Weather at the Needles Battery
The map on the left shows the hurricane of 1987 approaching the British Isles from the Bay of Biscay. The Battery, placed as
it is on a headland high above the sea, was exposed to the very worst of this storm and winds in excess of 160kph(100mph) were
recorded there. The circumstances that created this storm were exceptional, and fortunately occur very infrequently, but short
spells of ferocious weather do happen quite regularly.
Most bad weather is associated with depressions since they provide the esential ingredients: rising air for thick cloud and
rain, and steep pressure gradients for strong winds. Depressions are one of the atmospheric systems that give the UK its
changeable weather pattern. Sweeping in from the Atlantic Ocean, depressions bring conditions that can produce extreme weather.
Depressions are formed by the collision of dry polar air moving south and warm, moist, tropical air moving north. The collision
between the air masses forces the warmer air to rise - the rotation of the earth imparts a spiral movement to the rising air.
This upward movement results in the creation of a low pressure region on the earth's surface, more air is drawn in and rises,
deepening the depression.
The boundary between two different types of air mass is called a front. On a weather chart, the round (warm front) or
pointed (cold front) symbols on the front point in the direction of the front's movement. Fronts move with the wind, so they
usually travel from the west to the east. At a front, the heavier cold air undercuts the less dense warm air, causing the warm
air to rise over the wedge of cold air. As the air rises there is cooling and condensation, thus leading to the formation of
clouds. If the cloud becomes sufficiently thick, rain will form. Consequently, fronts tend to be associated with cloud and rain.
In winter, there can be sleet or snow if the temperature near the ground is close to freezing. A front which is moving in such a
way that the warm air is advancing to replace the cold air is called a warm front. As the warm front approaches, there is
thickening cloud and eventually it starts to rain. The belt of rain extends 100-200 miles ahead of the front. Behind the front
the rain usually becomes lighter, or ceases, but it remains cloudy. As a warm front passes, the air changes from being fairly
cold and cloudy to being warm and overcast (typical of warm air from the tropics travelling over the sea). Also there is a
clockwise change in wind direction, and the wind is said to 'veer'.
In the tearoom at the Old Battery is an anemometer which has a display linked to the Meteorological Office via a modem. This
shows the windspeed in knots and the wind direction. As recently as the 3rd November 2005 a weather system, accompanied by very
high winds, arrived over the English Channel and travelled in an easterly direction towards the Island. The wind was strong
enough to cause the cancellation of some of the ferries and those that continued to run must have had very rough crossings. The
winds peaked at almost 105kph(65mph) and gusts were considerably stronger. At midday the seas were striking the base of the
Needles Lighthouse and being carried by the wind to two-thirds its height. For a five day weather forecast and prevailing
conditions at Totland Bay click here.
Sir Francis Beaufort
The scale used to measure windspeed, usually referred to as 'Force' followed by a number, was devised by Sir Francis Beaufort
who joined the British Navy in 1787 as a thirteen-year-old cabin boy. At age twenty-two he had risen to the rank of lieutenant
and was serving aboard HMS Phaeton.
In 1805 he was given his first command and started to develop the first versions of the wind force scale. In 1838 Beaufort's
Wind Force Scale was introduced for use by the British fleet for all log entries. Beaufort was promoted to Rear Admiral in 1846
and he eventually retired from the Admiralty in 1855 after sixty-eight years of service. He died in 1857 but the system of
measuring windspeed he developed is still in use today.
| Force | Speed Knots |
Description | Beaufort's Criterion | Appearance Of Sea | |
|---|---|---|---|---|---|
| 0 | <1 | Calm | Calm | Sea like a mirror. | |
| 1 | 1-3 | Light Air | Just sufficient to give steerage way. | Ripples with the appearance of scales are formed without foam crests. | |
| 2 | 4-6 | Light Breeze | That in which a well-conditioned man-of-war with all sail set would go in smooth water from: | 1 to 2 knots | Small wavelets, still short but more pronounced. Crests have a glassy appearance and do not break. |
| 3 | 7-10 | Gentle Breeze | 3 to 4 knots | Large wavelets. Crests begin to break. Foam of glassy appearance. Perhaps scattered white horses. | |
| 4 | 11-16 | Moderate Breeze | 5 to 6 knots | Small waves, becoming longer; fairly frequent white horses. | |
| 5 | 17-21 | Fresh Breeze | That to which she could just carry in chase 'full and by'. | Royals, etc | Moderate waves, taking a more pronounced long form; many white horses are formed. Chance of some spray. |
| 6 | 22-27 | Strong Breeze | Single-reefed topsails and topgallant sails | Large waves begin to form; the white foam crests are more extensive everywhere. Probably some spray. | |
| 7 | 28-33 | Near Gale | Double-reefed topsails, jib, etc | Sea heaps up and white foam from breaking waves begins to be blown in streaks along the direction of the wind. | |
| 8 | 34-40 | Gale | Triple-reefed topsails, etc | Moderately high waves of greater length; edges of crests begin to break into the spindrift. The foam is blown in well -marked streaks along the direction of the wind. | |
| 9 | 41-47 | Strong Gale | Close-reefed topsails and courses | 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 | 48-55 | Storm | That which she could scarcely bear with close-reefed main topsail and reefed foresail. | 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 takes on a white appearance. The tumbling of the sea becomes heavy and shock-like. Visibility affected. | |
| 11 | 56-63 | Violent Storm | That which would reduce her to storm staysails. | Exceptionally high waves (small and medium sized ship might be for a time lost to view behind 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> | Hurricane | That to which no canvas could withstand. | The air is filled with foam and spray. Sea completely white with driving spray; visibility very seriously affected. | |
| Conversion Factors | |||||
| Knots - MPH | Multiply by 1.1508 | ||||
| Knots - KPH | Multiply by 1.852 | ||||
| MPH - KPH | Multiply by 1.6093 | ||||
| Celsius - Fahrenhite | Multiply by 1.8 and add 32 | ||||
Source:
Met Office http://www.metoffice.gov.uk
Meteorology At Sea by Ray Sanderson ISBN 0 540 07405 5
St. Vincent College http://www.stvincent.ac.uk/Resources/Weather/Depression