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Atmospheric tide
 
 
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Atmospheric tides are global-scale periodic oscillations of the atmosphere
Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low....
. In many ways they are analogous to ocean tides. Atmospheric tides can be excited by:



General Characteristics
The largest-amplitude atmospheric tides are mostly generated in the troposphere
Troposphere

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and almost all of its water vapor and particulate....
 and stratosphere
Stratosphere

The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down....
 when the atmosphere is periodically heated as water vapour and ozone
Ozone

Ozone or trioxygen is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O2....
 absorb solar radiation during the day.






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Encyclopedia


Atmospheric tides are global-scale periodic oscillations of the atmosphere
Atmosphere

An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low....
. In many ways they are analogous to ocean tides. Atmospheric tides can be excited by:

  • The regular day
    Day

    A day is a units of measurement of time equivalent to approximately 24 hours. It is not an International System of Units unit but it is accepted for use with SI....
    /night
    Night

    Night or nighttime is the period of time when the sun is below the horizon. The opposite of night is day . Time of day varies based on factors such as season, latitude, longitude and timezone....
     cycle in the insolation
    Insolation

    Insolation is a measure of solar radiation energy received on a given surface area in a given time. It is commonly expressed as average irradiance in watts per square meter or kilowatt-hours per square meter per day ....
     of the atmosphere
    Atmosphere

    An atmosphere is a layer of gases that may surround a material body of sufficient mass, by the gravity of the body, and are retained for a longer duration if gravity is high and the atmosphere's temperature is low....
  • The gravitational field
    Gravitational field

    A gravitational field is a scientific model used within physics to explain how gravitation exists in the universe. In its original concept, gravity was a force between point masses....
     pull of the Moon
    Moon

    The Moon is Earth's only natural satellite and the List of natural satellites by diameter satellite in the Solar System. The average centre-to-centre distance from the Earth to the Moon is km, about thirty times the diameter of the Earth....
  • Non-linear interactions between tides and planetary waves.


General Characteristics


The largest-amplitude atmospheric tides are mostly generated in the troposphere
Troposphere

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and almost all of its water vapor and particulate....
 and stratosphere
Stratosphere

The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down....
 when the atmosphere is periodically heated as water vapour and ozone
Ozone

Ozone or trioxygen is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O2....
 absorb solar radiation during the day. The tides generated are then able to propagate away from these source regions and ascend into the mesosphere
Mesosphere

The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. The mesosphere is located from about 50 km to 80-90 km altitude above the Earth's surface....
 and thermosphere
Thermosphere

The thermosphere is the layer of the earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization....
. Atmospheric tides can be measured as regular fluctuations in wind
WIND

The Global Geospace Science WIND satellite is a NASA science spacecraft launched at 04:31:00 EST on November 1, 1994 from launch pad 17B at Cape Canaveral Air Force Station in Merritt_Island%2C_Florida, Florida aboard a McDonnell Douglas Delta II 7925-10 rocket....
, temperature
Temperature

In physics, temperature is a physical property of a Physical system that underlies the common notions of hot and cold; something that feels hotter generally has the greater temperature....
, density
Density

The density of a material is defined as its mass per unit volume. The symbol of density is ....
 and pressure
Pressure

Pressure is the force per unit area applied to an object in a direction surface normal to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure....
. Although atmospheric tides share much in common with ocean tides they have two key distinguishing features:

  1. Atmospheric tides are primarily excited by the Sun
    Sun

    The Sun , a G V star, is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 98.6% of the Solar System's mass....
    's heating of the atmosphere whereas ocean tides are primarily excited by the Moon's gravitational pull. This means that most atmospheric tides have periods of oscillation
    Oscillation

    Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and Alternating current power....
     related to the 24-hour length of the solar day whereas ocean tides have longer periods of oscillation related to the lunar day
    Lunar day

    In space exploration, a lunar day is the period of time it takes for the Moon to complete one full rotation on its axis with respect to the Sun....
     (time between successive lunar transits) of about 24 hour
    Hour

    The hour is a unit of time. It is not an SI unit but is Non-SI units accepted for use with SI....
    s 51 minute
    Minute

    A minute is a unit of measurement of time or of angle.The minute is a Unit of measurement of time equal to 1/60th of an hour or 60 seconds. In the Coordinated Universal Time time scale, a minute occasionally has 59 or 61 seconds; see leap second....
    s.
  2. Atmospheric tides propagate in an atmosphere where density varies significantly with height
    Height

    Height is the measurement of vertical distance, but has two meanings in common use. It can either indicate how "tall" something is, or how "high up" it is....
    . A consequence of this is that their amplitude
    Amplitude

    Amplitude is the magnitude of change in the oscillating variable, with each oscillation, within an oscillating system. For instance, sound waves are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation....
    s naturally increase exponentially as the tide ascends into progressively more rarefied regions of the atmosphere (for an explanation of this phenomenon, see below). In contrast, the density of the oceans varies only slightly with depth
    Depth

    The term Depth may refer to: How deep something is. How far down.* Depth perception, 3d shapes* Depth of moral character* Depth in a well* Depth of a river...
     and so there the tides do not necessarily vary in amplitude with depth.


At ground level, atmospheric tides can be detected as regular but small oscillations in surface pressure with periods of 24 and 12 hours. However, at greater heights the amplitudes of the tides can become very large. In the mesosphere
Mesosphere

The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. The mesosphere is located from about 50 km to 80-90 km altitude above the Earth's surface....
 (heights of ~ 50 - 100 km) atmospheric tides can reach amplitudes of more than 50 m/s and are often the most significant part of the motion of the atmosphere.

