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MARINE METEOROLOGY. WILLI4M ALLINGHAM. NET BOOK.—This book is supplied to the Trade on terms which will not allow of Discount to the Public. The aim of these lecture notes is to provide training to the marine observers in . Software for Marine Meteorological Observers (TURBOWIN version ). Marine meteorology as the specialized, applied part of meteorology uses general meteorology objects of study for marine meteorology and oceanography.

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Kenn has taught Marine Meteorology to sailors for more than 15 years in The study of meteorology whether in the marine scene or ashore, is very much a. A handbook covering all aspects of marine meteorology from synoptic charts, depressions, anti-cyclones, tropical revolving storms, ice, forecasting mariners own. Manual on. Marine Meteorological Services. Volume I. (Annex VI to WMO Technical Regulations). Global Aspects. WMO-No. edition.

Weather ships[ edit ] The weather ship MS Polarfront at sea. Established during World War II, a weather ship , or ocean weather vessel, was a ship stationed in the ocean as a platform for surface and upper air meteorological observations for use in weather forecasting. They were used during World War II but had no means of defense, which led to the loss of several ships and many lives. They were primarily located in the north Atlantic and north Pacific oceans, reporting via radio. In addition to their weather reporting function, these vessels aided in search and rescue operations, supported transatlantic flights , [32] [33] acted as research platforms for oceanographers , [34] [35] [36] monitored marine pollution , [37] and aided weather forecasting both by weather forecasters and within computerized atmospheric models.

Established during World War II, a weather ship , or ocean weather vessel, was a ship stationed in the ocean as a platform for surface and upper air meteorological observations for use in weather forecasting. They were used during World War II but had no means of defense, which led to the loss of several ships and many lives.

They were primarily located in the north Atlantic and north Pacific oceans, reporting via radio. In addition to their weather reporting function, these vessels aided in search and rescue operations, supported transatlantic flights , [32] [33] acted as research platforms for oceanographers , [34] [35] [36] monitored marine pollution , [37] and aided weather forecasting both by weather forecasters and within computerized atmospheric models.

Research vessels remain heavily used in oceanography, including physical oceanography and the integration of meteorological and climatological data in Earth system science. Moored buoys have been in use since , [39] while drifting buoys have been used since Wind data from buoys has smaller error than that from ships. In use since , the weather satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth.

Satellites can be polar orbiting , covering the entire Earth asynchronously, or geostationary , hovering over the same spot on the equator. Beginning with the Nimbus 3 satellite in , temperature information through the atmospheric column began to be retrieved by satellites from the eastern Atlantic and most of the Pacific Ocean, which led to significant forecast improvements. See Photos 7 and 8. Turbulence can occur from a variety of causes anywhere in the troposphere. Fairly dry air gives a higher.

In stable atmosphere the cloud formed will be stratiform. In unstable atmosphere cloud will be cumuliform. Main causes of initial uplift of air 1 Thermal uplift has been described earlier in this chapter and is the result of the air temperature being raised through contact with a warmer surface.

The actual height to which this turbulence can extend depends on the nature of the surface and the force of the wind. When the horizontal inflow of air into an area exceeds the horizontal outflow. In such cases the weather on the lee side is relatively warm and dry.

Cape Town. More often than not the cloud structures of the warm front are of layer type. Orographic cloud can be either stratiform or cumuliform depending on whether the rising air is stable or unstable after passing the condensation level.

Frontal uplift is fully explained in Chapter For example. A similar effect sometimes occurs at Gibraltar. Cloud will not form unless the air is lifted above the condensation level. Orographic uplift of warm moist air can produce very heavy rain. Except in arid regions convergence is generally associated with much cloud and precipitation.

Values for temperature and height are not required. State what cloud types are associated with each. Name and describe the five main modes of initial uplift of air. Describe two situations in which it is commonly formed. Meteorologists refer to all of these phenomena as hydrometeors. Cloud, fog and mist are not classed as precipitation but are hydrometeors. The difference between rain and drizzle is only that the drops in drizzle are relatively very small diameter between 0.

They fall slowly and gently from low based stratus cloud. Unless the relative humidity is high beneath the cloud base the drops are likely to evaporate before reaching the surface. Rain and drizzle Formation Raindrops vary in size but they are all larger than the tiny droplets or ice particles of which clouds are composed; to turn these into rain appreciable convection vertical movement within the cloud is necessary. When convection is active within cloud the water droplets are carried up to greater heights and the process of cooling and condensation continues.

