Wind Strategy

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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’s Weather 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 1 Isobars 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:

1 degree of latitude = 60 nautical miles

Step 2 Scale 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, see Weather at Sea.

Step 3 Geostrophic 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, and this 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