Look upwind at the surface of the water: darker, rippled patches have more wind in them, and smoother, shinier patches have less. Gusts travel downwind towards you, so you can see them coming and get ready.
The wind is invisible, but its effect on the water is not. Wind roughens the surface into small ripples, and a rippled surface reflects less sky, so it looks darker. A gust shows as a dark patch moving across the water; a lull shows as a paler, smoother area. In very light air you may see small dark fans spreading out on glassy water, often called cat's paws.
To use this, look upwind regularly, not just at the sails. A gust you can see a few hundred metres away will reach you in a minute or so, which gives you time to tell the crew, get ready to ease the mainsheet, or plan to steer through it. A lull ahead tells you to expect the boat to slow and come upright.
Other clues help build the picture:
- White horses, waves with breaking white crests, start to appear at around 11 to 16 knots. When they cover the water, it is a fresh breeze.
- Boats on moorings lie head to wind (unless a strong tide is holding them), so they show the direction near the surface.
- Flags, smoke and other sails show both direction and changes in strength.
- The wind indicator at the masthead shows the apparent wind on your own boat.
Near land the wind is usually gustier and shiftier, because hills, buildings and trees break it up. It can also bend round headlands and funnel along valleys, so the wind close to a shore may come from a quite different direction from the wind out in open water.
A common mistake is to watch only the sails or the instruments, so that every gust arrives as a surprise. Make looking upwind a habit, every minute or two.
More in the book: Chapter 10, When the wind gets up