A proportion of the droplets will increase in size due to either:. Whatever the formation process rain is nearly always created in clouds of appreciable vertical extent. The greater the vertical thickness of the cloud the larger the raindrops. Thus drizzle may fall from quite shallow cloud. When the droplets are large and heavy enough to overcome the upward motion of air they will commence to fall. During descent through cloud they will continue to increase in size due to collision with the rising cloud droplets, until they fall as rain from the base of the cloud.

Some evaporation takes place in the warmer unsaturated air below the cloud base; if the falling drops are large enough in both size and number they will reach the surface. The dark vertical or trailing streaks of precipitation seen falling from the base of a cloud, and which do not reach the surface, are called virga or fallstreaks.

Classification of rain There are three main types: Sea surface temperatures undergo very little change in temperature during the course of a day see Chapter 2 , but moisture-laden air moving over a relatively very warm sea surface will often produce convectional rain, usually in the form of isolated showers, sometimes heavy with hail and thunder, especially in tropical regions.

It is usually heaviest on the weather slopes and may be very light or negligible on the leeward side. This type of rain can be exceptionally heavy and persistent if given suitable conditions: When sea winds cross a coast, surface friction on forested land is considerable and forms a barrier of air over which the oncoming air is forced to rise and sometimes causes precipitation.

Details are given in Chapter Snow and sleet Formation When water vapour condenses at temperatures well below freezing point it forms minute ice crystals which, during their very slow fall through cloud, build up a growth of feathery crystals forming snowflakes.

The size of snowflakes depends on temperature. In very low temperatures the ice crystals do not unite to form snowflakes, but may do so on reaching lower levels of the cloud where the temperatures are less cold. Thus the lower the temperature the smaller the snow flakes which reach the surface. For snow to reach the ground, air temperature near the surface must be lower than 3. Whether the snow lies or not depends mainly on the temperature of the surface on which it falls.

Heavy snow can also seriously affect visibility. Hail Hail falls from cumulonimbus cloud in the form of hard ice pellets of varying shapes and is often associated with thunderstorms. Formation Vigorous convection currents may carry supercooled water drops see Glossary up to a height where ice crystals are present and are supported by strong updrafts.

The ice particles grow in size by collision and coalescence with the supercooled water drops which freeze instantly on impact thereby forming pellets of white opaque ice called soft hail. When the pellets become large enough they will commence to fall and continue to grow. Due to the strong turbulent eddies, which are a characteristic of cumulonimbus clouds, some hailstones make several upward and downward journeys between the upper and lower levels of the cloud before finally falling to earth.

This would account for the concentric structure of very large hailstones which when cut in half may be seen to be made up of alternate layers of opaque and clear ice. In winter when the freezing level is well below the cloud base, above which all water drops must be supercooled, there will be no coating of clear ice on the hailstones. Size of hailstones On reaching the surface the size of hailstones depends mainly on the vertical extent of the cloud in which they are formed and the strength of the upcurrents within it.

Usually they measure only a few millimetres in diameter. In some hot, moist regions of the world hailstones larger than cricket balls and weighing 1 to 2 kg have been reported. Glazed frost This, as the name suggests, is a layer of ice which looks like glass.

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Rain or drizzle falling from the cloud associated with a warm front will freeze immediately on contact with the cold surface and other cold objects, coating everything with smooth clear ice. This form of ice can also be produced by fog droplets freezing onto cold objects. It is occasionally confused with black frost see Glossary.

Sea spray The most dangerous form of icing encountered at sea is produced by sea spray freezing onto the vessel. Ice from this source can accumulate very rapidly and can pose a severe threat to stability, particularly of small vessels. If the air temperature is below this, sea spray landing on the superstructure will freeze, producing a coating of ice.

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Significant amounts of spray are not generally present until wind speed reaches Force 5 and the rate of icing increases with increasing wind speed above this force. Dew A deposit of water formed by condensation on surfaces which have been cooled by radiation to a temperature below that of the dew point of the air.

Favourable conditions are a calm night with a clear sky and high relative humidity. The deposit is rough in appearance and grows to windward of the object. In temperate zones thunderstorms may occur at any time of the year during the passage of a cold front.

The latent heat released by condensation within the cloud boosts the upward convection. Its potential dangers to the mariner are its sometimes torrential rain. Causes of thunderstorms The conditions necessary for the formation of thunderstorms are: In the Mediter- ranean.

