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Vertical draft
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In meteorology, an updraft (British English: up-draught) is a small-scale current of rising air, often within a cloud.[1]
Overview
[edit]Vertical drafts, known as updrafts or downdrafts, are localized regions of warm or cool air that move vertically. A mass of warm air will typically be less dense than the surrounding region, and so will rise until it reaches air that is either warmer or less dense than itself. The converse will occur for a mass of cool air, and is known as subsidence. This movement of large volumes of air, especially when regions of hot, wet air rise, can create large clouds, and is the central source of thunderstorms. Drafts can also be caused by low or high pressure regions. A low pressure region will attract air from the surrounding area, which will move towards the center and then rise, creating an updraft. A high pressure region will attract air from the surrounding area, which will move towards the center and sink, spawning a downdraft.
Updrafts and downdrafts, along with wind shear in general, are a major contributor to airplane crashes during takeoff and landing in a thunderstorm. Extreme cases, known as downbursts and microbursts, can be deadly and difficult to predict or observe. The crash of Delta Air Lines Flight 191 on its final approach before landing at Dallas/Fort Worth International Airport in 1985 was presumably caused by a microburst, and prompted the Federal Aviation Administration (FAA) to research and deploy new storm detection radar stations at some of the major airports, notably those in the South, Midwest, and Northeast United States where wind shear affects air safety. Downbursts can cause extensive localized damage, similar to that caused by tornadoes. Downburst damage can be differentiated from that of a tornado because the resulting destruction is circular and radiates away from the center. Tornado damage radiates inward, towards the center of the damage.
The term "downdraft" can also refer to a type of backdraft which occurs through chimneys which have fireplaces on the lowermost levels (such as basements) of multi-level buildings. It involves cold air coming down the chimney due to low air pressure, and makes it hard to light fires, and can push soot and carbon monoxide into domiciles.
See also
[edit]- Terminal Doppler Weather Radar, a 5 cm Doppler weather radar design to detect wind shear near major airports in the U.S. (FAA) and abroad
- NEXRAD, a 10 cm radar in the U.S. that detects wind shear, but to a specific extent. (NOAA's National Weather Service)
- Low level windshear alert system
- Atmospheric thermodynamics
- Lee waves
- New Zealand National Airways Corporation Flight 441 — airplane crash linked to severe downdraft
- Rear flank downdraft and forward flank downdraft
- Thermal
References
[edit]- ^ Park, Chris (2007). A Dictionary of Environment and Conservation (1 ed.). Oxford: Oxford University Press. doi:10.1093/acref/9780198609957.001.0001. ISBN 9780198609957. Retrieved 14 August 2014.
External links
[edit]Vertical draft
View on Grokipediafrom Grokipedia
In meteorology, a vertical draft is a small-scale current of rising or sinking air driven by buoyancy differences relative to the surrounding atmosphere. Rising vertical drafts, known as updrafts, are caused by warmer, less dense air parcels, while sinking ones, known as downdrafts, involve cooler, denser air.[1] These drafts typically occur in convective processes and, when the air is sufficiently moist, lead to the condensation of water vapor into cumulus clouds or towering cloud structures.[2]
Vertical drafts play a central role in atmospheric convection, serving as the primary mechanism for transporting heat, moisture, and momentum vertically through the atmosphere.[3] They initiate and sustain the development of various weather phenomena, including fair-weather cumulus clouds, multicell thunderstorms, and severe supercell storms.[4] In thunderstorms, updraft velocities can range from approximately 20 to 40 miles per hour (32 to 64 kilometers per hour) in weaker systems, escalating to over 100 miles per hour (160 kilometers per hour) in intense supercells capable of producing large hail and tornadoes.[5][4]
