Wind Strategy E-Book

David Houghton

• Herausgeber: Fernhurst Books Limited
• Kategorie: Lebensstil
• Sprache: Englisch

Veröffentlichungsjahr: 2016

Beschreibung

The wind powers everything a sailor does and this book will help you to understand it. As a result you will be more prepared for your race, able to anticipate changes in the wind better and know what to do when they come. The first edition of this book was published in 1986, and it has been the go-to wind book for dinghy champions ever since. This new-look fourth edition is fully updated for modern forecasting and analyses a revised set of popular racing venues around the world: unveiling what to expect from the weather at over 25 regatta locations, it will get you ahead of the competition and powering up the leaderboard.

Leseprobe

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WIND STRATEGY

Dedicated to David Houghton1928 – 2015

Growing up sailing with my family around the Isle of Skye, off the often wild west coast of Scotland, instilled a passion in me for weather. I discoveredWind Strategyin my early teens when dingy racing, so it was a thrill ten years later to receive a call from the author himself.

David Houghton had been meteorologist to the British Sailing team at eight Olympics and had just returned from the very successful Sydney Olympics in 2000. He asked if I’d like to help him with his ninth in Athens. This coincided with Peter Harrison creating GBR Challenge, skippered by Ian Walker. I’d studied in Auckland at the National Institute of Water and Atmosphere (NIWA), working alongside Team New Zealand where Peter Blake showed me his weather team set up. I was inspired, and a few years later, very happy to be back in Auckland with GBR Challenge and Athens with the RYA Olympic Team with David to guide me.

David was an avid note taker. He would compile all his observations to create a ‘call book’ for each and every venue he visited or sent a remote forecast to. Looking through these notebooks, which he kindly gave to me, gave an insight into the wealth of knowledge which enabled David to first writeWind Strategy. The internet has changed many things, but it cannot change the weather and all his ideas, observations and theories still hold true today.

Having a mentor like David was indeed an honour: he was patient, full of good humour and many, many stories! He was passionate about sailing and educating sailors, not solely about producing the best forecast, but about disseminating his knowledge to enable you, the sailor, to head out onto the water with confidence in your own weather knowledge and wind strategy.

I miss him and hope this edition maintains his standards.

Fiona Campbell

WIND STRATEGY

David Houghton & Fiona Campbell

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Fourth edition published in 2016 by Fernhurst Books Limited 62 Brandon Parade, Holly Walk, Leamington Spa, Warwickshire, CV32 4JE, UK Tel: [phone] | www.fernhurstbooks.com

First edition published in 1986 by Fernhurst Books

Copyright © 2016 David Houghton & Fiona Campbell

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except under the terms of the Copyright, Designs and Patents Act 1988 or under the terms of a license issued by The Copyright Licensing Agency Ltd, Saffron House, 6-10 Kirby Street, London EC1N 8TS, UK, without the permission in writing of the Publisher.

Designations used by companies to distinguish their products are often claimed as trademarks. All brand names and product names used in this book are trade names, service marks, trademarks or registered trademarks of their respective owners. The Publisher is not associated with any product or vendor mentioned in this book.

This publication is designed to provide accurate and authoritative information in regard to the subject matter covered. It is sold on the understanding that the Publisher is not engaged in rendering professional services. If professional advice or other expert assistance is required, the services of a competent professional should be sought. The Publisher accepts no responsibilty for any errors or omissions, or for any accidents or mishaps which may arise from the use of this publication.

A catalogue record for this book is available from the British Library ISBN 978-1-909911-54-3 (paperback) ISBN 978-1-909911-85-7 (eBook) ISBN 978-1-909911-86-4 (eBook)

All photographs © Fernhurst Books Limited, except: p12, 33, 69, 77, 85 © Tom Gruitt; p18 © Bob Orsillo/Shutterstock; p29, 63, 64, 65, 67, 68, 83, 84 supplied by Pixabay; p37, 38 © Sealand Aerial Photography Ltd./Fernhurst Books Limited; p48 © D Waschnig/Shutterstock; p60 © brickrena/ Shutterstock; p72 © mokokomo/Shutterstock; p80 © Andrej Pol/Shutterstock; p81 © Bruno Fontes (BRA)

Front cover photograph © Tom Gruitt

Designed by Rachel Atkins

THE AUTHORS

WEATHER EXPERTS & SAILORS

David Houghtonwas a passionate meteorologist, spending over 30 years at the UK Met Office. He was a member of the Royal Meteorological Society since 1949. In 2000, David was awarded the Society’s Michael Hunt Award for “excellence in increasing the understanding of meteorology or its applied disciplines among members of the general public, including particular groups (e.g. sailors)”.

