Chapter3 Flashcards
Apart from special waves such as
tsunamis
Apart from special waves such as tsunamis, the only thing that produces the waves we see on our coasts is the
action of the wind blowing over the sea surface
Waves arriving on a coast can be generated by
- local wind, in ‘real time’, in which case the waves are called windsea, or
- they can be the result of a wind that blew over the surface of the ocean thousands of kilometres away, up to several days before, in which case they are termed swell or groundswell.
A swell
in the context of an ocean, sea or lake, is a series of mechanical waves that propagate along the interface between water and air and so they are often referred to as surface gravity waves.
Swell waves often have a
long wavelength
Swell waves often have a long wavelength but
this varies due to the:
- size,
- strength and
- duration of the weather system responsible for the swell and the size of the water body
Swell waves often have a long wavelength but this varies due to the size, strength and duration of the weather system responsible for the swell and the size of the water body, e.g.
wavelengths are rarely more than 150m in the Mediterranean
Swell waves often have a long wavelength but this varies due to the size, strength and duration of the weather system responsible for the swell and the size of the water body, e.g. wavelengths are rarely more than 150m in the Mediterranean. Swell wavelength, also,
varies from event to event
swells which are …………………………………… occure as a result of severe storms like ……………………..
longer than 700 m
tropical cyclones
Swells have a
narrower range of frequencies and directions than locally generated wind waves, because swell waves have dispersed from their generation area, taking on a more defined shape and direction.
Swell direction is the
direction from which the swell is coming
Swell direction is the direction from which the swell is coming. It is measured in
degrees (as on a compass)
Swell direction is the direction from which the swell is coming. It is measured in degrees (as on a compass), and often referred to in
general directions, such as a NNW or SW swell.
Large breakers one observes on a beach may result from
distant weather systems over a fetch of ocean
Five factors influence the formation of wind waves, which will go on to become ocean swell:
- Wind speed or strength relative to wave speed — the wind must be moving faster than the wave crest for energy transfer, stronger prolonged winds create larger waves
- The uninterrupted distance of open water over which the wind blows without significant change in direction (called the fetch)
- Width of area affected by fetch
- Wind duration — the time over which the wind has blown over a given area
- Water depth
All of these factors work together to determine the size of wind waves:
- Wave height (from trough to crest)
- Wave length (from crest to crest)
- Wave period (time interval between arrival of consecutive crests at a stationary point)
- Wave propagation direction
A fully developed sea has the ………………………………………… theoretically possible for a wind of a
maximum wave size
specific strength, duration, and fetch
A fully developed sea has the maximum wave size theoretically possible for a wind of a specific strength, duration, and fetch. Further exposure to that specific wind could only cause
a loss of energy due to the breaking of wave tops and formation of “whitecaps”
Waves in a given area typically have
a range of heights
Sea water wave is generated by
many kinds of disturbances such as Seismic events, gravity, and crossing wind
The generation of wind wave is initiated by the
disturbances of cross wind field on the surface of the sea water
Two distinct mechanisms have been proposed to explain the means by which the wind is capable of
generating waves and perturbations on the surface of oceans
Two distinct mechanisms have been proposed to explain the means by which the wind is capable of generating waves and perturbations on the surface of oceans:
The Kelvin–Helmholtz instability (after Lord Kelvin and Hermann von Helmholtz) can occur when
there is velocity shear in a single continuous fluid, or where there is a velocity difference across the interface between two fluids.
The Kelvin–Helmholtz instability (after Lord Kelvin and Hermann von Helmholtz) can occur when there is velocity shear in a single continuous fluid, or where there is a velocity difference across the interface between two fluids. An example is
wind blowing over water: The instability manifests in waves on the water surface. More generally, clouds, the ocean, Saturn’s bands, Jupiter’s Red Spot, and the sun’s corona show this instability.
KHI requires a minimum wind speed of
6 m s-1 to make waves grow against the competing effects of gravity and surface tension.
KHI requires a minimum wind speed of 6 m s-1 to make waves grow against the competing effects of gravity and surface tension. Thus, whilst KHI is relevant to the generation of
large wavelength perturbations
it is the Miles-Phillips Mechanism which is relevant to
low wind speeds, and short-wavelength perturbations.
Thus, whilst KHI is relevant to the generation of large wavelength perturbations, it is the Miles-Phillips Mechanism which is relevant to
low wind speeds, and short-wavelength perturbations.
Thus, whilst KHI is relevant to the generation of large wavelength perturbations, it is the Miles-Phillips Mechanism which is relevant to low wind speeds, and short-wavelength perturbations. In particular, the Miles-Phillips Mechanism involves a
resonant interaction between the surface of the water and turbulent fluctuations in the air.
To produce waves
the air moving over the surface of the water has to somehow transmit its energy to the water.
To produce waves, the air moving over the surface of the water has to somehow transmit its energy to the water. Just how this happens is a
very complicated process, still not well understood.