The strength and persistence of vertical drafts are influenced by environmental factors such as temperature gradients, humidity levels, and wind shear, which can tilt updrafts to separate them from downdrafts and prolong storm life cycles.[6] Strong updrafts elevate raindrops and ice particles into supercooled regions of the atmosphere, fostering hail growth through accretion and riming processes, with larger hailstones requiring correspondingly higher draft speeds.[5] In rotating supercells, persistent updrafts can organize into mesocyclones, contributing to the formation of tornadoes and other severe hazards.[7] Conversely, the eventual weakening of updrafts often triggers downdrafts—sinking columns of cooler air—that generate gust fronts, heavy rain, and damaging straight-line winds upon reaching the surface.[4]
Definition and Characteristics
Updrafts
An updraft is defined as a small-scale current of rising air in the atmosphere, driven by buoyancy resulting from air parcels that are warmer and thus less dense than the surrounding environment.[8] This buoyancy causes the parcel to accelerate upward, distinguishing updrafts from horizontal air movements.[9] Key characteristics of updrafts include vertical velocities typically ranging from 1 to 20 m/s, with spatial scales varying from tens of meters in narrow thermals to several kilometers in broader convective features.[10] They are often linked to the development of cumulus clouds, where rising moist air cools adiabatically and condenses, forming visible cloud structures.[11] A representative example occurs in fair-weather cumulus clouds, where localized updrafts with velocities around 5.5 m/s can propel air parcels to altitudes of up to 3 km, supporting cloud growth without leading to precipitation.[12] The fundamental buoyancy force propelling an updraft is expressed as where is the acceleration due to gravity, is the density difference (ambient minus parcel, positive for buoyant ascent), is the ambient density, and is the parcel volume.[13] Updrafts commonly pair with downdrafts as opposing vertical motions within convective circulations.[8]Downdrafts
A downdraft is defined as a small-scale column of sinking air that descends rapidly toward the ground due to the higher density of cooler air parcels compared to the surrounding environment.[14][15] Key characteristics of downdrafts include vertical velocities typically ranging from 1 to 25 m/s, though severe cases can reach up to about 30 m/s, driven by the density contrast that promotes descent.[15][16] These sinking currents often undergo evaporative cooling, which further increases the density difference and intensifies the downward motion, leading to the formation of gust fronts as the cool, dense air spreads horizontally upon hitting the surface.[15][17] Microbursts serve as a representative example of intense downdrafts, featuring descent rates averaging 10 m/s over horizontal scales of hundreds of meters.[17][18] The dynamics of downdraft descent are governed by the buoyancy force arising from the density difference, approximated in the simplified vertical momentum equation as where is the vertical velocity (negative for descent), is gravitational acceleration, is the downdraft air density, and is the ambient air density.[19] This equation highlights how negative buoyancy () accelerates the sinking motion, distinguishing downdrafts from updrafts in convective cells where positive buoyancy drives ascent.[20]Formation Processes
Thermal Convection
Thermal convection in the atmosphere arises from the uneven heating of the Earth's surface by solar radiation, which warms the ground and the overlying air through conduction. This creates parcels of air that are warmer and thus less dense than the surrounding cooler air, leading to buoyancy-driven ascent as these parcels rise to restore hydrostatic equilibrium.[21] The stability of this process depends on the atmospheric lapse rate, defined as the rate of temperature decrease with altitude. The atmosphere is unstable for dry processes when the environmental lapse rate exceeds the dry adiabatic lapse rate of approximately 9.8°C/km, promoting free convection where displaced air parcels continue to accelerate upward due to positive buoyancy. For moist convection, which is common in vertical draft formation, conditional instability occurs when the environmental lapse rate lies between the dry adiabatic lapse rate (~9.8°C/km) and the moist adiabatic lapse rate (~6°C/km). In this case, a parcel ascends dry and unsaturated until the lifting condensation level (LCL), after which latent heat release upon saturation can sustain or enhance buoyancy for further ascent. In detail, a surface air parcel absorbs heat from the warmed ground, expands due to decreased density, and begins to rise while undergoing adiabatic cooling at the dry adiabatic rate. As it ascends, the parcel remains warmer than its surroundings in an unstable environment, sustaining its upward motion until it reaches the lifting condensation level (LCL). Upon saturation at the LCL, latent heat release from condensation can enhance buoyancy if the atmosphere is conditionally unstable, allowing the parcel to continue rising along the moist adiabatic lapse rate. The dry adiabatic lapse rate is given by where is the acceleration due to gravity and is the specific heat capacity of dry air at constant pressure.