the knowl­edge

Many of us trust smart­phone weather apps, but they are just data, not a mea­sured sci­en­tific pre­dic­tion. Me­te­o­rol­o­gist Si­mon Keel­ing re­veals what you need for a real fore­cast

Yachting Monthly - - CONTENTS -

Weather apps need to be read with cau­tion to get a re­li­able weather fore­cast, says Si­mon Keel­ing

Weather fore­casts can seem mys­te­ri­ous. As a me­te­o­rol­o­gist I some­times feel like Severus Snape – the Dark Arts teacher at Hog­warts School of Witch­craft & Wizardry – cast­ing a spell to cre­ate a pre­dic­tion as to what the weather is go­ing to do.

So what ac­tu­ally goes into mak­ing a weather fore­cast for sail­ing, and how can you as a sailor in UK wa­ters make the most of them? Whilst this topic can seem hugely con­fus­ing, an amount of knowl­edge and some prac­ti­cal ap­pli­ca­tion can help you im­prove your own fore­cast­ing skills and in­ter­pret the data from weather apps.

CON­SIDER EACH EL­E­MENT

For starters I thought we’d look at what ac­tu­ally goes into mak­ing a weather fore­cast. Many of us are so used to glanc­ing at weather apps on our smart­phones that we never give a sec­ond thought to the data that has pro­duced that pre­dic­tion; that is, un­til the fore­cast goes wrong!

Ini­tially, ob­ser­va­tions from across the world are made at sta­tions on land, ships at sea and from weather bal­loons sent high into the at­mos­phere. The ob­ser­va­tions are all made at the same time. You can see the data from this at web­sites such as www.weath­eron­line.co.uk (click on ‘cur­rent weather’) and www.xcweather.co.uk.

As well as find­ing its way onto web­sites, this data is also in­gested into some of the world’s largest su­per­com­put­ers. These elec­tronic beasts de­vour num­bers, spew­ing out pre­dicted data based on equa­tions held within their vast brains.

How­ever, it’s im­por­tant for sailors to re­alise that not all of these equa­tions are per­fect. Some are, such as the law of thermo-dy­nam­ics, but oth­ers are ap­prox­i­ma­tions of how the at­mos­phere may

be­have, given a cer­tain set of cir­cum­stances. So there are in­built er­rors within the com­put­ers be­fore we have even started.

ER­ROR CAN BE EV­ERY­THING

I don’t want to make you wary of all fore­casts, but the mar­gins of er­ror can be quite large. That’s why I worry about sailors tak­ing too much no­tice of lo­ca­tion-based apps. Any winds given in these apps could have a mar­gin of er­ror of one Beau­fort force and more in cer­tain con­di­tions. Al­ways err on the side of cau­tion and add at least one Beau­fort force on top of what­ever speed the app is fore­cast­ing, thereby in­creas­ing your safety mar­gin. Not only that, as the fore­cast ex­tends out­wards in time these er­rors can be­come mag­ni­fied. So a 2- or 3-knot wind er­ror on a 3-hour fore­cast may be­come a 10-knot wind er­ror on the 24-hour fore­cast.

BUILD­ING THE FORE­CAST UP

Are you wor­ried yet? Well don’t be. Us me­te­o­rol­o­gists live with the knowl­edge of those un­cer­tain­ties ev­ery day. We see the same er­rors but ac­count for them in fore­casts such as the Ship­ping Fore­cast. Af­ter all, the fore­cast charts pro­duced by com­put­ers are only for guid­ance, it’s a me­te­o­rol­o­gist’s job to make sense of them. Each client has dif­fer­ing re­quire­ments for the fore­casts. For sailors it’s wind di­rec­tion and speed, sea state, vis­i­bil­ity and tem­per­a­ture which we fore­cast­ers are aware is crit­i­cal. For oth­ers, such as builders, it may be tem­per­a­ture and rain­fall which are im­por­tant. The fore­caster must amend the pre­dic­tion pro­duced by the com­puter to sat­isfy the needs of each cus­tomer.

When writ­ing fore­casts for sailors I use sim­i­lar in­for­ma­tion to that which you will see on an app. This is the raw, un­al­tered data be­ing spewed out by those su­per­com­put­ers. My first task, and the thing I sug­gest you do first, is to look up! Is the weather at the present time do­ing what the fore­cast says it will do? If not, that’s a sure sign not to trust the fore­cast. Af­ter all, if the next few hours is wrong how can you trust the fore­cast for the next 10 days?

Se­condly, I’ll look at what the weather is do­ing at nearby sta­tions. Is that ty­ing in with what the fore­cast is say­ing – is the wind di­rec­tion and speed how I’d ex­pect it to look at this time within the sur­round­ing 50 miles?

Thirdly, check the in­stru­ments on­board. You should have the min­i­mum of a barom­e­ter and the ma­jor­ity of us have a Mark I Eye­ball too. Use them both to ‘smell’ the weather. Is the barom­e­ter ris­ing or fall­ing as it should? What does the weather ac­tu­ally feel like? If the Ship­ping Fore­cast says driz­zle then the air should be feel­ing quite damp, even if it’s not rain­ing. Should it be a fair fore­cast, is the air feel­ing dry? If the con­di­tions aren’t do­ing what is pre­dicted then there’s a prob­lem with the fore­cast.

It’s all about cross-check­ing what the raw data is sug­gest­ing against what you are ac­tu­ally ex­pe­ri­enc­ing. That’s pretty much what the first part of a fore­cast­ing day is about. I’ll take a look at the cur­rent weather chart, draw on the iso­bars, as­sess what the weather is like over a large area and then use the data pro­duced by the com­puter to guide my fore­cast.

Above all, re­mem­ber the com­puter (or app) is a tool; a guide as to what the weather might do. Fore­cast­ing should be a LAF: Look, As­sess, Fore­cast. And re­mem­ber, the best aid to fore­cast­ing is you!

Plac­ing too much faith in weather apps could be risky

Dr Si­mon Keel­ing is a me­te­o­rol­o­gist and founder of www. weath­er­school.co.uk where he runs cour­ses for sailors and avi­a­tors.

Your weather app may have pre­dicted a gen­tle breeze, but it needs sen­si­ble in­ter­pre­ta­tion to come up with a real fore­cast where you are

Ev­ery yacht should still carry a barom­e­ter

Mon­i­tor­ing pres­sure changes is good sea­man­ship. Many dig­i­tal in­stru­ments have in­ter­nal barao­graphs

Check­ing live data from weather sta­tions is a good way to as­sess a fore­cast’s re­li­a­bil­ity Vast su­per-com­put­ers are the brains be­hind most fore­casts but their pre­dic­tions shouldn’t be taken as gospel The key to great fore­cast­ing is to bal­ance data from one or more sources with your own ob­ser­va­tions

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