Saturday, May 3, 2008

Sea State And Wind

The sea state descriptions that follow are for steady state conditions, which result when the wind has been blowing from the same direction for a relatively long time over a sufficiently long fetch (the distance the wind has blown across the water without interruption). For a given wind speed and duration, the longer the fetch, the greater the sea disturbance. Depth of water also effects the appearance of the sea waves running into shallow water become steeper and are more likely to break, which may result in an overestimate of wind speed.
The sea criterion is for use over relatively deep water only.
There are other conditions when wind speed may be higher or lower than indicated by the sea state. Heavy rain and floating ice will have a dampening effect on the sea surface. Wind blowing against (opposite) a tide or strong current causes a greater sea-wave height and sea-distur­bance than normal, while wind blowing in the same direction as a tide or strong current causes a smaller sea-wave height and sea-disturbance than normal. There is a lag period between the wind increasing or decreasing, and the sea wave height rising or falling. This is especially pronounced during a sudden change in wind speed. The presence of swell may cause more whitecaps to form, because wind waves have a greater tendency to break when superimposed on the crest of swell.
To distinguish sea from swell, remember that sea waves are generat­ed by the wind blowing at the time of observation, or in the recent past, in your local area. Swell waves have travelled into your area of observa­tion, after having been generated by winds in other areas (sometimes thousands of miles away). As sea waves move out from under the wind that produces them and become swell, their character changes. The crests become lower and more rounded, and they move in trains of simi­lar period and height. Swell is more symmetrical and uniform than sea,andwill have a longer period.
Alto­ - Cloud prefix meaning middle level.

Altocumulus - Middle level white or gray patch, sheet, or layer of cloud, composed of rounded masses, rolls, etc., which mayor may not be merged. Mainly composed of water droplets, some­times partly fibrous (ice).

Altostratus - Middle level grayish or bluish sheet or layer of striated, fibrous, or uniform appearance.

Anemometer - An instrument for measuring wind speed and direction. Typically 3 or 4 rotating cups measure speed, and a vane indicates direction.

Beaufort Wind Scale - A numerical scale of wind force originally designed by Admiral Francis Beaufort in the mid-19th century. It consists of sea-state descriptions correlated with ranges of wind speed.

Cirro - Cloud prefix meaning high level.

Cirrocumulus - High level thin white patch, sheet, or layer of cloud, composed of very small elements like ripples, grains, etc.

Cirrostratus - High level transparent, whitish cloud veil of fibrous (hairlike), or smooth appearance, often producing a halo.

Cirrus - High level clouds of ice crystals in the form of delicate white filaments, or white patches or narrow bands, with fibrous appearance of silky sheen.

Cloud Height - The height of the base of the cloud or cloud layer above the sea or land surface.

Cloud Layer - A grouping of clouds whose bases are at approximately the same level.

Convection - Vertical air movement in unstable air masses resulting in the development

of cumulus clouds.

Cloud Type - A cloud form identified as distinct according to the World Meteorological Organization.

Crest - The highest part of the wave.

Cumulonimbus - A heavy, dense cloud with considerable vertical extent, in the form of a mountain or huge tower. Part of the upper portion may be smooth or fibrous.

Cumulus - Detached clouds, generally dense and with sharp outlines, developing vertically in the form of rising mounds, domes, or towers, of which the bulging upper part may resemble a cauli­flower. The sunlit parts are brilliant white, the bases can be dark and nearly horizontal.

Fetch - The distance the wind has blown across the water without interruption.

Fracto - Cloud prefix meaning torn, ragged, or scattered appearance due to strong winds.

Freak Wave - A wave of great height and steepness, much higher than other waves in the prevailing sea or swell system.

Gust - Sudden brief wind increase followed by lull or slackening. One nautical mile per hour or .5 meters/second.

Lenticular - A type of cloud formed in the ascending portion of an airstream, which remains stationary while the air blows through it.

Nimbostratus - Gray cloud layer, often dark, thick enough to block out the sun, which appears diffuse by falling precipitation.

Ripple - A small wavelet that forms at wind speed of 1-3 knots.

Sea - Locally generated waves produced by the wind.

Sea Disturbance - Waves, whitecaps, spindrift, foam, etc. on the ,Sea surface.

Strato - Cloud prefix referring to cloud sheets or layers.

Stratocumulus - Lower level gray or whitish patch, sheet, or layer of cloud, usually with dark parts, with rounded masses or rolls, which may not be merged.

Stratus - Generally gray lower level cloud layer with a fairly uniform base, which may produce drizzle or snow grains.

Swell - Ocean waves which have traveled beyond the generating area, which have riot been produced by the local wind. They have longer periods than sea, and are more regular and uniform.

Trough - The lowest part of the wave.

True Direction - Direction measured in degrees clockwise from true north, where north is 0°.

Wave Height - Distance from trough to crest, averaged for the. better formed waves in the group.

Wave Length - Distance from trough to trough or crest to crest for adjacent waves.

Wave Period - Time, in seconds for the passage of successive wave crests. Normally computed as an average value for several waves.

White Cap - The breaking crest of a wave, usually white and frothy.

Wind - The horizontal motion of the air past a given point.

Wind Direction - The true direction from which the wind is blowing at a given location.

Wind Speed - The rate at which the air is moving horizontally past a given point. Wind speed estimates are usually made by relating the state of the sea to the Beaufort Scale of wind force.