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.

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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.

When covering the whole sky they give it a wavy appearance. Height of base is usually between about metres 1, feet and 1, metres 4, feet. See also altocumulus and cirrocumulus. Cumulus Cu Whitish cauliflower shaped clouds with a more or less flat base and appreciable vertical thickness.

They vary in size and vertical extent and can develop up to great heights see Cb. These clouds are a feature of the trade wind area but can occur anywhere. In certain conditions of light, parts of these look dark.

Base height is usually between about metres 1, feet and 1, metres 5, feet.

The thundercloud, a cumulus type of cloud but of great vertical extent. The top, instead of being rounded like a cauliflower, looks more mountainous or may grow into the shape of a fibrous looking anvil.

The base is generally flattish and often has fractostratus Fs clouds below it from which precipitation falls. If cumulonimbus cloud is very extensive it may resemble nimbostratus Ns.

Height of base is usually between metres 1, feet and 1, metres 5, feet. A layer or patches of flattish, globular shaped, fairly small clouds, white or grey in colour, often arranged in lines, very similar in appearance to stratocumulus but at a greater height and the individual cloudlets look smaller.

Base height above 2, metres 6, feet and not easy to estimate. See also strato- cumulus and cirrocumulus. A thin and apparently formless veil or sheet of cloud, usually grey in colour.

When present it often covers the whole sky, giving it a watery appearance; sun or moon shows through it with blurred outline. Sometimes it is thick enough to obscure sun or moon, in which case it is dark in colour.

See also cirrostratus. Cirrus Ci Nicknamed mares tails, feathery, fibrous or hairy clouds of delicate texture, high in the sky, usually coloured white and in various formations. Height of base above about 5, metres 1, feet. Popularly known as mackerel sky, the cloudlets are arranged usually in lines and somewhat resemble flocks of sheep. Base height above about 5, metres 18, feet. See also altocumulus and stratocumulus.

Cirrostratus Cs A diffuse and thin veil of cloud, whitish in colour somewhat similar to altostratus, but more diffuse. Only slightly blurs the outline of the sun and moon; it often produces a halo effect which altostratus does not.

Sometimes gives the sky a greyish or milky effect. See altostratus. Weather associated with cloud types With a few obvious exceptions, a particular cloudscape does not indicate very much by itself about impending weather changes; it needs to be considered against a background of the process of formation or dissipation of those particular clouds, also the wind and pressure changes that have occurred recently.

Ideally, a synoptic map is needed to get an overall picture of what is happening to the weather some distance away. It can be said, in very general terms, that at sea level the possible indications usually associated with certain cloud types are as follows, but complications may arise if high land is involved. Stratus No special significance but may affect visibility if very low.

Indicates a stable air mass, ie not much convection. Drizzle often falls from this cloud. Nimbostratus Can be termed the rain cloud. Considerable rain probable and prospects of bad weather generally. In some cases the rain does not reach the ground. Stratocumulus No special significance, not generally a bad weather cloud.

As with stratus, it indicates stability. Occasionally light rain or drizzle falls from this cloud. Cumulus Generally a fair weather cloud when small. When large they indicate unstable air with possibility of showers due to strong convection, and of sudden squally wind when nearby. If it thickens to windward it may indicate rain.

Altocumulus No special significance, not generally a bad weather cloud. Rain is likely when these clouds thicken to windward. If it grows in extent it may well indicate the approach of windy and bad weather generally.

Gives timely warning of a tropical storm, especially if followed by a falling barometer. If it turns to altostratus it probably indicates a coming depression or confirms the advent of a tropical storm. Thus column A exerts exactly the same force as column B and the rate at which pressure falls with height must be greater in the cold column.

In the lower levels the average rate at which pressure falls is approximately 1 hPa per Although the latter is still commonly used by the media it has been thought to be sufficiently important to use the preferred unit. In the mornings when the Earth is cool. After sunrise the land warms rapidly causing an increase in the temperature lapse rate. It follows therefore that the pressure must always decrease with height.