In temperate latitudes these conditions are most likely to be found in cols and shallow depressions. Thunder The sound resulting from the instantaneous expansion and contraction of the air is known as thunder.

Heavy rain and hail The formation of hail is described in Chapter 6. Lightning flash This is an electric spark on a gigantic scale. The rumbling effect which we hear is because the Hail and heavy rain.

It renders the air white hot along its channel. Lightning and thunder The intense activity within a Cb cloud results in the build up of tremendous electrical charges. Danger from lightning The risk of a steel ship being struck by lightning is not very great because her masts and other prominent features. Scientific investigations have shown that the upper part of a thunder cloud is charged with positive electricity.

Near the base there is often a small localised region which is positive. Light travels very rapidly and can be treated as though it arrived instan- taneously. Active cold fronts are unstable. Types of thunderstorm Heat thunderstorms These develop over land in warm. They are usually slight and dissipate rapidly on moving inland. The distance to a thunderstorm can be approximated by measuring the time in seconds between seeing the flash of light and the arrival of the sound.

Surface air flows in from all sides thus. Mountainous islands in the tropics are especially prone to these. In temperate latitudes they are most frequent in summer. There are various theories as to the mechanism resulting in the separation of charges within a Cb cloud. When the lightning stroke takes place between cloud and earth. They are most frequent in winter and are caused by a large lapse rate in polar maritime air.

Most of them have good experimental support but it is thought that several of the charging processes operate together and. The distance in miles is found by dividing this figure by 5 and the distance in kilometres is found by dividing the figure by 3.

Coastal thunderstorms This type can occur in any season. Frontal thunderstorms More common in winter in temperate latitudes.

They form at upper levels which leaves little room for development of Cb cloud. General Thunderstorms over the ocean. The arrival of the cold air is associated with a sudden veer in wind direction with a fierce squall and there are violent squalls with large changes in wind direction as the storm passes. The pressure generally rises at the forward edge of the storm and then there is a 'wake low' at the rear of the storm. They are rare in high latitudes due to low temperatures and consequent lack of moisture.

Occlusions sometimes produce thunderstorms. At night it is possible. Passage of a thunderstorm is associated with a sudden fall in temperature. This is due to cold air from high level being dragged down to the surface by falling precipitation.

Except in the doldrums only the frontal type are experienced. The precipitation is localised and may be very heavy. Terminology When the horizontal visibility lies between 1. The term fog is applied when the visibility. The necessary cooling referred to above is caused by: In certain circumstances it may also be caused by the evaporation of water vapour into the air.

Good visibility Good visibility is favoured by air temperatures which are below that of the underlying surface and by strong winds. Formation of fog Fog is formed by the cooling of a large volume of air below its dew point. Generally the daily change in sea surface temperature is less than 0. The warm. It is possible to estimate the likelihood of the formation of fog from obser- vations of air temperatures. There are only certain localities where such conditions are relatively prevalent.

The English Channel is often affected by advection fog when south-westerly winds reach the British Isles from the Azores in spring and early summer. The latter will be below the dew point of the air and normally the wind speed will be between 4 and 16 knots between Force 2 and 4 on the Beaufort scale. In ocean regions.

Admiralty Ocean Routeing Charts give information for each month of the year on: One is off the Grand Banks of Newfoundland where the cold Labrador Current causes a decrease in sea temperature.

A stronger wind will cause the cooling to be diffused through a greater depth of air and the dew point will not be reached. Under these conditions the land loses heat by radiation and cools the air close to the ground.

Usually associated with either a warm front or a warm occlusion when cold air meets warm moist air. Although it never actually forms over the sea. Cloudy skies overnight will reduce the effect of the radiation from the land.

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Since cold air is heavier than warm air. It is caused by the evaporation of relatively warm rain or drizzle which in turn cools the air through which it falls.

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Radiation fog This forms over land. If there is a gentle breeze blowing up to 5 knots. Radiation fog will be most dense around sunrise and normally disperses fairly rapidly as the land warms. Rapid evapora- tion takes place from the relatively warm sea surface into the colder air and condensation takes place.

It is recommended that further reference is made to the appropriate chapter of a specialist textbook dealing with radar. Radar range is likely to be more adversely affected by heavy rain than by fog.

Sand storms may extend up to miles out to sea and constitute a serious problem for the mariner. The watchkeeper must therefore exercise considerable caution when attempting to estimate either the distance from another sound source or its direction.

