The Weather at Sea, Explanations plus the Beaufort Scale

The Weather

Although only pleasure yachts now depend upon the vagaries of the wind to make their passages, an understanding of weather is still essential to every seaman. The aim of this article is to present a brief outline of the facts concerning weather which every seaman should know, and to indicate the necessity for accurate, careful and punctual records of wind, sea and sky, and of the readings of the barometer.

The Atmosphere

The phenomena that we describe as weather are caused by movements and other changing conditions of the atmosphere, or that layer of air which surrounds the Earth and contains oxygen, nitrogen and small amounts of other gases and also varying amounts of water vapour which cause cloud, rain and fog and make the temperature equable. The movements of the atmosphere are caused by the heat of the Sun, the rotation of the Earth, the distribution of land and sea, and some other more complex factors.
Atmospheric pressure. Air has a measurable weight or pressure. At sea level the pressure exerted by the air overhead is about 15lb per square inch, and fluctuations in this pressure can be accurately measured by means of the barometer. For various reasons atmospheric pressure is measured, not in pounds per square inch, but in dynes per square centimetre, commonly termed millibars, and the scales of all modern barometers are graduated in these units. The average pressure at sea level is about 1013 millibars. Three types of barometer are in general use, namely, the mercurial barometer, the aneroid barometer and the barograph. It has long been known that changes in atmospheric pressure are associated with changes of the weather, especially of the wind; hence the importance of keeping an accurate record of barometric readings. Broadly speaking, high pressure is associated with fair ( though not always sunny) weather and light winds, while low pressure is usually accompanied by strong winds and rain.

Wind and the Beaufort wind scale

Wind is the result of a difference of pressure between two adjacent areas. The relationship between wind direction and the distribution of pressure can be expressed as follows: If you face the wind when in the northern hemisphere, pressure will be low on your right hand and high on your left, and in the southern hemisphere it will be low on your left and high on your right.
Wind is always named and recorded by the direction from which it is blowing reckoned from True North; it should be logged to the nearest 10 Degrees of the compass. Its strength is estimated from the Beaufort Scale, given below.(For newcomers, a smack was a heavy wooden gaff rigged sailing vessel of around 35-45 ft, sailed by grizzled professional fishermen. The modern equavilent would perhaps be a heavy displacement yacht, crewed by a bunch of knowledgeable "beards". Lightly crewed family boats will be reefing at least one force earlier than mentioned in the table below.)

Beaufort Wind Scale

Wind Force (Beaufort) Speed in Knots Descriptive Term Coastal Criterion Sea Criterion
0 Less than 1 Calm Wind fills the sails of smacks, which then make good 1 to 2 knots. Sea like a Mirror
1 1 to 3 Light Breeze Fishing smacks just have steerage way. Ripples with the appearance of scales are formed, but without foam crests.
2 4 to 6 Light Breeze Wind fills the sails of smacks, which then make good 1 to 2 knots. Small wavelets, still short but more pronounced; crests have a glassy appearance and do not break.
3 7 to 10 Gentle Breeze Smacks begin to heel and make good 3 to 4 knots. Large wavelets; crests begin to break; foam of glassy appearance; perhaps scattered ' white horses'.
4 11 to 16 Moderate Breeze Good working breeze. Smacks carrying all sail and heel over considerably. Small waves, becoming longer; fairly frequent ' white horses'.
5 17 to 21 Fresh Breeze Smacks shorten sail. Moderate waves, taking a more pronounced long form; many ' white horses' are formed. (chance of some spray).
6 22 to 27 Strong Breeze Smacks have double reef in mainsails. Care required when fishing. Large waves begin to form; the white foam crests are more extensive everywhere. (probably some spray)
7 28 to 33 Near Gale Smacks remain in harbour, and those at sea lie to. Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind.
8 34 to 40 Gale All smacks make for harbour if near. Moderately high waves of greater length; edges of the crests begin to break into sprindrift. The foam is blown in well-marked streaks along the direction of the wind.
9 41 to 47 Strong Gale ==- High waves. Dense streaks of foam along the direction of the wind. Crests of waves begin to topple, tumble and roll over. Spray may affect visibility.
10 48 to 55 Storm ==- Very high waves, with long, overhanging crests. The resulting foam, in great patches, is blown in dense white streaks along the direction of the wind. On the whole, the surface of the sea takes a white appearance. The tumbling of the sea becomes heavy and shock-like. Visibility affected.
11 56 to 63 Violent Storm ==- Exceptionally high waves ( small & medium-sized ships might for a long time be lost to view behind the waves). 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. Visibility affected.
12 64 Plus Hurricane ==- The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected.