Throughout this book the authors have adopted the use of hPa. In Figure 1. These effects may be modified or masked at times by the direction and force of wind. Variation of pressure with height Atmospheric pressure at any level is the weight of the air above that level. As darkness approaches. Water vapour and cloud.

This is called the greenhouse effect. Absorption is very little. All incoming radiation ceases when darkness falls and the surface continues cooling through the night until sunrise when the whole cycle is repeated. Some of the incoming short-wave radiation from the sun is lost through absorption.

It explains why. Thus the diurnal 5. Diurnal range of surface temperature Soon after sunrise the incoming short-wave energy begins to exceed the outgoing long-wave emission. The surface temperature of the sun is something in the nature of 6. A thick cloud will reflect nearly 80 per cent of the radiation which it receives.

The temperature of the surface then starts to increase and generally reaches its maximum by about hours Local Time. All bodies.

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At very high temperatures a body emits both heat and light. Bodies which are good absorbers of heat are also good radiators and the converse is true. The hotter the body the shorter the wave length and the more intense is the emission. The actual wave length depends on the temperature of the radiating body.

In general. X and Y are two solar beams of equal intensity and having the same cross-sectional area. Fig 2. In Figure 2. The general pattern of diurnal variation in land temperature is often modified locally by weather. Beam Y is nearly vertical to the surface and its radiation is concentrated onto the relatively small area CD. The pecked arc FGH represents the upper limit of the atmosphere.

For instance. By contrast the temperature of the sea surface changes only a very little for the same amount of heat energy. The main factors governing air temperatures at sea are: The specific heat of water is higher than that of any other common substance. Thus the annual range. Air temperature near the surface tends to follow that of the surface below.

Hence there is a relatively high rise in temperature for a given amount of radiation. The reasons for this are: Hence the gain or loss of a given quantity of heat brings about a smaller change in temperature of sea than of land. Generally warmest within the tropics and subtropics. Dry soil. Dew point The temperature to which unsaturated air must be cooled to become saturated is the dew point.

Air which is unsaturated is often called dry air although it contains some water vapour. The higher the temperature the greater the quantity of water vapour the air can hold. The amount varies in both time and place. So long as air remains unsaturated and the amount of water vapour remains constant. It is supplied by evaporation from the sea surface and.

If unsaturated air is cooled sufficiently it will become saturated. Air which contains less water vapour than the maximum possible amount is said to be unsaturated. When the maximum possible amount is present the air is said to be saturated.

Saturated air The maximum quantity of water vapour which can be contained in a given volume of air is limited by temperature.

Further cooling will result in the excess water vapour being condensed into the liquid or solid state visible water droplets or ice crystals. Cloud and fog These should not be confused with water vapour. If air is saturated its temperature is its dew point. The following should be noted: The higher the initial dew point the greater the amount of water vapour condensed.

Exact values are of little or no concern to the mariner but the shape of the curve is of immense importance and this will become apparent in later chapters. It is expressed as a percentage. In those industrial areas where there is a very high degree of smoke pollution. They are the nuclei onto which water vapour condenses. Without their presence any condensation would be highly improbable. They are most abundant in the levels near the surface. Most of these particles are hygroscopic — that is.

Explain why. Explain why this is so. The present classification is an amplification and combination of these. It is important that a seafarer should be able to recognise the main cloud types because of their bearing upon existing and future weather. Cloud types are further divided into three groups based upon their height above the observer: A seafarer also needs to be able to recognise and name cloud types for inclusion in weather reports which are transmitted to a meteorological service.

This can be particularly useful when weather forecasts are for any reason unobtainable. The earliest cloud classification. Summary of cloud types The names and descriptions of the various cloud types have been agreed internationally by the World Meteorological Organisation.

The cloud formation in any locality is an important feature of most weather situations and provides the meteorologist with valuable information when preparing a forecast.

Height of base is usually between metres feet and metres 2. Description of cloud types see photos 1 to 16 Stratus St A more or less continuous layer or sheet of cloud. When it does touch the ground it is indistinguishable from fog. When broken up into patches by the wind it is called fractostratus Fs. Nimbostratus Ns A low. Height of base generally between metres feet and metres 2. The base is generally flattish and often has fractostratus Fs clouds below it from which precipitation falls.

See also cirrostratus. The top. See also strato- cumulus and cirrocumulus. When present it often covers the whole sky. Altostratus As A thin and apparently formless veil or sheet of cloud. They vary in size and vertical extent and can develop up to great heights see Cb. Sometimes it is thick enough to obscure sun or moon.

When covering the whole sky they give it a wavy appearance. Height of base is usually between metres 1. Height of base is usually between about metres 1. These clouds are a feature of the trade wind area — but can occur anywhere.

Base height is usually between about metres 1. Cumulus Cu Whitish cauliflower shaped clouds with a more or less flat base and appreciable vertical thickness. Base height above 2.

Height of base above about 5. In certain conditions of light. See also altocumulus and cirrocumulus. They are often arranged in lines or groups orientated in one or two directions.

Altocumulus Ac A layer or patches of flattish. If cumulonimbus cloud is very extensive it may resemble nimbostratus Ns. Cumulonimbus Cb The thundercloud. Only slightly blurs the outline of the sun and moon. If it thickens to windward it may indicate rain. Stratus No special significance but may affect visibility if very low. Considerable rain probable and prospects of bad weather generally. Stratocumulus No special significance. It can be said. Altocumulus No special significance. Weather associated with cloud types With a few obvious exceptions.

Cumulus Generally a fair weather cloud when small. As with stratus. Sometimes gives the sky a greyish or milky effect. Base height above about 5.

Occasionally light rain or drizzle falls from this cloud. See also altocumulus and stratocumulus. Drizzle often falls from this cloud. When large they indicate unstable air with possibility of showers due to strong convection. See altostratus. In some cases the rain does not reach the ground.

Cirrostratus Cs A diffuse and thin veil of cloud. Nimbostratus Can be termed the rain cloud. Rain is likely when these clouds thicken to windward. Indicates a stable air mass. Cirrostratus Gives a fair indication of the approach of rain. If it follows cirrus it may well indicate approach of a depression.

If it disperses it probably has no significance. Gives timely warning of a tropical storm. What does this type often portend? If it turns to altostratus it probably indicates a coming depression or confirms the advent of a tropical storm.

Cirrus If it grows in extent it may well indicate the approach of windy and bad weather generally. Adiabatic processes in the atmosphere Atmospheric pressure decreases with height. In both cases no interchange of heat takes place between the body of air and its environment. If the same body of air is subjected to a reduction in pressure it undergoes expansional cooling as opposed to thermal cooling.

Moist air gives a relatively low cloud base. In the latter case. Cloud formation in brief 1 When unsaturated air is forced to rise it will expand and cool adiabatically. For an example in the former case.

When a body of air is subjected to an increase in pressure it undergoes compressional heating as opposed to thermal heating. Thus if a body of air rises through the surrounding air ie its environment it undergoes a reduction in pressure and is cooled adiabatically. Saturated adiabatic lapse rate SALR Upward motion of saturated air results in condensation of excess water vapour.

The process of condensation releases the latent heat of vaporisation which. Thus the SALR increases with height. See Figure 5. As the rising air gains height above the condensation level see Glossary the amount of water vapour is progres- sively reduced.

Refer now to Figure 5. Fig 5. The height at which this takes place is governed almost entirely by the shape of the ELR curve. Atmospheric stability and instability See Stability in the Glossary. Upward motion will be arrested at the level where the temperature of the rising air reaches that of the environment. The graph shows that the rising air at each successive level becomes progressively cooler and therefore denser and heavier than the surrounding air. Z represents a specimen of air at the level WX.

If a body of air at the surface becomes warmer than the surrounding air it will commence to rise through the environment and.

Case 1 Stable air Figure 5. It is important to note that. If the displacing force ceases to act. A surface inversion may be caused through radiation cooling of a land surface at night. It is generally associated with an anticyclone. Thus the downward motion is stimulated and will continue after the displacing force ceases to act.