This is one type of fog which may also be associated with strong winds since it requires a continual supply of cold air. When caused by smoke or dust particles it is described as haze. Use of radar in fog Meteorological factors may affect the normal expected range of radars. If humidity decreases with height.

It is most common in Arctic and Antarctic waters and in the Baltic but it can also occur off the eastern coasts of continents in winter. Causes of the latter range from forest fires. On the other hand.

Why is this so? Isobars An isobar is a line. Horizontal movement of the air is caused by differences in pressure between one point at that level and another. It follows. Isobars are spaced at intervals of one or more hPa. The isobaric patterns which they form enable us to recognise definite pressure systems such as depressions. Unlike the force of gravity which acts vertically downwards.

In modern meteorology pressure is expressed in hPa or millibars. Cause of wind It is important at this stage to remember that atmospheric pressure at any point is exerted equally in all directions. Surface pressure at any one point varies continually. This difference in pressure produces a pressure gradient force.

Fig 9. Calculation of this effect is simplified by using an imaginary force. The path of the air appears to be deflected to the right in the northern hemisphere and to the left in the southern hemisphere. If the isobars are straight and parallel. The geostrophic wind speed This may be found by means of a geostrophic wind scale printed on a synoptic weather chart. Relationship between pressure gradient and wind speed The pressure gradient is the change in pressure with distance.

Pressure gradient is described as steep when the isobars are close together and slack when they are widely spaced. The gradient wind This wind flows parallel to curved isobars. The geostrophic wind speed is found from the curved lines. The greater the pressure gradient the closer the isobars and the stronger the wind. A resultant force is needed to allow the air to travel on a curved path. This results in the gradient wind speed being less than the geostrophic wind speed when circulating around low pressure and more than geostrophic when circulating around high pressure.

Diurnal variation of wind speed at the surface This is caused by diurnal variation in convection currents. Thus the reduction in surface wind force is less by day.

In the middle latitudes in summer the land becomes warmer than the surrounding sea.

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If an area. The diurnal variation of wind speed over sea areas is negligible. During the day. At night. This is because the specific heat capacity of water is higher than that of land. This reduces the total quantity of air over the warm area and so causes pressure to fall at the surface less air. Similarly an inflow of air into an upper level of a cold region will cause surface pressure to rise. See Monsoon in Glossary. Thus surface heating is strong in equatorial latitudes and very weak in polar regions.

Surface pressure is low over north-west India in summer and very high over Siberia in winter. Pressure at an upper level within the column then becomes higher than in the surrounding air at the same upper level. See Figure 9. At this upper level. The air moving towards the areas of lower pressure would be deflected due to the Coriolis force.

At the polar caps they are nearly horizontal during the half-year that the sun is above the horizon. The continent of Asia shows a very marked example of the above.

Idealised pressure distribution and wind circulation on a uniform globe If the surface of the Earth were uniform. The subtropical high pressure belts. The winds. Bear in mind that Figures 9. Take special note of the seasonal changes over Asia and compare the general flow of isobars in the North Indian Ocean with the North Atlantic and North Pacific. Trade winds The Trade winds blow more or less constantly except when monsoons prevail throughout all seasons at a mean speed of around 14 knots and are generally strongest in the late winter.

In the central areas of these anticyclones light variable winds and calms with fine. The Trade wind areas follow slightly the annual movement of the sun. The modification is more significant in the northern hemisphere. These oceanic highs move north and south a little. They are. See Figures 9. The southern hemisphere has a small total land area in comparison to the great expanse of ocean and the wind circulation more nearly conforms to the ideal pattern.

World pressure distribution and prevailing winds Figures 9.

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Vessels which are dependent only on sail for their propulsion can be delayed for long periods in these regions. The pressure gradient extends beyond the North Indian Ocean into the southern hemisphere. The resulting wind circulations tend to persist throughout their particular seasons and are called monsoons. In the North Indian Ocean and western part of the North Pacific the Trade winds disappear completely during the period of the south-west monsoon.

In the southern hemisphere the westerlies blow right round the world with great consistency and frequently attain gale force which gives them the name of Roaring Forties. This fluctuates seasonally. The most developed monsoons occur over southern and eastern Asia. They are further characterised by very heavy convectional rain and thunderstorms. The doldrums of the North Atlantic remain north of the equator throughout all seasons. Winds of the temperate zones Westerly winds predominate on the poleward sides of the oceanic highs.

