PRE­DICT AND AVOID FOG

Get­ting caught out by fog is an in­evitable right of pas­sage for ev­ery yachts­man, but it can be pre-empted and avoided if you know what to look for, ex­plains Si­mon Keel­ing

Yachting Monthly - - CONTENTS -

Cop­ing with fog isn’t fun, but recog­nise the warn­ing signs and you won’t be taken by sur­prise

Fog. Fog patches. Mist. Haze. All are words we all hear in the Ship­ping and In­shore Wa­ters fore­cast, but did you know that each of them car­ries a strict def­i­ni­tion and that any sailor can have a de­cent at­tempt at fore­cast­ing them? All you need is a lit­tle knowl­edge about why and how they form.

HAZE

So, let’s start with haze. This is de­fined as a re­duc­tion in vis­i­bil­ity caused by some type of pol­lu­tant in the at­mos­phere. This in­cludes dust, smoke and even salt re­leased from a rough sea. It’s a hard one for the sailor to pre­dict, but there are cer­tain sit­u­a­tions in which haze is more com­mon. Watch for a flow from the con­ti­nent, usu­ally on an east or south-east breeze. This im­ports pol­lu­tion from the in­dus­trial ar­eas of France, the Low Coun­tries and Ger­many. A steady breeze of say Force 3 or Force 4 is per­fect for haze; any­thing stronger and the tur­bu­lence caused tends to dis­perse the pol­lu­tion par­ti­cles, any­thing less and the par­ti­cles tend to set­tle at ground level.

MIST

Next is mist, which is de­fined as re­duc­tion in vis­i­bil­ity caused by wa­ter droplets. Of course, fog is also caused by wa­ter droplets in the at­mos­phere, but the dif­fer­ence be­tween mist and fog is the ac­tual vis­i­bil­ity as­so­ci­ated with each. In mist the vis­i­bil­ity is greater than 1,000 me­tres and usu­ally less than 5km.

Misty con­di­tions for sailors tend to fall into two cat­e­gories. Con­sider the mist that drifts over an in­land ma­rina or river in­let. This is usu­ally caused by cold air flow­ing off the land over warmer wa­ter. Con­den­sa­tion oc­curs as the wa­ter vapour which is usu­ally in­vis­i­ble in the at­mos­phere can no longer hide it­self. Winds need to be light in or­der for mist to form; look for a speed of F2 or less.

Sec­ond is the mist which forms over more open wa­ter. This gen­er­ally oc­curs when the at­mos­phere is car­ry­ing a fair amount of wa­ter vapour. Look out for warm air flow­ing over a colder sea. A typ­i­cal wind di­rec­tion for mist to form in the UK is a south­west­erly aird­flow. Wind speeds for this type of mist for­ma­tion tend to be around Force 4.

FOG

Now the big­gie; fog! Again let’s think about two dif­fer­ent forms of fog. Firstly, there’s the fog form­ing over the in­lets and in­land mari­nas. This is es­sen­tially the same as the mist for­ma­tion but oc­curs when the air mass is more moist. Watch for a wind speed of around a Force 2 as this pro­vides the per­fect con­di­tions for fog for­ma­tion, es­pe­cially af­ter a cold night. The wind blow­ing off­shore is also key. Usu­ally this type of fog does not ex­tend far off­shore and is usu­ally fairly easy to sail away from.

The main type of fog that af­fects our sail­ing is that which is caused by a moist, trop­i­cal mar­itime air mass. This is air, which has orig­i­nated close to the trop­ics, per­haps in the area of high pres­sure around the Azores. It gets trans­ported north-east­wards and may well start its life com­pletely free of cloud. That’s be­cause at warm tem­per­a­tures the air can hold plenty of wa­ter as an in­vis­i­ble vapour.

As the air gets blown north-east­wards it flows over cooler At­lantic wa­ters. Even­tu­ally the tem­per­a­ture of the air mass is such that it can no longer hold the wa­ter as an in­vis­i­ble vapour, and so it con­denses. Usu­ally, the wind is strong enough and the lower few hun­dred feet of the at­mos­phere warm enough to lift the con­densed air into cloud.

How­ever, if the air mass is suf­fi­ciently wet, the air can come soaked through, sim­i­lar to a sponge. This en­ables fog to ex­ist from the sur­face and is the rea­son it can ex­ist even when the wind is blow­ing at gale force. Re­mem­ber, too, that if the wind is blow­ing on­shore, the lift­ing ef­fect of the wind hit­ting a high cliff or coastal hill can be enough to cool the air suf­fi­ciently to al­low fog to form; hence coastal fog.

So how can the sailor bet­ter fore­cast fog? Well, it may sound ob­vi­ous but is there any men­tion of fog or mist in the Ship­ping and In­shore Wa­ters fore­cast? That should set alarm bells ring­ing that if you do not have fog present at the cur­rent time, it could form.

What about wind di­rec­tion? The most likely wind di­rec­tion for fog for­ma­tion in the UK and Ire­land is a south­west­erly. Speeds of around Force 4-Force 5 are per­fect. Check on the sur­face pres­sure chart and see where the air that is af­fect­ing the area you are in­ter­ested in sail­ing in has orig­i­nated — it’s eas­ier than you may have thought: re­mem­ber that winds flow along the iso­bars; clock­wise around high pres­sure, an­ti­clock­wise around ar­eas of low pres­sure. Fol­low iso­bars back­wards, against the wind. Is the air com­ing from any­where in the sub trop­ics, close to the Azores high? This will likely be warm air, which if it has tracked north­wards over cooler wa­ter is likely to be fog­ging out.

It may be that fog is form­ing in an area of lighter winds. When this type of fog oc­curs the wind is of­ten very light be­cause the air has orig­i­nated close to the Azores high, drifted north and then stag­nated close to our coasts.

So the key for a sailor when pre­dict­ing fog is; check the wind di­rec­tion and speed, look where the air has come from, think about how much mois­ture there is in the air and lis­ten for the clues in the Ship­ping and In­shore Wa­ters fore­casts.

Dr Si­mon Keel­ing is a Royal Me­te­o­ro­log­i­cal So­ci­ety me­te­o­rol­o­gist and the founder of Weather School

Don’t be fooled by good sail­ing winds. Fog at Forces 4-5 off­shore isn’t un­suai

Synop­tic charts are a use­ful tool for any sailor. Could you spot the fog risk on this chart?

A moist air mass and low winds fol­low­ing a cold night. Per­fect con­di­tions for coastal fog

Spot the high form­ing over the north At­lantic. As this moves east, it is an early warn­ing olf pos­si­ble fog

A trop­i­cal air mass from the Azores head­ing East may well be bring­ing fog with it

Prime con­di­tions for fog can some­times be spot­ted on a synop­tic chart

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