Glossary

Beaufort Wind Force Scale

A thirteen-step scale of wind speeds (Force 0 to Force 12) based on observations of the effects at sea and on land.

Coriolis effect

The apparent tangential acceleration of an object moving towards or away from an axis around which it is moving.

Reynolds number

A dimensionless number that indicates the relative importance of inertial and viscous forces and also the likelihood of turbulent flow. Reynolds numbers are frequently used to compare flow patterns.

amphidromic point

A point with no tidal rise or fall. Also called a tidal node.

amphidromic system

A system (e.g. the North Sea or the Sound of Jura) containing an amphidromic point.

amplitude

The maximum extent of a vibration, oscillation or periodic function, measured from the position of equilibrium or centre line. The amplitude of a sinusoidal curve is half the difference between the maximum and minimum values of the curve.

anticyclones

The large-scale atmospheric rotation around an area of high pressure. Anticyclonic rotation is clockwise in the northern hemisphere and anti-clockwise south of the equator.

atmospheric boundary layer

The region of the atmosphere, up to about 1000 m above ground level, in which interaction with the ground significantly affects wind speed and direction.

break

A water wave breaks when it changes from approximately sinusoidal in shape to hooked prior to the crest collapsing. Waves may break as a result of wind action, moving into shallower water or a combination of both.

crest

The top of a water wave.

currents

Bulk flow in the atmosphere or oceans.

cyclones

The large-scale atmospheric rotation around an area of low pressure. Cyclonic rotation is anti-clockwise in the northern hemisphere and clockwise south of the equator. A cyclone is also the name for a hurricane when south of the equator.

depression

An area of atmospheric low pressure. Also called a low.

design wave

The likely worst-case wave height, used as a factor in the design of structures that interact with the sea.

design wind speed

The likely highest wind speed, used as a factor in the design of structures that interact with the atmosphere.

double tides

Two high tides in close succession with a small drop in between (or two low tides in close succession with a small rise in between).

drag coefficient

A non-dimensional form of drag: the drag force produced as a fraction of the product of stagnation pressure and a characteristic area. See also lift coefficient.

ebb

The receding or downward-going tide.

fetch

The distance over which wind can build up waves at sea.

flood

The approaching or upward-going tide.

geostrophic winds

Winds above the atmospheric boundary layer, driven by pressure gradients and Coriolis forces. Also called gradient winds.

gradient winds

See geostrophic winds.

ground level to reference height

The reference height of 10 m used for modelling wind variation in the atmospheric boundary layer.

high tides

The time at which the sea reaches its greatest depth in a particular tidal cycle; also the depth at that time.

isothermal height

See tropopause.

low

See depression.

low tides

The time at which the sea reaches its smallest depth in a particular tidal cycle; also the depth at that time.

neap tides

The time of lowest tidal range, when moon and sun work in opposition.

obliquity

The tilt of the Earth’s axis relative to a normal to the plane in which it orbits the sun.

orbit

The path followed by a body moving round another under the influence of gravity.

peak

The highest point of a wave.

ripples

Surface waves in water (or any other liquid) with an amplitude much smaller than the undisturbed depth.

slack water

A time when tidal currents are zero, usually coinciding with high or low tide.

spring tides

The time of highest tidal range, when moon and sun work together.

stratosphere

The upper part of the atmosphere.

surges

Rapid changes of sea level due to tidal effects.

swell

Long-wavelength oceanic waves.

tidal bulge

One of two areas of increased water depth, caused by the gravitational attraction of the sun and moon, which travel around the world and cause tides.

tidal currents

Horizontal flows of water caused by tidal depth changes.

tidal node

See amphidromic point.

tidal range

The difference in height between a high tide and the preceding or following low tide.

tidal wave

An ocean wave, normally caused by an undersea earthquake, which on approaching land causes a sea level change comparable to that caused by tides.

trade winds

The relatively constant east–west winds that blow in the tropical zones north and south of the equator.

tropopause

The top of the troposphere, above which the atmospheric temperature (in the stratosphere) is effectively constant. Also called isothermal height.

troposphere

The lower part of the atmosphere, in which most weather systems exist.

trough

The lowest part of a surface wave.

tsunami

Japanese term for a tidal wave, derived from ‘tsu’ (harbour) + ‘nami’ (wave).

wave periodic time

The time it takes for any point to experience a full wave cycle.

wave speed

The speed at which a travelling wave advances.

wavelength

The spatial distance over which a periodic waveform repeats (e.g. the distance between successive peaks or successive troughs).

waves

Any regular oscillation of a continuous medium.

westerlies

The relatively constant west–east winds that blow in the temperate zones further north and south of the equator than the tropical zones.