When judging the force and direction of the wind at sea, due allowance must be made for the speed of the boat, and the direction of the wind must be obtained by looking at the waves and not at the exhaust smoke, a flag or masthead pendant. Make it a habit to estimate true wind direction and strength from the waves. Masthead wind instruments will give you an "apparent" wind direction and strength. When the direction of the wind changes in a clockwise direction (e.g. from S.W. To W.) it is said to veer; when it changes in an anti-clockwise direction ( e.g. from S.W. To S.) it is said to back.


As already stated, the heat of the sun is a primary cause of motion of the atmosphere. An important consequence is that the weather is intimately bound up with changes of temperature; the wind may blow warm or cold in ways with which we are all familiar.
Different substances require different amounts of heat to produce a given change in temperature. The amount of heat required to raise the temperature of water is two or three times greater than that required to raise the temperature of an equivalent volume of land by the same amount, and conversely, water yields from two to three times more heat than land when the temperature falls. The sea is therefore usually cooler than the adjacent land surface in summer or in hot regions, and warmer in winter or in cold regions.
Temperature is measured by means of a thermometer. At sea, both the temperature of the air and that of the sea surface are important, and different thermometers are provided for measuring each.


The amount of water vapour present in the air varies from place to place and from time to time, and is an important factor in determining the nature of of the weather. The amount of water vapour that the air can hold in an invisible state is limited, and it varies with the temperature. Warm air can hold more water vapour than cold air; when the air contains as much water vapour as it can hold at a given temperature it is said to be 'saturated'. If the temperature is lowered further, or more water vapour is added to the air, the surplus condenses into minute but visible drops of water, which constitute cloud, mist or fog.
Humidity is measured with a hygrometer, the commonest form of which is the wet and dry bulb thermometer. When the air is dry the wet bulb reading is several degrees below that of the dry bulb, but when it is saturated both will read the same and fog may then be expected.


As explained in the preceding paragraph, when air is cooled below its saturation point the surplus water vapour condenses into visible droplets of water. Such condensation occurs in the atmosphere when there is cooling due to uplift and results in the formation of cloud. It is largely upon the manner of uplift that the type of cloud depends.
Air is often heated in its lower layers by its passage over a relatively warm land or sea surface. This causes it to expand, become lighter and rise ( in the same way that warm water rises in the hot-water system of a house). As the air rises it expands further because of the decrease in pressure, and cools because of its expansion. If this cooling is continued beyond the point at which the air is saturated, cloud results. Clouds formed in this way, are of the cumulus type, with flat, horizontal bases and rounded cauliflower heads which sometimes spread out at great heights into the shape of an anvil.
Where two masses of air from different sources and with different characteristics meet, boundary lines are formed where the colder and heavier air runs under the warmer and lighter air and compels it to rise. As in the previous case, the expansion and cooling of this air results in the formation of cloud, which spreads as a more or less continuous layer of stratiform cloud.
Air may also be forced to rise up the sides of mountains and high ground towards which the wind is blowing. The consequent expansion and cooling of the air again leads to the formation of status-type cloud if the ascent of the air is sufficient to cause condensation.

