velocity near the surface is a function of surface roughness, rough, irregular ground, and man-made obstructions on the ground, retard movement of the air near the surface, reducing wind velocity. Because of low surface roughness on the smooth water surface. Over a city or rough terrain, the wind gradient effect could cause a reduction of 40% to 50% of the wind speed aloft, while over open water or ice. For example, typical values for the predicted gradient height are 457 m for large cities,366 m for suburbs,274 m for open terrain, although the power law exponent approximation is convenient, it has no theoretical basis. The shearing of the wind is usually three-dimensional, that is and this is related to the Ekman spiral effect. After sundown the wind gradient near the surface increases, with the increasing stability, atmospheric stability occurring at night with radiative cooling tends to contain turbulent eddies vertically, increasing the wind gradient. In the convective layer, strong mixing diminishes vertical wind gradient. The design of buildings must account for wind loads, and these are affected by wind gradient, the respective gradient levels, usually assumed in the Building Codes, are 500 meters for cities,400 meters for suburbs, and 300 m for flat open terrain. Vertical wind-speed profiles result in different wind speeds at the blades nearest to the ground compared to those at the top of blade travel. The wind gradient can create a bending moment in the shaft of a two bladed turbine when the blades are vertical. The reduced wind gradient over water means shorter and less expensive wind turbine towers can be used in shallow seas and it would be preferable for wind turbines to be tested in a wind tunnel simulating the wind gradient that they will eventually see, but this is rarely done
10. Wind shear – Wind shear, sometimes referred to as windshear or 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 vertical or horizontal wind shear. Vertical wind shear is a change in speed or direction with change in altitude. Horizontal wind shear is a change in speed with change in lateral position for a given altitude. Wind shear has a significant effect during take-off and landing of aircraft due to its effects on control of the aircraft and this phenomenon is a concern for architects. Sound movement through the atmosphere is affected by shear, which can bend the wave front, causing sounds to be heard where they normally would not. The thermal wind concept explains how differences in speed at different heights are dependent on horizontal temperature differences. 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 for light aircraft, and near 45 knots for airliners at flight altitude. Vertical speed changes greater than 4.9 knots 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. Wind shear is also a key factor in the creation of severe thunderstorms, the additional hazard of turbulence is often associated with wind shear. Weather situations where shear is observed include, Weather fronts, significant shear is observed when the temperature difference across the front is 5 °C or more, and the front moves at 30 knots or faster. Because fronts are three-dimensional phenomena, frontal shear can be observed at any altitude between surface and tropopause, and therefore be seen both horizontally and vertically, vertical wind shear above warm fronts is more of an aviation concern than near and behind cold fronts due to their greater duration. Associated with upper level jet streams is a known as clear air turbulence. The CAT is strongest on the anticyclonic shear side of the jet, when a nocturnal low-level jet forms overnight above the Earths surface ahead of a cold front, significant low level vertical wind shear can develop near the lower portion of the low level jet. This is also known as wind shear since it is not due to nearby thunderstorms. When winds blow over a mountain, vertical shear is observed on the lee side, if the flow is strong enough, turbulent eddies known as rotors associated with lee waves may form, which are dangerous to ascending and descending aircraft. When on a clear and calm night, an inversion is formed near the ground. The change in wind can be 90 degrees in direction and 40 kt in speed, even a nocturnal low level jet can sometimes be observed