I read this book recently and though that it had relevant information which would be not normally associated directly with diving as we are not on the water quite as long as sailors. However the information is just as beneficial for us fair-weather seafarers!! Thank you to David Houghton a top British meteorologist and the author of “the weather at sea”, from which the article below is taken. We have full reference details at the end of this article if you would like to get this book from the library or purchase.
What is that black cloud to windward – does it indicate a squall, a gale, a thunderstorm, for or is it just a patch of dirty, smoke-laden air? There are a number of questions you can ask which will at least help toward an answer, and may rule out some hazards, as follows -
- What was the latest weather forecast?
- What is the barometer doing?
- Has the wind changed? Is there any swell?
- What have the higher clouds been doing? have they been typical of those ahead of a front or trough?
- Did you notice which way there were moving?
- Are you with 10 miles of land?
All of this information you should have in your log. It is important to refer to the facts, not your crew’s idea’s or hunches as to what may have happened recently.
Gales
Gales which are due to depressions (squalls are very short-lived in comparisson and are considered separately) do not spring up without warning, and with information in your log which will enable you to answer the above questions you will never be caught unawares. Watch out in particular for:
The weather forecast. If the possibility of a gale has been mentioned keep a listening watch on Radio 4 or the nearest coast radio station.
Your barometer. A fall in pressure of over 8mb in three hours is almost certain to be followed by a gale whatever the wind is to start with, and a fall of over 5mb in three hours by a force 6. If the wind is already force 7 you must expect a gale within an hour, but if it is only force 3 when your barometer starts falling at a this rate you have at least 3-4 hours before a gale arrives. A very rapid rise in pressure after a trough has passed is also indicative of a gale, and the same rates of pressure change apply – a rise of over 6 mb in 3 hours for a force 8, and over 5mb in 3 hours for a force 6 (you must make allowances for your own movement, either toward or away from the low pressure – Buys Ballot’s law will tell you which way you are going relative to the pressure centre). These rapid pressure changes are certain signs of a gale.
If the wind is both backing and increasing it indicates a trough approaching, and you should find that the barometer is falling as well. If the black cloud of the trough is almost upon you the signs are of a temporary squall rather than a prolonged gale.
Waves on the sea produced by the local wind tell you no more that the wind itself, but swell waves can be a useful predictor. If they are increasing from the direction in which you know there is a depression an increase in wind is likely. Since swell waves take a long time to generate they usually indicate the approach of a large area of stronger winds which are likely to last for days rather than hours.
High clouds increasing from the direction of lower pressure and travelling from a direction well veered on the surface wind (e.g. southerly surface wind and northwesterly upper wind) are a sign of inceasing wind. The faster high clouds are travelling the greater the likely increase in surface wind during the next 6-12 hours.
If the wind is blowing approximately along the coast so that with your back to the wind the coast is on your right-hand side, a force 6-7 wind either in harbour or 8 or more miles of the coast means a gale force 8 in a band within about 5 or 6 miles of the coast.
Squalls
Two different types of squalls are met in practice one due to thunderstorms which we deal with below, and one associated with either a cold front or a trough of low pressure in relatively cold air. Typically some miles ahead of such a trough you may be able to see a line of towering cumulus clouds, and along the line of the trough itself the clouds are dense and black with patches of heavy rain. Absence of swell waves, only a slow fall in pressure and no preceding increase in high cloud moving from a well veered direction all suggest a very temporary squall lasing not more than about half an hour, if that. The best advise is to reef and make for the lightest part of the cloud. Having weathered the squall you can usually assume that another one is unlikely for 3-4 hours.
Thunderstorms
The dare mass of threatening cloud associated with a mature thunderstorm is often heralded by a decrease in wind and an almost glassy sea. These features clearly distinguish it from a depression or trough. The barometer may move up and down quite quickly and erratically by one or two millibars, but again the absence of a longer period fall distinguishes this from an approaching depression. The best rule to avoid the worst of the wind is to leave the storm to port. Squalls associated with the storm usually move steadily outwards from the centre with the wind ahead of them in the opposite direction, that is blowing in towards the storm.