These stormy areas are easily identified on satellite images. America and Australia. Monsoons Large land masses become heated in summer and.

The reverse takes place in winter. A winter monsoon is cool and dry with mainly fine weather. They occur to a lesser degree in West Africa. Towards the western sides of the oceans the Trade winds tend to flow nearly parallel to one another and finally become easterly in direction. Northern Australia and Indonesia. In the China Sea this summer monsoon is less strong than in the Indian Ocean and the rainfall is comparatively slight.

More often than not it flows between south and east rather than south-west. Winds are south-easterly in winter and north-westerly in summer. The prevailing winds are north-westerly in winter and south-westerly in summer. Typhoons occur frequently. Examples of other monsoons When reading the following refer to Figures 9. In the North Indian Ocean this monsoon is dry and usually brings fine and clear weather.

Tropical cyclones occur in the Indian Ocean and Bay of Bengal. Along the coast of China and Indo-China the pressure gradient is steep and the winds stronger. East coast of Brazil A north-east monsoon blows from September to March. Between January and April. From February to April such periods may persist for over a week. They are disturbed by travelling depressions. In northern winter a large anticyclone is situated over Siberia and the north- east monsoon. In the Indian Ocean it blows as a strong wind reaching gale force at times.

West coast of Africa — Gulf of Guinea. During its long passage over the warm sea it picks up a vast quantity of moisture and gives very heavy orographic rain on the windward coasts of India. The local name for these periods is Crachin. A south-west monsoon blows from June to September. They are experienced in temperate latitudes during warm summer weather but rarely exceed Force 3 and may extend 10 to 15 miles on either side of the coastline.

The air. The warm air over the land rises and is replaced by air flowing in from over the sea. Under such conditions in summer months the land heats up rapidly during the day whilst the sea remains cool. Conversely the wind force along the coast may be considerably increased when the gradient is favourable. At night the process is reversed. Land and sea breezes a diurnal effect occur most frequently and are more pronounced in countries where solar heating is powerful.

In the tropics they sometimes reach Force 5 and may be felt 20 miles from the coast. This sea breeze generally becomes appreciable after midday but in very warm weather may commence earlier if conditions are otherwise favourable.

See Anabatic and Katabatic winds in the Glossary. The air over the sea is displaced by the land breeze and forced upwards. The most favourable conditions for land and sea breezes are anticyclonic. Being local and temporary. If the existing pressure gradient is steep and unfavourable it will completely mask the land and sea breezes.

In tropical areas the land and sea breeze effect is almost routine. The land breeze is much weaker than the sea breeze. Higher up it flows back to the land.

A brief description of each is given in the Glossary. Vendavales East coast of Spain and Gibraltar Strait In the vicinity of the observer both the current wind and sea surface may be calm but there may be experienced a distinct wave motion.

They often have an oily appearance and may have originated thousands of miles away. Characteristics of waves The overall characteristics of sea waves are quite complex but the simple wave is described in these terms: In the open ocean the size of the waves depends also upon the depth of the water.

By contrast. These waves are known as swell waves. Up to a limit. Sea waves and swell Waves caused directly by the wind blowing at the time of observation are known as sea waves.

All this happens to be rather a convenient arrangement for the mariner because. Quite simply. The shape of this wave is described as a trochoid. The height is not specifically related to the other factors because when the steepness exceeds about 1 in 13 the wave will break.

The circular motion decreases rapidly with depth. This shows in elevation a cross-section through a simple wave. Figure In such cases it may be difficult to distinguish sea from swell and synchronism may cause some of the waves to be very large. Wave complications In the open ocean. When a wind starts to blow. The numbered arrows indicate the motion of a particle of water. Wave trochoids Although each wave has a forward motion.

Another complication arises when sea waves and swell waves are present together. Below the surface the water particles take up similar orbits diminishing in size with the depth until. This fact is important in handling a ship in heavy seas. If the period of roll is greater than the period of the wave. Since waves travel in groups. Wave groups Wind-generated waves travel in groups. When the period of roll is less than the period of the wave the ship will tend to align her decks with the slope of the wave.

The behaviour of a vessel depends to a great extent on her period of roll and pitch. A violent motion may result but little water will be shipped.

On very small craft. In large merchant ships the period of roll is greatly in excess of the longest wave periods. A dangerous condition can arise with the waves abeam when the period of roll is the same as the period of the wave.

Synchronisation may result in the ship being rolled over. Fig As the front part of the wave slows the water begins to pile up.

It is important to remember that in relatively shallow and enclosed areas such as the North Sea and the Baltic. When approaching a beach at an oblique angle a wave tends to change its direction so that the advancing edge or front becomes parallel to the beach. Tsunamis are extremely rare in the Atlantic but have been recorded. These waves could be expected to have maximum heights of about 0.

A long swell. The high seas of the Roaring Forties for example are generated by fairly consistent strong winds of virtually unlimited fetch. Indeed the word tsunami is derived from Japanese and means harbour wave. The waves which are produced have small heights and long wavelengths in the deep ocean and travel very rapidly away from their source. Tsunami These waves were also known as tidal waves. Their speed of travel is related to the square root of the depth of the water so the tsunami begins to slow down as it reaches shallow coastal waters.

The maximum wave height recorded to date is 25 metres. Their cause is sudden. Since December Ocean waves in shoaling water When an ocean wave comes into shallow water. The size of waves depends also on the duration of blow. A warning service has been set up to detect possible tsunami- producing events.

Initially the waves are short and steep but if the wind continues to blow from the same direction they gradually become longer. The tsunami. The Pacific is prone to tsunamis because of the seismic activity which takes place there. Tidal races can be hazardous as the seas produced tend to be confused. Very steep and dangerous waves are sometimes experienced with south- westerly winds off the east coast of South Africa south of Durban.

Investigations are still continuing into the apparent complete disappearance of a number of ships. This can produce a wave of unusual height in an otherwise moderate sea. Tidal streams A tide flowing against the wind weather tide will often cause waves to heap up and break at the crest. Practical value of wave data Information about wave performance in the oceans is needed for the following purposes: A lee tide tends to flatten the sea. Such areas are indicated on charts and in sailing directions.

The very strong currents experienced in some tidal races eg the Portland Race and the Alderney Race can make it very difficult to con low-powered craft. Freak waves When swell and waves are moving in different directions. The waves in a race often arrive from several directions with little warning. A more exact method of describing waves when coding weather reports for sending to meteorological services is to report their estimated height and period. Similar phrases are customarily used in weather bulletins for shipping to describe actual and forecast waves.

The following tables giving descriptions and approximate equivalent heights of sea and swell waves have been agreed by the WMO for international use. These tables are not used for coded weather reports and are only intended for guidance. The synoptic maps. Such observations are admittedly difficult to make with any accuracy from the high bridge of a fast-moving ship.

Although these criteria no longer appear. Probable wave heights and probable maximum wave heights have been added only as a rough guide to show what may be expected in sea areas remote from land. State of sea photographs for estimating wind speeds The colour photographs between pages 70 and 71 illustrate the appearance of the sea corresponding to the Beaufort wind scale. Their purpose is to assist observers in estimating the wind speed when making weather reports.

The description of the sea is according to the Sea Criterion laid down by the World Meteorological Organisation. Watchkeepers should thoroughly familiarise themselves with the scale.

The appearance of the sea may be affected also by fetch see Glossary. In enclosed waters. The scale provides a practical means of estimating the force of the wind from the appearance of the sea. Probably some spray — are unaltered. Wind speeds are stated for a height of 10 metres above sea level. Crests 0. Crests begin to break.

Foam of glassy 0. Perhaps scattered white horses. Chance of some spray. Probably some spray. The foam is blown 7. Dense streaks of foam along the direction 7 gale of the wind.

Crests of waves begin to topple, tumble 10 and roll over. Spray may affect visibility. On the whole the surface of the sea takes a white appearance. The tumbling of the sea becomes heavy and shocklike. Visibility affected. Small and medium-sized The sea is completely covered with long white patches of foam lying along the direction of the wind.

Everywhere the edges of the wave crests are blown into froth. Sea completely 14 cane white with driving spray; visibility very seriously affected. In enclosed waters, or when near land with an offshore wind, wave heights will be smaller, and the waves steeper.

Figures in brackets indicate probable max height of waves. Very few ships carry an anemometer, and this would only indicate the relative wind aboard a moving ship.

What is needed is the true wind force and direction and the Beaufort scale provides the best method of making this important observation. Both these observations are relatively easy to make in daylight but difficult on a dark night, especially with light winds, particularly in a fast ship; care and experience and common sense are needed, using the feel of the wind on the face or wetted finger, first of all to determine the force and direction of the relative wind.