He worked as an advisor to Olympic, Admiral’s Cup, America’s Cup and Round the World Race sailing teams for over 30 years in many different parts of the world. This provided him with the wherewithal to hone and refine the concepts, arguments and rules of thumb in this book.

David’s unique double expertise as a sailor and weather forecaster makes his books invaluable aids to all sailors. BothWeather at SeaandWind Strategyhave been in print since 1986 and are regarded as classic weather texts for the cruising and racing sailor respectively.

Fiona Campbellis a successful meteorologist with a passion for the environment and a love of sailing. She graduated from the University of Reading, with her degree thesis based on a study of the winds in the Hauraki Gulf, New Zealand.

Fiona took over from David the responsibility for advising the UK’s top sailing teams at Olympic, European and World regattas, as well as GBR and other America’s Cup challenges. She has also worked to help boat designers, competitors and race organisers, such as Cowes Week, to optimise their performance.

Fiona worked alongside weather legend David Houghton for many years and became co-author ofWind Strategyfor the 2005 edition – a book which she had loved and used herself when learning to race in the North West Scotland training squad. She has relished the opportunity to update the text again for this latest edition.

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CONTENTS

FOREWORD

CHAPTER 1

The Wind-Wise Sailor

CHAPTER 2

The Sailor’s Wind

CHAPTER 3

Wind Facts: Coasts, Lakes & Islands

CHAPTER 4

Wind Facts: Wind Bands, Water Temperature & The Tide

CHAPTER 5

Wind Facts: Gusts & Lulls

CHAPTER 6

Wind Facts: Southern Hemisphere

CHAPTER 7

The Sea Breeze

CHAPTER 8

Sea Breeze With Gradient Wind

CHAPTER 9

Afternoon Winds: Gradient Wind Onshore

CHAPTER 10

Lakes, Mountains, Valleys & Peninsulas

CHAPTER 11

As The Sun Goes Down

CHAPTER 12

Afternoon & Evening Winds: Southern Hemisphere

CHAPTER 13

Gravity Waves, Billows & Surges

CHAPTER 14

The Message Of The Clouds

CHAPTER 15

Light Airs

CHAPTER 16

Obstacles In The Wind

CHAPTER 17

Water Currents

CHAPTER 18

Waves

CHAPTER 19

Dangerous Waves

CHAPTER 20

At The Regatta

CHAPTER 21

Which Sails?

CHAPTER 22

Popular Racing Venues

KEY QUESTIONS

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FOREWORD

I first met David Houghton aged 17 at an RYA Youth training weekend where he was delivering his weather lecture to young sailors. As a mad keen sailor and enthusiastic geographer at school and university I would hang on his every word. I loved the fact that understanding more about the weather helped to explain what we witnessed around us every day and I loved the fact that understanding the weather helped me to win races. In 2002 I employed Fiona Campbell as our meteorologist for the GBR America’s Cup Challenge in Auckland. Her enthusiasm for the weather and her accurate forecasting was a huge help to our training and in the races we won. She truly picked up the mantle from David and added her modern scientific approach to David’s years of experience.

Both David and Fiona will tell you that I was always the one asking difficult questions and staying behind after the weather briefings to better understand the reasons behind their forecasts. I loved the fact that both David and Fiona would send real weather balloons into the sky each morning to better understand the gradient wind. I will never forget the first race of the Sydney 2000 Olympic Games where David strongly believed the gradient wind would go left outside of Sydney Harbour that day. I grilled him on it after the briefing and in that first race Mark Covell and I set up to win the pin and smash the left hand side of the course. Not exactly conservative tactics for race 1 of the Olympics but we one-tacked the beat and lead around every mark as a result – we won that race thanks to David and because we understood the weather better than the other competitors. It played a big part in us winning our silver medal.

Racing around the world 3 times in the Volvo Ocean Race has given me the chance to sail through entire weather systems and experience many different weather and wave patterns. It has given me an even better appreciation of the wind and weather and especially the effects of topography and clouds on our surface winds. It is true that the clouds really can be ‘signposts in the sky’ and they are crucial to your racing strategy.

