Ondes de pression au-dessus de la Mer d'Arabie

meteorology Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...

, an undular bore is a

wave In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (r ...

disturbance in the Earth's

atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. ...

and can be seen through unique cloud formations. They normally occur within an area of the atmosphere which is stable in the low levels after an

outflow boundary An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air ( outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually ...

or a

cold front A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern ...

moves through. In

hydraulics Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid coun ...

, an undular bore is a gentle bore with an undular

hydraulic jump A hydraulic jump is a phenomenon in the science of hydraulics which is frequently observed in open channel flow such as rivers and spillways. When liquid at high velocity discharges into a zone of lower velocity, a rather abrupt rise occurs in ...

pattern at the downstream ( subcritical) side.

In meteorology

Overview

Undular bores are usually formed when two air masses of different temperatures collide. When a low level boundary such as a cold front or outflow boundary approaches a layer of cold, stable air, it creates a disturbance in the atmosphere producing a wave-like motion, known as a

gravity wave In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media when the force of gravity or buoyancy tries to restore equilibrium. An example of such an interface is that between the atmosphere an ...

. Although the undular bore waves appear as bands of

clouds In meteorology, a cloud is an aerosol consisting of a visible mass of miniature liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals ma ...

across the sky, they are

transverse waves In physics, a transverse wave is a wave whose oscillations are perpendicular to the direction of the wave's advance. This is in contrast to a longitudinal wave which travels in the direction of its oscillations. Water waves are an example of t ...

, and are propelled by the transfer of energy from an oncoming storm and are shaped by gravity. The ripple like appearance of this wave is described as the disturbance in the water when a pebble is dropped into a pond or when a moving boat creates waves in the surrounding water. The object displaces the water or

medium Medium may refer to: Science and technology Aviation *Medium bomber, a class of war plane * Tecma Medium, a French hang glider design Communication * Media (communication), tools used to store and deliver information or data * Medium of ...

the wave is travelling through and the medium moves in an upward motion. However, because of gravity, the water or medium is pulled back down and the repetition of this cycle creates the transverse wave motion. The undular bore's wavelength can measure peak to peak and can travel to . The medium it travels through is the

. There are several varying types of ‘‘bores’’ in different layers of the atmosphere, such as the mesospheric bore which occurs in the

mesosphere The mesosphere (; ) is the third layer of the atmosphere, directly above the stratosphere and directly below the thermosphere. In the mesosphere, temperature decreases as altitude increases. This characteristic is used to define its limits: it ...

Occurrences

Rare but not unknown in a great many locations, the waves appear with some predictability and regularity in the Gulf of Carpentaria during Spring. They have been seen as frequently as six days in a row according to reports by the two pilots who have most experience with soaring these sometimes enormous examples of the undular bore, known in Australia as the Morning Glory cloud.

In hydraulics

Undular bore Araguari River-Brazil-USGS-bws00026

The term "bore" is also used to describe positive surges advancing in shallow waters. When the surge's

Froude number In continuum mechanics, the Froude number (, after William Froude, ) is a dimensionless number defined as the ratio of the flow inertia to the external field (the latter in many applications simply due to gravity). The Froude number is based on ...

is less than 1.4 to 1.7 (i.e. above unity and below a number somewhere in the range 1.4 to 1.7), the advancing front is followed by a train of well-defined free-surface undulations (called "''whelps''"). The surge is then called an undular surge or undular bore. The undulations form a

standing wave In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect ...

pattern, relative to the undular bore front. So, the

phase velocity The phase velocity of a wave is the rate at which the wave propagates in any medium. This is the velocity at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave (for example, ...

(propagation velocity relative to still water) of the undulations is just high enough to keep the undulations stationary relative to the bore front. Now, in

water wave In fluid dynamics, a wind wave, water wave, or wind-generated water wave, is a surface wave that occurs on the free surface of bodies of water as a result from the wind blowing over the water surface. The contact distance in the direction of t ...

s, the

group velocity The group velocity of a wave is the velocity with which the overall envelope shape of the wave's amplitudes—known as the ''modulation'' or ''envelope'' of the wave—propagates through space. For example, if a stone is thrown into the middl ...

(which is also the energy-transport velocity) is less than the

. Therefore, on average, wave energy of the undulations is transported away from the front, and contributing to the energy loss in the region of the front. A related occurrence of positive surges is the

tidal bore Tidal is the adjectival form of tide. Tidal may also refer to: * ''Tidal'' (album), a 1996 album by Fiona Apple * Tidal (king), a king involved in the Battle of the Vale of Siddim * TidalCycles, a live coding environment for music * Tidal (servic ...

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environme ...

References

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External links