[24] In fair-weather conditions, this mechanism generates persistent updrafts in the form of thermals, which are buoyant bubbles of rising air.[25]Orographic and Frontal Lifting
Orographic lifting occurs when prevailing winds force air masses upward over topographic barriers such as mountains, resulting in adiabatic expansion and cooling as the air rises. This mechanical ascent reduces the air temperature at rates governed by the dry or moist adiabatic lapse rates, typically 9.8°C per kilometer for dry air and about 6°C per kilometer for saturated air, potentially leading to condensation when the dew point is reached and initiating vertical drafts in the form of updrafts.[26][27] The intensity of this forced ascent is approximated by the vertical velocity , where represents the horizontal wind speed and denotes the slope of the terrain; this linear relationship highlights how stronger winds or steeper slopes enhance upward motion.[28] Vertical speeds in orographic lifting are thus directly influenced by wind velocity and terrain gradient, often generating stationary mountain waves on the leeward side that feature alternating updrafts and downdrafts. These waves can contribute to downdraft formation in the lee-side regions through descending air motions.[27][29] Frontal lifting arises from the convergence of contrasting air masses along weather fronts, where warmer, less dense air is compelled to ascend over cooler, denser air beneath. This synoptic-scale forcing promotes adiabatic cooling and moisture convergence, fostering the development of vertical drafts through sustained upward motion.[30] In warm fronts, the gradual override of cold air by advancing warm air produces broad areas of forced ascent, while cold fronts involve more abrupt lifting as cold air wedges under warm air, intensifying vertical velocities.[31]Role in Atmospheric Phenomena
Thunderstorm Development
Thunderstorms progress through three primary stages in their lifecycle, each dominated by distinct patterns of vertical drafts. In the initial cumulus stage, strong updrafts driven by thermal instability lift warm, moist air parcels, leading to rapid vertical growth and the formation of towering cumulus clouds that can evolve into cumulonimbus structures.[8] These updrafts transport low-level moisture aloft, where cooling and condensation release latent heat, further intensifying the ascent and building the cloud's anvil and overshooting tops.[11] As the storm reaches the mature stage, a balance emerges between persistent updrafts and emerging downdrafts, with precipitation beginning to form and fall. Updrafts continue to sustain the storm's core, but rain and ice particles induce evaporative cooling, generating downdrafts that descend alongside the updrafts, often separated by wind shear. These downdrafts spread cool air outward at the surface, forming gust fronts or outflow boundaries that can trigger new convective cells. This stage is marked by intense weather, including heavy rainfall, lightning, and hail, as the drafts interact dynamically.[32][8] In the dissipating stage, downdrafts prevail as the updraft supply of warm, moist air is cut off by the spreading cold outflow and stabilization of the atmosphere. The storm weakens, with vertical motion diminishing and precipitation tapering off, though residual downdrafts may persist briefly. Throughout the lifecycle, vertical drafts are central: updrafts fuel growth and precipitation formation, while downdrafts facilitate dissipation and boundary propagation. In severe cases, such as supercell thunderstorms, updraft cores can reach speeds approaching 50 m/s, sufficient to loft large hailstones and produce extreme rainfall rates by prolonging hydrometeor residence time in the cloud.[11][32][33] The theoretical maximum updraft speed in thunderstorms can be approximated using parcel theory, which relates buoyancy to vertical acceleration: Here, is the updraft speed, is gravitational acceleration, is the depth over which the parcel rises (often the cloud layer thickness), is the potential temperature excess of the parcel relative to its environment, and is the mean potential temperature. This equation derives from integrating the buoyancy force along the ascent path, assuming minimal entrainment and drag, and highlights how thermal instability drives intense vertical motion essential for storm development./14:_Thunderstorm_Fundamentals/14.03:_Section_4-)Tornado and Severe Weather Formation
Intense vertical drafts play a pivotal role in the formation of tornadoes and other severe rotational weather within supercell thunderstorms, primarily through the process of vorticity tilting. In supercells, strong updrafts interact with environmental horizontal vorticity—generated by vertical wind shear—by tilting it into the vertical plane, thereby creating a mid-level mesocyclone characterized by organized rotation on scales of 2–10 km. This tilting mechanism converts streamwise horizontal vorticity components into vertical vorticity, initiating cyclonic rotation aloft as the updraft rises. The vertical component of vorticity, denoted as , is mathematically expressed as , where and are the horizontal wind components in the x and y directions, respectively; this vertical vorticity is then amplified by the updraft through vertical stretching, enhancing rotational intensity.[34][35][36][37][38] Persistent updrafts are essential for sustaining this rotation, as they continuously advect and stretch the tilted vorticity, preventing dissipation and allowing the mesocyclone to descend toward the surface. Rear-flank downdrafts (RFDs), descending air masses on the storm's rear flank, further contribute to tornadogenesis by enhancing low-level wind shear; they create baroclinic zones that generate additional horizontal vorticity near the ground, which strong updrafts then tilt into vertical rotation, focusing it into a concentrated vortex. The development of the RFD is often a key precursor to tornado formation, as it modulates buoyancy and convergence at the updraft's base, promoting the necessary low-level shear for vortex intensification. These downdraft-updraft interactions distinguish supercell tornadogenesis from non-rotational severe weather.[37][35][39][40][41] Examples of this dynamic are evident in high-intensity tornadoes rated on the Enhanced Fujita (EF) scale, where updraft speeds exceeding 40 m/s have been linked to EF-3 and stronger events through numerical simulations of supercell environments. For instance, the EF-5 Joplin, Missouri, tornado of May 22, 2011, which caused 161 fatalities and $2.8 billion in damage, was embedded in a rapidly intensifying supercell where vertical draft interactions drove the mesocyclone's descent and tornadogenesis, as analyzed in post-event radar and modeling studies. Such cases underscore how draft-induced vorticity amplification can produce destructive vortices with path lengths over 30 km and widths up to 1 km.[42][43]Measurement and Modeling
Observational Techniques
Ground-based Doppler radars are widely used to detect and quantify vertical drafts by measuring radial velocity shifts in precipitation or hydrometeors within the radar beam. These shifts arise from the Doppler effect, where the frequency of the returned radar signal changes based on the component of target motion toward or away from the radar, allowing inference of vertical wind speeds in updrafts and downdrafts up to several tens of meters per second.[44][12] For instance, in convective storms, positive radial velocities indicate updrafts when the beam is oriented near vertical, while negative values signal downdrafts, providing velocity profiles with temporal resolutions on the order of minutes.[45] Aircraft and balloon probes offer in-situ measurements of vertical wind components, capturing high-frequency fluctuations in real-time as platforms traverse atmospheric layers. Research aircraft equipped with gust probes use inertial navigation and differential GPS to compute vertical winds from airspeed and platform motion, achieving resolutions of 0.1 m/s or better over flight paths spanning kilometers.[46] Dropsondes deployed from research aircraft via systems like the Airborne Vertical Atmospheric Profiling System (AVAPS) and weather balloons with GPS sondes estimate vertical air motion by correcting descent or ascent rates for buoyancy and environmental winds, revealing draft intensities in the troposphere during profiles at 5-10 m/s.[47][48] These methods excel in resolving fine-scale turbulence and drafts within boundary layers or storm cores, complementing remote sensing by providing ground-truth data.[46] Satellite observations infer vertical drafts indirectly through infrared imagery, where colder cloud-top temperatures indicate overshooting updrafts penetrating the tropopause. Geostationary satellites like GOES measure brightness temperatures in the 10-12 μm window channel, with deviations from expected values signaling rapid ascent rates of 10-50 m/s in deep convection, as warmer surface or cloud-base temperatures relative to tops imply stronger buoyancy-driven motion.[49] This technique covers vast areas but lacks the vertical resolution of in-situ methods, typically estimating updraft speeds via thermodynamic retrievals with uncertainties around 20%.[50] A key advancement in ground-based observations is dual-Doppler radar synthesis, which combines radial velocity data from two or more radars to reconstruct three-dimensional wind fields, including vertical drafts. By solving the geometric intersection of beams and applying variational methods to minimize errors, this approach resolves horizontal and vertical wind components at spatial resolutions of 100-500 m, enabling detailed mapping of draft structures in supercells or thunderstorms over domains of tens of kilometers.[51][52] Such syntheses have quantified updraft cores exceeding 20 m/s with horizontal accuracies of 1-2 m/s.[53]Numerical Simulations
Numerical simulations play a crucial role in understanding vertical drafts by solving the governing equations of atmospheric fluid dynamics on computational grids, enabling predictions of updraft and downdraft behaviors that are difficult to observe directly. These models incorporate physical parameterizations to represent subgrid-scale processes, allowing researchers to explore the initiation, evolution, and interactions of vertical motions in convective systems.[54] Cloud-resolving models (CRMs), such as the Weather Research and Forecasting (WRF) model, simulate vertical drafts at horizontal grid resolutions of 1-10 km, capturing mesoscale convective structures while resolving individual cloud elements.[54] For finer-scale details, large-eddy simulations (LES) employ grids below 1 km to explicitly resolve turbulent eddies and microphysical processes within drafts, often nested within coarser CRM domains to bridge scales.[55] These simulations incorporate moist thermodynamics, including phase changes and latent heat release, alongside turbulence parameterizations to replicate the interactions between updrafts and downdrafts, such as entrainment and detrainment that influence draft intensity and longevity.[56] The core dynamics are governed by the Navier-Stokes equations for momentum, adapted for compressible, moist air: where is the velocity vector, is density, is pressure, is gravity, and includes buoyancy terms from temperature and moisture perturbations, along with diffusive and subgrid forces.[57] Validation of these models often involves comparing simulated vertical velocities and draft structures against radar observations, with initial conditions derived from observational data to ensure realism.[58] For instance, idealized convection studies using CRMs have demonstrated good agreement in updraft core sizes and downdraft propagation speeds when benchmarked against dual-Doppler radar retrievals of supercell thunderstorms.[59]Environmental and Human Impacts
Effects on Aviation Safety
Vertical drafts, particularly in the form of clear air turbulence (CAT) associated with mountain waves, pose significant hazards to aviation by inducing unpredictable vertical accelerations that can lead to loss of control or structural stress on aircraft. Mountain waves form when stable air flows over mountainous terrain, creating oscillating updrafts and downdrafts that propagate vertically, often resulting in severe turbulence invisible to pilots without visual cues. Aircraft encountering these waves may experience sudden vertical gusts causing accelerations up to 2g, which can exceed design limits for smaller planes and result in passenger injuries or airframe damage.[60][61] Microburst downdrafts, intense localized downdrafts from convective activity, represent another critical threat, rapidly accelerating aircraft downward and causing sudden altitude loss during takeoff or landing. These downdrafts can produce wind shear with vertical velocities exceeding 10 m/s, drastically reducing airspeed and lift, potentially leading to stalls. A notable example is the 1994 crash of USAir Flight 1016, where a DC-9 encountered a microburst-induced downdraft during approach to Charlotte Douglas International Airport, resulting in the aircraft colliding with trees and claiming 37 lives; the National Transportation Safety Board determined the primary cause was the crew's penetration of a thunderstorm producing the microburst.