Rain, Snow & Hail

If the cooling process which gives rise to cloud-formation is continued the droplets of which the cloud consists grow in size until they become too heavy to remain suspended in the air, and they therefore fall to the ground as very fine rain or drizzle. If the droplets are held aloft by upward currents of air they grow by agglomeration and eventually fall as raindrops. When very strong up-draughts exist, as in large cumulus-type clouds, the drops become very large before falling as heavy showers of rain or, under certain conditions, as hail. The latter is caused by very strong up-draughts carrying the raindrops above the freezing-level, where they turn into continually growing balls of ice which eventually fall to the ground as hailstones. Thunderstorms are often accompanied by hail, because they also are caused by very strong up-draughts extending to great heights.
If the temperature at which condensation actually takes place is below freezing-point the droplets form as ice crystals, which grow and become snowflakes. Snowflakes often melt and change to raindrops before reaching the ground; as a rough guide, snow rather than rain is likely if the air temperature at ground or sea level is below 3 deg C (37 deg F).

Fog & Mist

The cause of fog and mist is fundamentally the same as that of cloud, i.e. the cooling of air below the temperature at which it is saturated. In cloud the resulting condensation into visible particles takes place aloft, but in fog or mist the condensation occurs on the surface of the sea or ground. The difference between fog and mist is merely one of degree. Visibility of under 1000 metres (approximately 5 cables) constitutes a fog. Mist is recorded when the visibility is over 1000 metres but is perceptibly reduced by the presence of water droplets.
In the open sea almost all fog is caused by warm, moist air blowing over a relatively cold sea surface, whereby the lower layers of the air are chilled. These sea fogs are therefore most common in spring and early summer, at which seasons the sea surface temperature is lowest. They are also particularly prevalent in the vicinity of cold ocean currents.
Over the land, on clear nights with little wind, a sharp drop in temperature occurs after sunset as a result of the Earth radiating its heat away into space. The air in contact with the ground is cooled, condensation takes place and fog is formed. Such land-formed fogs are known as radiation fogs, and are most common in the winter with its long nights; they are thickest in the latter part of the night and early morning. As the Earth becomes heated during the day such fogs become thinner and often disperse. They frequently affect harbours, especially in smoky industrial areas, and may drift seaward under the influence of offshore winds, thus giving rise to coastal fog. If the sky is cloudy, fogs of this type are improbable, because the clouds act as a blanket and to a large extent prevent the Earth losing heat by radiation; neither do they occur with strong winds, which thoroughly stir the air and so prevent any of it remaining long enough in contact with the cold ground.
As fog can only occur when the air is saturated, or very nearly so, the hygrometer (wet and dry bulb thermometers) provides a useful guide to its probability.

Pressure Systems

The relationship between pressure distribution and wind direction has already been explained previously. From this relationship it follows that in the northern hemisphere the wind circulates round an area of low pressure in a counter-clockwise direction, and in the southern hemisphere in a clockwise direction. These areas of low pressure are known as depressions or lows, and they are generally associated with cloudy, unsettled weather and strong winds. Depressions usually move fairly quickly and almost always from a westerly towards an easterly point. In the northern hemisphere the approach of such a system is therefore heralded by the wind backing towards a southerly point. Its approach is also foreshadowed by a falling barometer, an increase and thickening of cloud and, later, by rain.
It also follows from this pressure / wind relationship that the circulation round an area of high pressure is clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere.
Such an area of high pressure is known as an anticyclone or high. Like depressions, anticyclones generally move in an easterly direction; but in the eastern parts of the oceans the movement is usually slow and erratic, and an anticyclone may remain nearly stationary for days at a time: Consequently the weather often remains much the same for considerable periods, and then changes slowly. Anticyclonic weather is often fine and sunny though at times overcast and gloomy weather prevails; winds, however, are only light or moderate in strength and rain is unlikely; radiation fog often forms over the land at night and affects harbours and coastal areas, and in winter it may persist throughout the day.

This material has been adapted from various Admiralty Manuals of Seamanship, published between the 30's and the 50's...In many ways it is of more use to the small craft mariner than more modern material.

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