Thus the initial forced motion will be stimu- lated and the air will continue to rise after the displacing force has been removed. Case 2 Unstable air Figure 5. So we take. If the dis- placing force should cease to act the air specimen will start to float upwards and finally come to rest at its original level.

When these clouds move quickly across the sky. Photo by Mrs M I Holmes. Watch how your barometer behaves from now on. Photo by C S Broomfield. If cirrostratus increases to cover the whole sky it may foretell the approach of a depression. Watch for the appearance of altostratus with lowering base and falling barometer. When cirrostratus covers the sun or moon. Cirrocumulus is usually seen in association with patches of cirrus from which it often develops.

The associated weather is usually fair and quiet and the cloud movement very slow. Photo by W S Pike. Photo by R K Pilsbury. The cloud is thin enough for a watery sun or moon to be visible. When altostratus thickens to windward it may indicate the approach of a depression and. Watch your barometer. Here they completely cover the sky. Photo by S Jebson. Sometimes several layers of this cloud type can be seen at different levels.

When altocumulus thickens to windward and loses its rounded shapes. Breaks are often visible in the cloud layer. These rolls or patches are often arranged in a fairly regular pattern of wavy lines or groups. Occasional light rain or drizzle sometimes falls from this cloud. Nimbostratus is frequently associated with fronts. The stratus fractus appears as ragged pieces of cloud below the main cloud layer. If the clouds remain small. Photo by J F P Galvin. Their flat bases look dark by comparison with their gleaming white tops which are usually well rounded and often show a hard outline against a clear blue sky.

When the tops begin to lose their rounded cauliflower-like appearance and become fibrous. Cumulus clouds often tend to flatten out or disperse at sunset when convection ceases. Photo by Cleo Irving. Sudden hard squalls and a large change or reversal in wind direction may occur in the vicinity of well-developed cumulonimbus clouds. Such clouds often cover a wide area. Heavy showers of rain. With still further development their fibrous tops spread out like an anvil. Photo by R B Tucker.

The clouds in this picture resemble a pile of plates. Orographic cloud is continuously forming at the windward edge and evaporating downwind although it appears to be stationary. V the temperature to which a sample of surface air is raised by solar radiation. Refer now to Figures 5. The cloud thus formed could reach to a very great height. The air specimen of temperature V will rise through its environment cooling at the DALR until it reaches its dew point at the condensation level CL at which height it is saturated and.

First now consider Figure 5. Note that the values of T. V and the ELR remain unchanged. The degree of stability or instability depends not only on the shape of the ELR curve but also on the height of the condensation level which is governed by the dew point. Comparing Figures 5. In this Fig 5. V and the ELR are the same in both figures.

All upward motion of air will be arrested at the level of the inversion QR. Under these conditions cloud of great vertical develop- ment can be expected.

The atmosphere is thus very unstable. If the dew point is low enough to give a condensation level above QR the sky will be cloudless. Figure 5. Thus the air becomes stable at a height well below that of the condensation level and cloud cannot form.

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A higher base gives stratocumulus. Layer type cloud will be formed. Surface heating by the sun is strong and the dew point is high. This is because the SALR increases with altitude and the curve eventually meets that of the ELR at a level at which upward motion of air ceases and is thus the maximum height to which cloud can develop.

It gives altocumulus cloud at medium heights and cirrocumulus at high levels. Air flowing horizontally over a rough surface sets up horizontal and vertical eddy currents which occur mainly in the lowest metres of the troposphere. The greater the degree of instability the greater the amount of cumuliform cloud. When surface air is forced up to a height above the condensation level cloud will form. Very moist air gives low stratus. 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.

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.

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.

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Although it never actually forms over the sea. Cloudy skies overnight will reduce the effect of the radiation from the land. 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. 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. 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.

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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. 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.

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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.Further cooling will result in the excess water vapour being condensed into the liquid or solid state visible water droplets or ice crystals.

Surface pressure is low over north-west India in summer and very high over Siberia in winter. The synoptic maps. The total cloud cover exceeded the areal extent of precipitation bv a factor of 4. The process of condensation releases the latent heat of vaporisation which.

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

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