If the relative wind seems to be about 15 knots from abeam, then the true wind is on the quarter, about 20 knots fresh breeze. Woods Hole Oceanographic Institute has an interesting website at http: Length, Period, Height, Speed. An air mass may be described as a huge body of homogeneous air covering thousands of square miles; throughout the air mass temperature and humidity are more or less uniform in any one horizontal plane.

An air mass could be broadly classified, therefore, as warm or cold and moist or dry; the terms are relative. Classification of air masses is described later. The lower levels of the atmosphere automatically assume the characteristics of the underlying surface. Thus an air mass originating over very cold land in winter would be cold and dry, whereas an airstream approaching the land after a long sea passage from warmer latitudes would be relatively warm and moist.

Source regions The area in which an air mass originates is called the source region. The principal source regions are the large anticyclonic areas which lie to the north and south of the disturbed westerlies, ie the polar highs covering the polar caps, the oceanic sub-tropical highs and the continental highs; the latter being the anticyclones which develop over large land masses during winter months.

Other parts of the world may become source regions for short periods. In all these areas the pressure gradient is generally slight and the horizontal movement of air is slow, thus allowing plenty of time for the surface charac- teristics temperature and humidity to penetrate upwards to considerable heights. General classification of air masses This is broadly based on the source regions, and the terms used to describe a particular type of air mass may seem a little confusing at first.

For example, so- called Polar air does not originate from the polar caps but from subpolar regions. Air coming from polar regions is called Arctic air or Antarctic air, as appropriate. Similarly so called Tropical air does not flow from tropical latitudes.

An airstream flowing from between the Trade wind belts is classified as equatorial air. These main types are sub-classified as maritime or continental; the former originating over the sea and being moist in character, the latter flowing from dry land and generally fairly dry, but it is important to remember that the history of an air mass can change its characteristics. The table of Air Mass Classification given below is general.

Almost any area of the world can occasionally act as a source region. Characteristics of an air mass The characteristics of an air mass are governed by three factors: As an air mass moves away from its source region it assumes the characteristics of the surface over which it is passing; thus warm dry air moving over a cold sea will pick up moisture and gradually become cooler in the layers near the surface.

Modifications to the surface temperature may alter the stability of the air mass. Air mass weather Air mass characteristics are based on the following general principles: Cold air moving over a warm surface 1 Becomes heated at the surface by contact.

The height to which convection currents will go depends on a number of factors which are explained in Chapter 4. See Figure Warm air moving over a cold surface 1 Becomes cooled at the surface by contact.

This diffuses the cooling upwards from a few feet to a height of metres. Photo by G J Simpson. Photo by G Bartlett. Foam of glassy appearance. There is the possibility of scattered white horses. There is the chance of some spray. There is probably some spray.

The foam is blown in well marked streaks along the direction of the wind. Crests of waves begin to topple. Photo by: J P Laycock. Dense streaks of foam along the direction of the wind. On the whole. Visibility is affected. Photo by J P Laycock. The resulting foam. The tumbling of the sea becomes heavy and shock-like. Small and medium-sized ships might be for a time lost to view behind the waves.

The sea is completely white with driving spray. Photo by J F Thomson. If the air is dry or fairly dry. They should be studied in conjunction with the following descriptions of weather generally associated with each type. Details of specific air mass types Figures Bear in mind that the air mass charac- teristics described apply equally to both northern and southern hemispheres. Types of fog and their causes are discussed in Chapter 8.

Polar maritime Pm air Cold air from higher latitudes moving over a relatively warm surface. See Chapter 4. This same airstream.

It thus assumes the charac- teristics of polar maritime air. In summer the polar continental air mass will remain dry and cloudless as it moves over land which is warmer than at the source but.

Tropical continental Tc air Very warm and dry at source. The weather then becomes similar to that of polar maritime air but much colder and more intense in character. Moving into higher latitudes it becomes cooled in the lower layers and remains dry whilst passing over land.

Cumulus or cumulonimbus clouds form and squally showers of rain or hail occur. A stable air mass in which very widespread advection fog. Tropical maritime Tm air Warm and very moist air moving into higher latitudes passes over a sea surface which becomes progressively cooler. Little change takes place during its passage over cold land. Clear skies can generally be expected but.

Because of its low temperature the moisture content is low. Orographic rain at high coastlines is common.