[62][63] Statistical data from aviation authorities highlight the frequency and impact of turbulence linked to vertical drafts. The Federal Aviation Administration receives approximately 65,000 pilot reports annually of moderate or greater turbulence over the United States, many attributable to vertical wind variations from mountain waves and convective drafts, with severe cases involving vertical accelerations reaching 2g and causing approximately 58 serious injuries each year as of 2023.[64][65] Low-level wind shear from downdrafts is particularly dangerous near airports, where changes in horizontal wind speed () exceeding 15 knots over 1 km can reduce aircraft lift by altering relative airflow, increasing stall risk during critical phases of flight.[61][66] To mitigate these risks, modern aircraft are equipped with onboard wind shear detection systems, such as the Predictive Windshear System (PWS), which uses forward-looking Doppler radar to identify hazardous vertical velocity changes greater than 6 m/s—corresponding to moderate turbulence thresholds—and issue timely alerts to pilots for evasion maneuvers. Ground-based systems like the Terminal Doppler Weather Radar (TDWR) complement these by detecting microbursts with vertical components over 8 m/s, providing airport advisories to delay operations. These technologies have significantly reduced wind shear-related accidents since their widespread adoption in the 1990s.[67][68][69]Influence on Wildfire Spread and Air Quality
Vertical drafts play a critical role in wildfire dynamics by driving the ascent of heated air, smoke, and embers within fire plumes, which enhances combustion efficiency and promotes rapid fire spread. Updrafts, fueled by the buoyancy of hot gases from burning vegetation, loft embers and firebrands over significant distances, enabling spot fires that extend the fire perimeter beyond the main front.[70] In intense megafires, these updrafts can exceed 50 m/s, rivaling those in severe thunderstorms and intensifying fire behavior through increased oxygen supply and preheating of fuels.[71] For example, during the 2016 Pioneer Fire in Washington State, plume updrafts reached approximately 58 m/s, contributing to extreme fire growth and ember transport.[72] The height achieved by these plumes is often estimated using plume rise models based on buoyancy flux. A common formulation for buoyant plume rise, such as Briggs' equation for neutral conditions, is Δh = 21.4 F^{3/4} / u for F < 55 m^4 s^{-3} (where F is buoyancy flux in m^4 s^{-3}, u is wind speed in m s^{-1}), accounting for the initial vertical momentum imparted by the fire's heat release and its dilution through atmospheric entrainment, providing a basis for predicting how high smoke and embers are injected into the atmosphere.[73] In terms of air quality, vertical drafts govern the dispersion and concentration of wildfire pollutants such as particulate matter (PM_{2.5}) and volatile organic compounds. Downdrafts, often associated with subsiding air around plume peripheries or nocturnal stability, mix smoke downward and trap it near the surface, leading to elevated ground-level pollutant concentrations and degraded local air quality, particularly in valleys or under inversions.[72] Conversely, strong updrafts facilitate vertical mixing within the planetary boundary layer and loft smoke into the free troposphere, reducing near-surface impacts but enabling long-range transport over hundreds of kilometers, as seen in smoke from western U.S. fires affecting air quality in the eastern U.S. and even Europe.[72] Extreme updrafts can also induce the formation of pyrocumulus clouds when moist air is drawn into the plume and condenses, potentially evolving into pyrocumulonimbus storms. These fire-induced clouds, reaching heights of 10–15 km, generate lightning through charge separation in the convective updrafts, which can ignite additional fires downwind and complicate suppression efforts. A notable case occurred during the 2009 Black Saturday fires in southeast Australia, where pyrocumulonimbus over the Kinglake complex produced lightning strokes that started a new fire approximately 100 km away.[74]References
- http://www.atmo.[arizona](/page/Arizona).edu/students/courselinks/fall10/atmo551a/AdiabaticLapseRate.pdf
- https://www.weather.gov/media/[aviation](/page/Aviation)/afp/stability_clouds.pdf