oceanographic characteristic

LDD 10102INTRODUCTION TO SHIP

TECHNOLOGY

OCEANOGRAPHIC CHARACTERISTIC

SECTION 1

CHAPTER 1

OCEANOGRAPHIC CHARACTERISTIC

WIND

WAVE

TIDES

ICEBERG

CURRENT

MARINE INDUSTRY

WINDDefinitions:-

Wind is a large scale movement of air caused by differences in atmospheric pressure between localities.

Strong winds can add to the resistance a ship make manoeuvring difficult. The wave characteristics depend upon the strength of wind, its duration and the distance over which it acts, which is called its fetch. Beam wind will make a ship heel and create waves. Wind can be measured using Beaufort Wind Scale.

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Beaufort Number(force)

Wind SpeedWave Height (feet)

WMO* description

Effects observed on the seaknots mph

0 under 1 under 1 - Calm Sea is like a mirror

1 1 - 3 1 - 3 0.25 Light air Ripples with appearance of scales; no foam crests

2 4 - 6 4 - 7 0.5 - 1 Light breeze Small wavelets; crests of glassy appearance, not breaking

3 7 - 10 8 - 12 2 - 3 Gentle breeze Large wavelets; crests begin to break; scattered whitecaps

4 11-16 13-18 3½ - 5 Moderate breeze

Small waves, becoming longer; numerous whitecaps

5 17-21 19-24 6 - 8 Fresh breeze Moderate waves, taking longer form; many whitecaps; some spray

6 22-27 25-31 9½-13 Strong breeze Larger waves forming; whitecaps everywhere; more spray

7 28-33 32-38 13½-19 Near gale Sea heaps up; white foam from breaking waves begins to be blown in streaks

8 34-40 39-46 18-25 Gale Moderately high waves of greater length; edges of crests begin to break into spindrift; foam is blown in well-marked streaks

9 41-47 47-54 23-32 Strong gale High waves; sea begins to roll; dense streaks of foam; spray may begin to reduce visibility

10 48-55 55-63 29-41 Storm Very high waves with overhanging crests; sea takes white appearance as foam is blown in very dense streaks; rolling is heavy and visibility is reduced

11 56-63 64-72 37-52 Violent storm Exceptionally high waves; sea covered with white foam patches; visibility further reduced

12 64 and over

73 and over

45 and over

Hurricane Air filled with foam; sea completely white with driving spray; visibility greatly reduced

Figure : Beaufort Wind Scale

Gale

Storm

Strong gale

Violent storm

Hurricane

Wind Measurement

Wind direction is measured in a clockwise direction from North. 0˚/ 360˚ (N),45˚(NE),90˚(E),135˚(SE),180˚(S),225˚(SW),270˚ (W),315˚ (NW).

Wind velocities are measured in a horizontal plane, although a definite vertical gust component exists.

The term ‘velocity’ indicates direction and speed. Anemometer was used to measure speed and the direction of the

wind.

Figure: Wind Direction

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Figure 3: Anemometer

DigitalAnalog

3 Basic Wind Forms

There are three basic wind forms experienced around coastline:-

Frontal

Wind

Pressure

Wind

Land and Sea breezes.

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Frontal Wind CHAPTER 1 (Sec. 1)

This can be the most severe of all, although it usually doesn’t last long. Associated mostly with the onset of a cold front, this wind will blow very

hard and very strong initially and come in with a sudden impact It will blow itself out very quickly, the worst often being over in less than

an hour.

Figure 4: Frontal Wind

If the cold front is severe, there is usually prior warning in the form of sky signs. A build up of cumulonimbus clouds, often with lightning and thunder, appears in the Southern and Western skies

This is the proverbial calm before the storm, because within minutes the frontal wind will race down in a line across the water, at speeds probably somewhere between 40 and 60 knots.

Temperature will drop considerably as the cold air comes in, there may be rain, hail, lightning and thunder, and the wind will blow hard for probably 20 minutes to half an hour before it begins to ease. It will probably blow itself out within the hour.

Pressure Wind Formed by the circulation of air around pressure systems, these are

the most common winds of our everyday life. They are fairly predictable in that they rotate around high and low pressure systems.

On the weather map the isobars, or lines of pressure, indicate their direction and approximate intensity:

winds circulating clockwise around a high-pressure system in the Northern Hemisphere

winds circulating anti-clockwise around a low-pressure system in the Northern Hemisphere

vice versa in the Southern Hemisphere.

Northern Hemisphere Isobars

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Weather Report (Satellite Image)Weather Report (Satellite Image)

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Land & Sea Breezes

Land Breezes

Sea Breezes

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Sea Breeze

A sea-breeze (or onshore breeze) is a wind from the sea that develops over land near coasts. It is formed by increasing temperature differences between the land and water which create a pressure minimum over the land due to its relative warmth and forces higher pressure, cooler air from the sea to move inland. Generally, air temperature gets cooler relative to nearby locations as one moves closer to a large body of water.

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Land Breeze

At night, the land cools off quicker than the ocean due to differences in their specific heat values, which forces the dying of the daytime sea breeze. If the land cools below that of the adjacent sea surface temperature, the pressure over the water will be lower than that of the land, setting up a land breeze as long as the environmental surface wind pattern is not strong enough to oppose it. If there is sufficient moisture and instability available, the land breeze can cause showers or even thunderstorms, over the water.

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Wind Vector Wind Vectors indicate wind

direction and speed. The black arrows plotted on this

image are wind vectors These vectors indicate direction

and intensity of the wind. The vectors point in the direction to

which the wind is blowing. (This image, winds are primarily blowing from west to east).

Intensity of the wind is conveyed through the size of the vector. The longer the arrows, the stronger the winds.

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