As the race courses we sail on have become shorter and shorter and the boats have become faster and faster it is easy to think that the weather and your wind strategy is no longer important. It is true to say that boat handling and starting are even more important than ever but in every race you still need a strategy. If you go the wrong way you will not win the race. That is why professional teams spend a lot of money on forecasting and that is why every America’s Cup and major Olympic team will have a weather forecasting team of their own. The wind and how that wind reacts close to the shore are still fundamental and you ignore the theory behind it at your peril. Every top racing sailor should make it their business to understand the theory behind it.

It is too easy now to look at a weather app or online forecast and feel like that is sufficient to base your weather strategy on when racing. I would encourage every racing sailor to not be satisfied with this. These ‘forecasts’ are improving but are based on models that generally have poor temporal and spatial resolution. They might tell you what clothes to wear that day but they will not explain why the left or right hand side of the course may be favoured at any point in time.

Weather is multidimensional and rarely simple but in this book both David and Fiona expertly explain what can be a complicated subject in an easily understandable format. You will remember or learn something every time you pick up this book. I can guarantee that something you learn in this book will help you improve your racing results. Remember it all and you may no longer be the sailor in the bar explaining how ‘unlucky’ you were when the wind shifted against you!

Wishing you fast and happy sailing!

Ian Walker

2 x Olympic silver medallist, Olympic gold medal winning coach, the first British Volvo Ocean Race winning skipper, 4 x World Champion, 2 x America’s Cup challenger, YJA Yachtsman of the Year 2000 & 2015

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CHAPTER 1

The Wind-Wise Sailor

It has long been assumed that a helmsman competing on home waters has an advantage over a visitor because years of practice have imparted a ‘seat of the pants’ appreciation of the behaviour of the local wind. The confidence of the ‘seat of the pants’ sailor rests in the past. Every decision about a windshift is based on the argument ‘it happened last time’, or ‘in the same month X years ago’.

The confidence of the wind-wise sailor, on the other hand, rests in an appreciation of the causes of bends and bands in the wind whereby accumulated experience at a variety of venues increases racing skill. Because the weather demonstrates an almost infinite number of variations, there will inevitably be occasions when the ‘seat of the pants’ sailor is caught out, having never seen anything like it before. The wind-wise sailor, however, will identify a reason for the unusual event and is likely to sail better through making well-founded decisions. To be right every time is hardly possible, but knowledge increases with every new observation as new pieces are added to the total picture of weather wisdom.

Although every sailing venue is different, the forces which create and control the wind are in principle the same everywhere. There is a scientific reason for every windshift and bend, and virtually all those which are important to the racing sailor can be understood by the application of basic and straightforward principles of meteorology. Taking a laptop in a racing dinghy is not an option, and numerical modelling of mesoscale wind systems in support of dinghy racing is little short of taking a sledgehammer to crack a nut.

The best and only realistic solution is the development of simple conceptual models of wind behaviour such that every reasonably intelligent sailor can recognise what is happening while racing, identify the causes of the wind patterns experienced and make informed on-the-water decisions.

Similarly with clouds: there are very many variations on the theme of lines and bands of cloud, and indeed great artists have for centuries found them a never-ending source of inspiration. For the sailor every cloud and every cloud pattern conveys a message of some sort concerning the origin, movement, and stability of the air it represents. Chapter 14 looks at the messages which are capable of translation into tactical advice.

When David Houghton first wrote this book, National Meteorological Services did not make many detailed wind observations in coastal waters. By and large the only observers were sailors. Their observations reported following a day’s racing, in their log book or by word of mouth, originally formed the mainstay of this study. Increasing numbers of weather stations and availability of data on the internet has helped to repeatedly prove the basic principles David developed and which are detailed in this book.

The following chapters are a result of some 50 years of study of sailing venues all over the world, working closely with sailors involved in world-class racing from round-the-buoys to round-the-world events. Most of the basic principles are presented in terms of simple conceptual models of wind behaviour. The principles are the same in both hemispheres but the rules of thumb and the geometry of the models differ from the Northern to the Southern Hemisphere. So the main arguments are developed for the Northern Hemisphere, followed by a couple of chapters summarising the differences which apply for the Southern Hemisphere.

Large scale weather systems are explained in David Houghton & Libby Greenhalgh’sWeather at Sea, also published by Fernhurst Books, which includes guidance to the understanding, interpretation and construction of weather maps.

You need a weather map, not just a spot forecast from the latest app, to give an overall picture of what the gradient wind is doing and what changes are expected; a first and essential stage in deducing the finer details of what to expect during a race.

