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Atmospheric tides (sometimes known as air tides or atmospheric oscillations) are global-scale periodic atmospheric oscillations. In many ways they are analogous to ocean tides. Atmospheric tides can be caused by: This article is about tides in the ocean. ...
- The regular day/night cycle in the solar heating of the atmosphere
- The gravitational pull of the moon (lunar tides)
- Non-linear interactions between tides and planetary waves.
Atmospheric tides are mostly generated in the troposphere and stratosphere. The tides are then able to propagate away from these source regions and ascend into the mesosphere and thermosphere. Although atmospheric tides share much in common with ocean tides, two key distinguishing features are that: Rossby (or planetary) waves are large-scale motions in the ocean or atmosphere whose restoring force is the variation in Coriolis effect with latitude. ...
Atmosphere diagram showing the mesosphere and other layers. ...
Atmosphere diagram showing stratosphere. ...
The mesosphere (from the Greek words mesos = middle and sphaira = ball) is the layer of the Earths atmosphere that is directly above the stratosphere and directly below the thermosphere. ...
The thermosphere is the layer of the earths atmosphere directly above the mesosphere and directly below the exosphere. ...
i) The atmospheric tides are primarily excited by the Sun's heating of the atmosphere whereas ocean tides are primarily excited by the Moon's gravitational field. This means that most atmospheric tides have periods of oscillation related to the 24-hour length of the solar day whereas ocean tides have longer periods of oscillation related to the lunar day (time between successive lunar transits) of about 24 hours 51 minutes.
ii) The atmospheric tides propagate in an atmosphere where density varies significantly with height. A consequence of this is that their amplitudes naturally increase exponentially as the tide ascends into progressively more rarified regions of the atmosphere (for an explantion of this phenomenon, see below). In contrast, the density of the oceans varies only slighthly with depth and so there the tides do not necessarily vary in amplitude with depth.
Note that There is also a very small contribution from the gravitational pull of the sun, but its effect is weaker than that of the moon and is negligible compared to the thermal solar tides.
At ground level atmospheric tides can be detected as regular but small oscillations in surface pressure. However, at greater heights the amplitudes of the tides can become very large. In the mesosphere (heights of ~ 50 - 100 km) atmospheric tides can be the largest-amplitude motions present, reaching amplitudes of more than 50 m/s. The tides can be measured as regular fluctuations in wind, temperature and density. As waves propagate upwards the density of atmosphere decreases exponentially, the amplitude of the wave increases correspondingly so that energy is conserved. The amplitude of a wave at a height of z is described by the equation:
A = A0exp( − z / 2H)
where A0 is the initial amplitude of the wave, z is height and H is the scale height of the atmosphere. To meet Wikipedias quality standards, this article or section may require cleanup. ...
Following this growth with height atmospheric tides have much larger amplitudes in the middle and upper atmosphere than they do at ground level. Atmospheric tides often dominate the motion of the mesosphere and lower thermosphere region (~80 - 120 km). The mesosphere (from the Greek words mesos = middle and sphaira = ball) is the layer of the Earths atmosphere that is directly above the stratosphere and directly below the thermosphere. ...
The thermosphere is the layer of the earths atmosphere directly above the mesosphere and directly below the exosphere. ...
Solar Atmospheric Tides
The largest amplitude atmospheric tides are generated by the periodic heating of the atmosphere by the Sun - the atmosphere is heated during the day and not heated at night. This regular diurnal (daily) cycle in heating generates tides that have periods related to the solar day. It might initially be expected that this diurnal heating would give rise to tides with a period of 24 hours, corresponding to the heating's periodicity. However, observations reveal that large amplitude tides are generated with periods of 24 and 12 hours. Tides have also been observed with periods of 8 and 6 hours, although these latter tides generally have smaller amplitudes. This set of periods occurs because the solar heating of the atmosphere occurs in an approximate square wave profile and so is rich in harmonics. When this pattern is decomposed into separate frequency components using a fourier transform, as well as the mean and daily (24-hr) variation, significant oscillations with periods of 12, 8 and 6 hrs are produced. Tides generated by the gravitational effect of the sun are very much smaller than those generated by solar heating. Solar tides will refer to only thermal solar tides from this point. A square wave is a kind of basic waveform. ...
In mathematics, the Fourier transform is a certain linear operator that maps functions to other functions. ...