Arctic maritime Am air Originates over ice and snow surfaces and is thus very cold at all levels. Widespread advection fog often encountered over the relatively cool waters of the north- east Pacific and the Grand Banks of Newfoundland area in the North Atlantic. In summer. Warm polar maritime wPm air or returning polar maritime rPm air A polar maritime air mass. Equatorial E air masses Warm.

On such occasions it is called returning polar maritime air or warm polar maritime air See Figures It then undergoes cooling in the surface layers. An air mass originating in desert regions may carry quantities of fine dust for thousands of miles. See Figures Describe its initial characteristics and the changes you would expect as it moves eastwards across the Atlantic Ocean.

Depression or low An area of low barometric pressure surrounded by an area in which the pressure is relatively high. Depressions are of greatly varying intensities and are usually associated with bad weather — ie much cloud and precipitation with strong or gale force winds. These seven distinctive isobaric forms are: The isobars are roughly circular or oval in shape and.

It should be noted that in both hemispheres the surface wind flows slightly in towards the central area see Convergence in Glosssary where the worst weather is usually encountered. Depressions tend to move towards areas of low or falling pressure and to steer round high pressure regions. Despite the availability of official weather forecasts. The isobars are roughly circular or oval in shape. The term depression is commonly applied to cyclones in latitudes which lie outside the tropics but may also be used to describe a weak tropical cyclone.

In the southern hemisphere the wind flows clockwise round the low pressure centre. In the northern hemisphere the wind circulates in a clockwise direction round the centre of high pressure. Note that the isobars are closer together near the centre.

Anticyclone A region of high pressure surrounded by an area of relatively low pressure. As a depression develops the pressure gradient becomes steeper and the winds stronger. A weakening or dying depression is said to be filling up. High pressure systems are usually slow moving by comparison with other systems and often remain stationary for long periods.

When the primary depression is old and filling up. Land and sea breezes see Chapter 9 are marked.

Weather is usually quiet. In winter the weather may be one of two types: General characteristics The pressure gradient is slight. A secondary depression A secondary depression is one which forms within the isobaric pattern of another primary depression. Note that the isobars are more widely spaced near the centre of high pressure and that the surface wind tends to flow outwards from the centre. An anticyclone is said to intensify as the pressure within the system rises.

See Anticyclonic gloom in Glossary. In summer the weather is generally dry. Fronts Fronts. Note that secondary depressions often develop into much more vigorous systems than their primaries. The weather associated with a trough is generally cloudy with precipitation.

Trough of low pressure This is distinguished on the weather chart by a system of isobars which appear sharply curved concave towards low pressure along a line called the trough line within a depression. The secondary in Figure See Line squall in Glossary. When the isobars of a depression or a tropical cyclone are circular the term trough refers to a line drawn through the centre of the system at right angles to the line of progression of the centre. A trough may be termed deep or shallow according to whether the curva- ture of the isobars is acute or gentle.

The isobars assume their greatest curvature along the axis of the ridge. It is generally associated with the fair weather of the anticyclone. In Figure Straight isobars An atmospheric pressure distribution in which the isobars run in more or less parallel straight lines across a large area. It is usually the outlying portion of a large and distant depression or anticyclone. It is easy to see why the winds are variable within the col area.

It is generally asso- ciated with light variable winds. In winter it moves southwards and extends from Florida to south-west England. Such a boundary is represented on the weather chart by a line called a front. The polar front The polar front marks the boundary between polar and tropical air masses in the Atlantic and Pacific Oceans. Air mass boundaries When two air masses of differing characteristics meet they do not mix freely but remain separated by a boundary called the frontal surface.

There is a similar front in the North Pacific.

The Arctic front The Arctic front marks the transition between Arctic air and polar maritime air. In the North Atlantic its mean position in summer is from Newfoundland to Scotland. The main frontal zones The positions of frontal zones marking the boundaries between the principal air masses fluctuate constantly whilst their mean positions move north and south with the seasons.

When two airstreams with different temperatures converge and meet. Some mixing of the air masses does take place but only along this boundary which is really a narrow zone of transition often referred to as the mixing zone. Its range of movement is small over the oceans but may be very large over the continents. See Convergence in Glossary. The sequence of events is described in the following paragraphs: Under suitable conditions a small wave forms on the polar front.

The intertropical convergence zone ITCZ The intertropical convergence zone lies within the tropics and is a broad zone of separation between the NE and SE Trades which flow equatorwards from opposite hemispheres.

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