Iain Percy & Andrew Simpson: Wind-wise sailors who worked with David Houghton & Fiona Campbell

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CHAPTER 2

The Sailor’s Wind

Anything moving requires energy to start it off, and in most cases to keep it going. The wind is no exception. Air moves around the Earth in response to heating by the sun. Equatorial regions receive the most heat, polar regions the least. The major wind systems of the world are all the result of heated air rising over equatorial regions and being replaced by colder air from polar regions. The zone where the major cold and warm winds meet is commonly known as the polar front, and is the birthplace of many of the larger weather systems – the depressions and anticyclones – of temperate latitudes.

The traditional (and easiest) way to map the movements of air around the world is to plot the values of pressure, or weight of air, at the Earth’s surface. Such weather maps with their lines of equal pressure – isobars – have been in use for over a hundred years, ever since the invention of the electric telegraph. Weather satellites have provided pictorial evidence of the size, shape and main characteristics of depressions and anticyclones, the clouds acting as dye in the air to map out their development and decay.

A snippet of a weather map

In its simplest form: Heated air rises over the equator to be replaced by cooler air from the poles

Due to land and the Earth’s rotation, this simple cell model (left) is modified (above)

The Pressure Gradient Wind

To map the winds over an area of hundreds of kilometres there is still no substitute for the surface pressure pattern, because there is a direct relationship between the wind and the gradient of surface pressure. Wherever there is a pressure gradient a wind blows with a strength directly proportional to that gradient. If the earth was not rotating, the wind would blow straight across from high pressure to low pressure – as you might expect. But because of the Earth’s rotation it blows across the pressure gradient (except near the equator), one way in the Northern Hemisphere and the other way in the Southern. You can best remember which way using Buys Ballot’s Law which states that in the Northern Hemisphere, if you stand with your back to the wind, the low pressure is on your left-hand side (below).

Buys Ballot’s Law

To enable the sailor to take full advantage of this relationship between pressure gradient and wind, many weather maps are printed with a scale in one corner called the ‘geostrophic scale’. An example follows. Take a pair of dividers, set the points at right angles to adjacent isobars over the area of interest, then transfer their distance apart to read (on the scale) the wind speed for the appropriate latitude. Note that for a given isobar spacing the wind is much stronger in low latitudes than in high latitudes.

Example:Geostrophic Scale

The geostrophic scale is the universal scale for obtaining wind speed in knots from a weather map with isobars at 4 millibar intervals.

Step 1

Measure the distance apart of adjacent isobars on any weather map for the area you want.

Step 1Isobars on a weather map

Step 2

On the scale, set your dividers to the distance you measured in Step 1. In this case: 100 nautical miles at 50° N. If no distance scale is provided use the relationship between distance and latitude:

Step 2Scale of nautical miles

Step 3

Transfer your dividers with the setting obtained in Step 2 to the geostrophic wind scale, again for the appropriate latitude. Interpolating between the two vertical lines gives a wind speed of 32 knots. For a more detailed introduction to weather maps, weather systems and the pressure gradient wind including the all-important Earth-turning (Coriolis) Force, seeWeather at Sea.

Step 3Geostrophic wind scale in knots for 4 millibar intervals

Local Winds

The global picture of the creation, movement and interaction of warm and cold air masses is repeated on virtually every scale down to that of the garden bonfire, where the hot air carries the smoke upwards and is replaced by colder air moving in around the sides (below). This essentially simple picture of air movement into and upwards from a bonfire typifies what happens continually all over the world. Variations in heating and cooling of the land and sea due to variations in cloudiness, topography, time of day, colour of the land, angle of the sun, etc, all feature in the production of local winds.

The creation, movement and interaction of warm and cold air masses

Thinking about the pattern of air movement around a fire will help you tune into the air movements due to temperature differences between land and sea, or between one side of a valley and another – changing as the sun moves round. You can also contemplate the airflow into and out of a typical cumulus cloud, and the origins of the gusts and lulls in the wind to which we constantly have to tack. You may also be able to make a reasonable shot at predicting the onset of a new breeze, or the next stage in the evolution of the one you have got. The purpose of this book is to make you a wind-wise sailor, capable of the best possible decision at every stage of a race.