Near the coast thunderstorms are more likely in the afternoon and early evening when land temperatures are at a maximum. Over the open ocean thunderstorm frequency is highest at night when cloud-top temperatures are their lowest, this creates conditions of minimum stability over the sea.
For safety from lightening remember that tall, free-standing objects tend to be focal points for the electrical discharge to earth. So make sure tha the conduction route (metal) through to the water is continuous.
FOG
Two main types of fog are met at sea; fog which has formed over land on a cool, clear night and drifted out over the water; and sea fog which forms over the sea itself when relatively warm moist air moves over colder water. Drifting patch’s of land fog tend to lift to 3-6 meters or so above the sea surface and gradually break up as they move away from the coast. The warmer the water compared to the temperature of the air over the land, the quicker the fog disperses. SO if you are in fog in harbour and the forecast is for for over land clearing during the morning, you can safely sail out to sea expecting the fog to be gone by the time you return to port.
Sea Fog
Sea fog forms typically in tropical maritime air as it moves northwards over colder waters. Expect it for instance in warm, moist southwesterly winds blowing in from the Azores or further south. The critical factors are the dewpoint of the air and the temperature of the sea surface.
The dewpoint of any particular mass of air is defined as the temperature to which it must be cooled for condensation to occur, that is for fog to form. Air from warm sea areas to the southwest arrives with a high dewpoint, often higher than the sea surface temperature around Britain, so it becomes characteristically foggy. Air originating over dry land or cold seas has a low dewpoint, usually well below the sea surface temperature around Britian and is not associated with fog.
When the dewpoint value is marginal the incidence of fog relates to the variation in sea temperature. In winter and spring the sea is coldest inshore so fog forms more frequently inshore than out to sea. In summer and autumn the sea is coldest away from the shore so fog forms more frequently out to sea. There are always variations in sea temperature from place to place and consequently variation in sea fog. Many changes in sea temperature are due to the tide, so the position and extent of fog banks after with changes in the tide.
If the dewpoint of the air is everywhere well above sea temperature, widespread of extensive fog is forecast. It is only a little above sea temperature, and in some cases may not be so, fog banks are forecast. If the dewpoint of the air is above the sea temperature only in a few places fog patches are forecast, and coastal fog in winter and spring.
If you are caught in widespread fog there is little you can do other than sail to the lee of the nearest land or wait for the next cold front. The sample of sea surface temperature isotherms for a typical five period in July illustrates how variable the temperature is. If the dewpoint of air moving in on southwesterly winds was 15 degrees C, fog would be widespread south of Cornwall and in the Bristol Channel, but the coastal waters off northern France would be largely clear.
As the sea fog gets older its characteristics gradually change: it becomes colder and heavier, and immune to variations in sea temperature. The `haar` which sometimes envelopes the coasts of north and east Scotland is of this type. It often starts life in warm, moist southerly winds moving over colder seas around Ireland. By the time it reaches Scotland a day or more later it has cooled many degrees by radiation from the top of the fog) and becomes a very wet and persistent fog with few if any breaks except in the lee of the land.
Steep Waves
A change in wind direction, or a change in tide may mean a change to a wind-against-tide situation. Beware! This can lead very quickly to a dangerously steep sea. The reason is as follows.
The height of the waves depends on:
- the strength of the wind
- for how long and over what distance it has been blowing
When there is a current running the wave height remains at the value appropriate to the wind strength, but distance apart (wavelength) changes. It is reduced when the current is running against the wind. A shorter wavelength means a steeper wave.
Currents:
- off headlands such as Portland Bill
- over ledges such as the Needles
- through races such as the Little Russel
- through narrows such as Corryvrechan
can be strong enough to reduce the wavelength by over 50% in an opposing wind, giving very steep and dangerous seas. Conversely the increase in wavelength when tide and wind are running in the same direction will make life a lot easier.
This article is taken from Weather at Sea by David Houghton, fourth edition;ISBN 1 904475 16 7