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Anabatic wind
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An anabatic wind, from the Greek anabatos, verbal of anabainein meaning "moving upward", is a warm wind which blows up a steep slope or mountain side, driven by heating of the slope through insolation.[1][2] It is also known as upslope flow. These winds typically occur during the daytime in calm sunny weather. A hill or mountain top will be radiatively warmed by the Sun which in turn heats the air just above it. Air at a similar altitude over an adjacent valley or plain does not get warmed so much because of the greater distance to the ground below it.
The air over the hill top is now warmer than the air at a similar altitude around it and will rise through convection. This creates a lower pressure region into which the air at the bottom of the slope flows, causing the wind. It is common for the air rising from the tops of large mountains to reach a height where it cools adiabatically to below its dew point and forms cumulus clouds. These can then produce rain or even thunderstorms.[2]
Anabatic winds are particularly useful to soaring glider pilots who can use them to increase the aircraft's altitude. Anabatic winds can be detrimental to the maximum downhill speed of cyclists. Conversely, katabatic winds are down-slope winds, frequently produced at night by the opposite effect, the air near to the ground losing heat to it faster than air at a similar altitude over adjacent low-lying land.
See also
[edit]References
[edit]- ^ Marine Meteorological Glossary Archived December 11, 2008, at the Wayback Machine
- ^ a b "American Meteorology Society Glossary". Archived from the original on 2009-02-26. Retrieved 2009-01-06.
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Anabatic wind
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Definition and Characteristics
Core Definition
Anabatic wind derives its name from the Greek term anabatos, the verbal adjective of anabainein, meaning "moving upward," which aptly describes the phenomenon of warm air rising along inclined surfaces.[6][7] Scientifically, an anabatic wind is defined as a local wind system characterized by upslope airflow driven by buoyancy forces arising from the heating of the ground surface, which warms the overlying air and causes it to ascend along slopes, predominantly occurring during daylight hours when solar radiation is active.[4][8] This process establishes it as a thermally direct circulation, wherein warmer air rises and cooler air descends in a localized pattern, distinguishing it from broader synoptic-scale wind systems that are often influenced by pressure gradients or Coriolis effects rather than direct thermal buoyancy.[9] These winds typically operate on a local scale, spanning from tens of meters to several kilometers horizontally and vertically, with characteristic speeds ranging from 1 to 10 m/s under standard conditions, though intensities can reach up to 10-15 m/s in cases of strong heating and steep topography.[8][10][4]Key Physical Properties
Anabatic winds exhibit a distinct temperature profile characterized by warmer air near the sloping surface compared to the overlying atmosphere, driven by radiative heating that creates buoyancy. This results in a temperature excess of up to 5°C near the surface relative to ambient air, with gradients typically decreasing with height over the slope, often spanning a shallow layer of tens to hundreds of meters.[11] In observed cases, potential temperature decreases with height in the unstable boundary layer near the slope, with surface layers showing the strongest gradients due to direct solar heating.[12] The velocity structure of anabatic winds features a jet-like profile, with maximum speeds occurring close to the surface and tapering off aloft. Typical speeds range from 2 to 5 m/s, with peaks often located within the lowest 10-20% of the planetary boundary layer depth, such as 20-40 m above the slope in daytime conditions.[12] Shear layers form at heights of 10-100 m, where momentum flux divergence contributes to the flow's deceleration higher up.[11] These winds flow upslope in a direction perpendicular to the terrain contours, closely following the slope orientation and hugging the surface due to the buoyancy force.[11] Anabatic winds typically persist for 4-12 hours during the day, initiating in the morning as heating begins and peaking around midday before weakening in the late afternoon as solar input diminishes.[12] Turbulence levels in anabatic winds are moderate to strong, with enhanced mixing in the convective boundary layer due to thermal instability; turbulent kinetic energy often reaches 0.25-0.75 m²/s² near the surface and increases with height in the mixed layer.[12] This turbulence promotes vertical transport of heat and momentum, though it remains less intense than in fully developed convective conditions.[11]Formation Mechanisms
Thermal Driving Forces