Drag & Stability

We have mentioned how the pressure gradient drives the wind, andthis pressure gradient wind is found at a height of about 500 metres, above the influence of surface drag or friction. We have also seen how air warmed at the ground rises to be replaced by colder air from aloft. The extent to which this happens depends upon the stability or buoyancy of the air which strongly influences the ability of the wind to overcome surface friction. Let’s look at these factors in turn:

Drag:The rougher the surface the greater the drag. A smooth sea exerts minimum drag, a forest gives near maximum drag. Drag influences the speed of the wind: the greater the drag the slower the wind for a given pressure gradient. It also influences the wind direction, backing the wind from the pressure gradient direction in the Northern Hemisphere and veering it in the Southern Hemisphere. “Backing” means the wind direction swings anticlockwise, “veering” means the change is clockwise.

Drag influences the wind direction differently in the Northern & Southern Hemispheres

Over a smooth sea the surface wind is only about 15° back from the wind at 500 metres. Over a forest the difference may be 40° or even more.

Surface wind over smooth sea & rough land

A modern town with a variety of high rise buildings presents obstacles to the wind rather than a simple friction effect. These are discussed in Chapter 16.

Stability:The critical factor in determining the stability of the air at ground or sea level is the temperature of the surface. To overcome drag there has to be a continual transfer of momentum downwards. This process is seriously hindered when the air is stable, and encouraged when it is unstable (buoyant). Air that is warmed at the earth’s surface becomes unstable and rises to be replaced by colder air from above. Air that is cooled at the earth’s surface becomes stable and resists any attempt to make it rise. Unstable air is continually overturning and transferring momentum downwards, minimising the effect of surface friction. In stable air there is little interaction between the air near the surface and the air higher up, merely the drag of the air on itself, which is often insufficient to keep it going at the surface, so that frequently the air near the ground stops moving altogether (below).

Visibility is a good indicator of how well the air is mixed by overturning. In unstable air visibility is typically good. In stable air pollution is trapped near the ground and it is typically hazy with poor visibility.

The stability of the air impacts the presence of surface wind

Change In Wind Over Land Between Day & Night

Over land the rise and fall in temperature between day and night causes changes, often major changes, in the wind. There are, of course, sea breezes and land breezes which we will consider in later chapters. But aside from these the mere change in surface temperature means a change in wind as the air near the ground goes through a diurnal cycle of heating and cooling. From sunrise through to mid-afternoon the air near the ground becomes increasingly unstable, and as the downward transfer of momentum increases so does the wind. As the sun goes down the temperature falls and the wind decreases. After dusk if there is little or no cloud the surface temperature falls quickly, the air becomes very stable and the surface wind soon dies.

These considerations are as important for inland sailors as for those on coastal waters. The wind strength over a small lake is largely determined by what happens over the land around. Near coasts the influence of the changing land temperature is particularly noticeable when the gradient wind is blowing offshore.

If the wind is strong – 25 to 30 knots or more – there is usually enough mechanical turbulence to keep the air well-mixed and maintain a downward transfer of momentum throughout the night as well as the day. This effectively prevents not only a fall in temperature at night but also a rise by day, and the wind is more constant.

On occasions when the wind dies away at night and cold stable air becomes established near the surface over land, it can be difficult to shift. Where the cold stable air is protected by hills from the wind above, for instance in a deep valley, it can persist for several days or until cloud and rain arrive to help move it.

A typical record of windspeed over 24 hours due solely to the rise and fall in temperature as the sun rises and sets

Change In Wind Over The Sea Between Day & Night

At sea the surface temperature varies little from day to night – a degree or two at most – since the specific heat of water is much greater than that of land and also because mixing is fairly continuous. In many places the largest variations in sea surface temperature over a period of a few hours are associated with tidal movements.

Under skies covered in low cloud, however, a significant change in wind between day and night is experienced due to the rise and fall in temperature of the top of the low cloud. Under persistent low cloud the sea surface temperature remains constant; there is no sun to warm it and no clear sky at night to allow it to cool. But the top of the cloud will warm by day and cool at night. The colder the cloud top the greater the temperature drop from the sea surface to the top of the cloud, so the more unstable the air and the stronger the wind. The warmer the cloud top the smaller the temperature drop from the surface, so the more stable the air and the lighter the wind. Thus we have a reversal of the typical diurnal variation in wind over land with the strongest wind in the early hours and the lightest in mid-afternoon. A good example is found offshore at San Diego where over a cold sea we find a persistent layer of stratocumulus cloud with the average wind varying from about 13 knots in the early morning to 6-8 knots in the afternoon.

View above persistent stratocumulus cloud

The temperature of the top of low cloud impacts the stability of the air over the sea, and thus the wind strength