Solar energy is absorbed throughout the atmosphere some of the most significant in this context are water vapor at (~0 - 15 km) in the troposphere, ozone at (~30 to 60 km) in the stratosphere and molecular oxygen and molecular nitrogen at (~120 to 170 km) in the thermosphere. Variations in the global distribution and density of these species results in changes in the amplitude of the solar tides. The tides are also affected by the environment through which they travel, so changes in the lower atmosphere can effect tides observed in the mesosphere and lower thermosphere region. It has been suggested that multiple sections of steam be merged into this article or section. ...
Atmosphere diagram showing the mesosphere and other layers. ...
For other uses, see Ozone (disambiguation). ...
Atmosphere diagram showing stratosphere. ...
The thermosphere is the layer of the earths atmosphere directly above the mesosphere and directly below the exosphere. ...
The mesosphere (from the Greek words mesos = middle and sphaira = ball) is the layer of the Earths atmosphere that is directly above the stratosphere and directly below the thermosphere. ...
The thermosphere is the layer of the earths atmosphere directly above the mesosphere and directly below the exosphere. ...
Solar tides can be separated into two components: migrating and non-migrating.
Migrating Solar Tides Migrating tides are sun synchronous - from the point of view of a stationary observer on the ground they propagate westwards with the apparent motion of the sun. As the migrating tides stay fixed relative to the sun a pattern of excitation is formed that is also fixed relative to the sun. Changes in the tide observed from a stationary viewpoint on the earths surface are caused by the rotation of the Earth with respect to this fixed pattern. Seasonal variations of the tides also occur as the Earth tilts relative to the Sun and so relative to the pattern of excitation. [1]
The migrating solar tides have been extensively studied both through observations and mechanistic models. [2]
Non-Migrating Solar Tides Non-migrating tides can be thought of as global-scale waves with the same periods as the migrating tides however, non-migrating tides do not follow the apparent motion of the sun. Either they do not propagate horizontally, they propagate eastwards or they propagate westwards at a different speed to the sun. These non-migrating tides may be generated by differences in topography with longitude, land-sea contrast and surface interactions. It has been suggested that Geomorphometry be merged into this article or section. ...
The primary source for the 24-hr tide is in the lower atmosphere where surface effects are important. This is reflected in a relatively large non-migrating components seen in longitudinal differences in tidal amplitudes. Largest amplitudes have been observed over South America, Africa and Australia. [3] In contrast the 12-hr tide is thought to be primarily generated higher in the atmosphere and so has a relatively small contribution from non-migrating components.
Lunar Atmospheric Tides Atmospheric tides are also produced through the gravitational effects of the Moon. Lunar (gravitational) tides are much weaker than solar (thermal) tides and are generated by the motion of the Earth's oceans (caused by the Moon) and to a lesser extent the effect of the Moon's gravitational attraction on the atmosphere.
Dissipation Atmospheric tidal dissipation may come from many mechanisms. A wave that loses amplitude is said to dissipate. ...
Tidal amplitudes maximize in height in the mesosphere and lower thermosphere region, above about 90 - 100 km, the decreasing density of the air causes energy transfer by wave motion to become less efficient due to molecular diffusion damping the tides. Turbulence caused by breaking gravity waves at these heights causes a drag on tidal flow and causes further damping of the tides. Wave breaking can also occur through convective and dynamic instabilities. Interactions between tides and other atmospheric motions also dissipates energy front the tides. Above the thermosphere, tides are dissipated by the radiation of energy into space and drag caused by charged particles in the ionosphere. This article or section does not adequately cite its references or sources. ...
Gravity waves refer to different (but basically similar) concepts. ...
Effects of Atmospheric Tide Understanding atmospheric tides is essential in understanding the atmosphere as a whole. Modeling and observations of atmospheric tides are needed monitor and predict changes in the Earths atmosphere.
See also It has been suggested that Theory of tides be merged into this article or section. ...
It has been suggested that this article or section be merged into Tide. ...
The mesosphere (from the Greek words mesos = middle and sphaira = ball) is the layer of the Earths atmosphere that is directly above the stratosphere and directly below the thermosphere. ...
The thermosphere is the layer of the earths atmosphere directly above the mesosphere and directly below the exosphere. ...
Notes and references - ^ Global Scale Wave Model UCAR
- ^ http://www.hao.ucar.edu/modeling/gswm/refs.html
- ^ Hagan, M.E., J.M. Forbes and A. Richmond, 2003: Atmospheric Tides, Encyclopedia of Atmospheric Sciences
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