Anabatic winds are primarily initiated by the absorption of solar radiation on inclined terrain surfaces, such as rock or soil, which warms the ground and transfers heat to the overlying air through sensible heat flux.[13] This process is most effective during daytime under clear skies, when incoming solar radiation is maximized, leading to a gradual development of upslope flows that peak near solar noon.[14] The buoyancy mechanism arises as the heated air near the surface becomes less dense than the surrounding ambient air, generating a positive pressure gradient that drives the flow upslope. This buoyancy acceleration can be expressed as , where is the acceleration due to gravity, is the temperature difference between the heated air and the ambient air, and is the reference ambient temperature.[15] The resulting density contrast creates a hydrostatic pressure imbalance, compelling the warmer air to rise along the slope while cooler air from below is drawn in to replace it.[16] Surface characteristics significantly influence the intensity of heating and thus the strength of anabatic flows, with darker surfaces exhibiting lower albedo that absorb more solar radiation compared to lighter or reflective ones.[13] Similarly, drier slopes promote greater sensible heat flux into the air by minimizing evaporative cooling, whereas vegetated or moist surfaces reduce heating efficiency through increased latent heat loss and shading effects.[13] Initiation of sustained anabatic winds requires a minimum slope angle of approximately 0.1° to generate the necessary along-slope buoyancy component, though steeper angles enhance flow development.[14] Clear atmospheric conditions are essential to provide sufficient insolation for surface warming, typically occurring diurnally during periods of weak synoptic forcing.[13]Atmospheric Conditions Influencing Development
Anabatic winds develop most effectively under anticyclonic synoptic conditions, where high-pressure systems foster clear skies and minimal large-scale forcing, allowing thermal contrasts to dominate local circulations.[14] Weak background winds, typically below 5 m/s aloft, further enhance these flows by reducing interference from synoptic-scale advection, while low humidity in such environments reduces the likelihood of early cloud formation and precipitation, permitting unimpeded upslope motion.[14][17] Slope orientation plays a key role in modulating anabatic wind intensity, as aspects receiving maximum solar insolation experience greater surface heating and thus stronger buoyancy-driven flows. In the Northern Hemisphere, south-facing slopes absorb more direct sunlight throughout the day, leading to enhanced upslope winds compared to north-facing slopes, which receive reduced radiation and exhibit weaker or intermittent flows.[18][14] Within the planetary boundary layer, low-level convergence induced by surrounding terrain topography can amplify anabatic inflows by drawing in ambient air toward heated slopes, thereby sustaining the upslope circulation. Conversely, wind shear from upper-level flows may disrupt development if sufficiently strong, introducing turbulence that erodes the thermal structure essential for buoyancy.[14][19] Seasonal variations significantly influence anabatic wind occurrence and strength, with flows peaking during summer months due to higher solar angles and intensified day-night heating contrasts that maximize surface sensible heat flux. In contrast, winter conditions or polar regions feature diminished insolation, resulting in weaker or absent anabatic circulations as radiative forcing fails to overcome persistent stable layers.[14] Cloud cover and precipitation act as primary suppression factors for anabatic winds by intercepting incoming solar radiation and reducing the net energy available for surface heating, thereby halting or weakening upslope development. These conditions diminish the diurnal temperature gradient, often confining flows to negligible speeds or preventing their initiation altogether.[14] Anabatic winds typically integrate with the diurnal cycle, reaching maximum intensity around solar noon when heating is optimal.[14]Comparisons with Related Phenomena
Differences from Katabatic Winds
Anabatic winds flow upslope along inclined terrain, driven by daytime solar heating that warms the air near the surface, creating buoyancy and lower pressure aloft, whereas katabatic winds descend downslope, propelled by nighttime radiative cooling that increases air density near the ground.[14][20] This thermal contrast results in anabatic winds transporting warmer air upward, often with a temperature excess of 2–7°C relative to the surrounding atmosphere over depths of 10–100 m, while katabatic winds carry cooler, denser air downward, producing a temperature deficit of 3–7°C over shallower layers of 1–20 m.[14] In mountainous valleys, anabatic and katabatic winds form a complementary diurnal cycle: anabatic flows dominate during the day under solar heating, typically initiating after sunrise and peaking in the afternoon, while katabatic flows prevail at night following sunset, as cooling promotes downslope drainage.[14][5] This opposition creates a regular reversal in wind direction along slopes, with anabatic upslope motion giving way to katabatic downslope flow, enhancing local circulation in sloped terrain.[14] Anabatic winds are generally weaker and more variable in intensity, with typical speeds of 1–5 m/s peaking at heights of 10–50 m, influenced by factors like slope angle and ambient winds, though they can reach up to 10 m/s in stronger convective conditions.[14][4] In contrast, katabatic winds often exhibit greater persistence and can achieve higher speeds of 1–4 m/s in typical settings, peaking low at 1–15 m, but escalating to 20 m/s or more in glacial or polar environments where cold, dense air accelerates downslope.[14][21]| Aspect | Anabatic Winds | Katabatic Winds |
|---|---|---|
| Direction | Upslope (toward higher elevations) | Downslope (toward lower elevations) |
| Primary Cause | Surface heating and buoyancy | Radiative cooling and density increase |
| Typical Time | Daytime (solar heating) | Nighttime (nocturnal cooling) |
| Temperature Effect | Warmer air rises (excess 2–7°C) | Colder air sinks (deficit 3–7°C) |
| Typical Speed | 1–5 m/s (up to 10 m/s, variable) | 1–4 m/s (up to 20 m/s in glacial areas) |