The Weather at Sea, Explanations plus the Beaufort Scale
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.
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
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
average pressure at sea level is about 1013 millibars.
types of barometer are in general use, namely, the mercurial barometer, the
aneroid barometer and the barograph.
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
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.
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
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
Wind fills the sails of smacks,
which then make good 1 to 2 knots.
Sea like a
1 to 3
Fishing smacks just have steerage
Ripples with the appearance of
scales are formed, but without foam crests.|
4 to 6
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
7 to 10
Smacks begin to heel and make
good 3 to 4 knots.
Large wavelets; crests begin to
break; foam of glassy appearance; perhaps scattered ' white
11 to 16
Good working breeze. Smacks
carrying all sail and heel over considerably.
Small waves, becoming longer;
fairly frequent ' white horses'. |
17 to 21
Moderate waves, taking a more
pronounced long form; many ' white horses' are formed. (chance of some spray).
22 to 27
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
28 to 33
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
34 to 40
All smacks make for harbour if
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.|
41 to 47
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.|
48 to 55
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.|
56 to 63
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.|
The air is filled with foam and
spray. Sea completely white with driving spray; visibility very seriously
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
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.
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
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.
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
Humidity is measured with a hygrometer, the commonest form of which is
the wet and dry bulb thermometer.
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
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.
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.
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.
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
may also be forced to rise up the sides of mountains and high ground towards
which the wind is blowing.
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
Rain, Snow & Hail
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.
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).
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
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
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.
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.
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.
relationship between pressure distribution and wind direction has already been
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.
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
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.
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