The reason for this dramatic growth in amplitude from tiny fluctuations near the ground to oscillations that dominate the motion of the mesosphere lies in the fact that the density of the atmosphere decreases with increasing height. As tides or waves propagate upwards, they move into regions of lower and lower density. If the tide or wave is not dissipating, then its kinetic energy
Kinetic energy

The kinetic energy of an object is the extra energy which it possesses due to its motion. It is defined as the mechanical work needed to accelerate a body of a given mass from rest to its current velocity....
 density must be conserved. Since the density is decreasing, the amplitude of the tide or wave increases correspondingly so that energy is conserved. The amplitude of a wave at a height of z can thus be described by the equation:

where is the initial amplitude of the wave, is height and is the scale height
Scale height

A scale height is a term often used in scientific contexts for a distance over which a quantity decreases by a factor of e . It is usually denoted by the capital letter H....
 of the atmosphere.

Following this growth with height atmospheric tides have much larger amplitudes in the middle and upper atmosphere than they do at ground level.

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
Square wave

A square wave is a kind of non-sinusoidal waveform, most typically encountered in electronics and signal processing. An ideal square wave alternates regularly and instantaneously between two levels....
 profile and so is rich in harmonics. When this pattern is decomposed into separate frequency components using a fourier transform
Fourier transform

In mathematics, Fourier analysis is a subject area which grew out of the study of Fourier series. The subject began with trying to understand when it was possible to represent general functions by sums of simpler trigonometric functions....
, 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.

Solar energy is absorbed throughout the atmosphere some of the most significant in this context are water vapor
Water vapor

Water vapor or water vapour , also aqueous vapor, is the gas phase of water . Water vapor is one Phase of the water cycle within the hydrosphere....
 at (~0 - 15 km) in the troposphere
Troposphere

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% of the atmosphere's mass and almost all of its water vapor and particulate....
, ozone
Ozone

Ozone or trioxygen is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic O2....
 at (~30 to 60 km) in the stratosphere
Stratosphere

The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down....
 and molecular oxygen and molecular nitrogen at (~120 to 170 km) in the thermosphere
Thermosphere

The thermosphere is the layer of the earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization....
. 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.

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 Earth's 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.

The migrating solar tides have been extensively studied both through observations and mechanistic models.

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
Topography

Topography is the study of Earth's surface shape and features or those ofplanets, Natural satellite, and asteroids. It is also the description of such surface shapes and features ....
 with longitude, land-sea contrast and surface interactions.

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
South America

South America is the southern continent of the Americas, situated entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere....
, Africa
Africa

Africa is the world's second-largest and second most-populous continent, after Asia. At about 30.2 million km? including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area....
 and Australia
Australia

Australia, officially the Commonwealth of Australia, is a country in the southern hemisphere comprising the Australia of the world's smallest continent, the major island of Tasmania, and numerous list of islands of Australia in the Indian Ocean and Pacific Oceans....
. 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


Damping
Damping

Damping is any effect, either deliberately engendered or inherent to a system, that tends to reduce the amplitude of oscillations of an oscillatory system....
 of the tides occurs primarily in the lower thermosphere region, and may be caused by turbulence
Turbulence

In fluid dynamics, turbulence or turbulent flow is a fluid regime characterized by chaotic, stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time....
 from breaking gravity wave
Gravity wave

In fluid dynamics, gravity waves are waves generated in a fluid medium or at the Interface between two media which has the restoring force of gravity or buoyancy....
s. A similar phenomena to ocean waves breaking on a beach
Beach

File:MiamiSouthBeachPanoramaEdit.jpgA beach is a geology landform along the shoreline of a body of water. It usually consists of loose particles which are often composed of Rock , such as sand, gravel, shingle beach, pebbles, or cobble....
, the energy
Energy

In physics, energy is a scalar physical quantity that describes the amount of Work_ that can be performed by a force. Energy is an attribute of objects and systems that is subject to a conservation law....
 dissipates into the background atmosphere. Molecular diffusion
Diffusion

Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion....
 also becomes increasingly important at higher levels in the lower thermosphere as the mean free path
Mean free path

In physics the mean free path of a particle is the average distance covered by a particle between subsequent impacts....
 increases in the rarefied atmosphere.

Effects of Atmospheric Tide

The tides form an important mechanism for transporting energy input into the lower atmosphere from the upper atmosphere, while dominating the dynamics of the mesosphere and lower thermosphere. Therefore, understanding the atmospheric tides is essential in understanding the atmosphere as a whole. Modeling and observations of atmospheric tides are needed in order to monitor and predict changes in the Earth's atmosphere.

See also

  • Tide
    Tide

    Tides are the rising of Earth's ocean surface caused by the tidal forces of the Moon and the Sun acting on the oceans. Tides cause changes in the depth of the marine and estuary water bodies and produce oscillating currents known as tidal streams, making prediction of tides important for coastal navigation ....
  • Earth tide
    Earth tide

    Earth tide is the sub-meter motion of the Earth of about 12 hours or longer caused by Moon and Sun gravitation, also called body tide which is the largest contribution globally....
  • Mesosphere
    Mesosphere

    The mesosphere is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the thermosphere. The mesosphere is located from about 50 km to 80-90 km altitude above the Earth's surface....
  • Thermosphere
    Thermosphere

    The thermosphere is the layer of the earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization....