A very important part of prerace planning is figuring out the wind, its direction and strength – short the weather basics. In sailboat racing knowing which way and how strong the wind blows is like knowing the numbers of tomorrow’s lottery already today. To understand the wind we need to take a look at the helicopter’s perspective and learn about the whole process – the weather.
Anything moving needs energy to get started and kept going. The wind is no exception. Air moves around the earth – the sun contributes the energy. On the earth the equatorial regions receive the most heat and the polar regions the least. These two zones are the reasons for the world’s major wind systems. The air always moves from the cold region to the warmer one. At the heated region the cold air gets heated as well, rises, cools and moves back to the colder region.
The easiest way to map the movements of air (=wind) around the world is to plot the values of pressure. The values of pressure can be defined as the weight of air. Lines with equal pressure are called isobars. You find them on weather maps. Isobars with higher numbers float around a high pressure system (high) and isobars with low pressure float around a low pressure System (low).
The Gradient Wind
The easiest way to map the wind over a large area is the surface gradient pattern. Wherever there is a pressure gradient the wind strength is directly proportional to that gradient.
The gradient wind is a balance of the pressure gradient force, the centrifugal force and the coriolis effect. A geostrophic wind becomes a gradient wind when the wind begins flowing through curved height contours. The curving motion introduces a centrifugal (outward fleeing) force. If the earth was not rotating the wind would blow straight from high to low pressure. Because of the earth’s rotation it blows across the pressure gradient.
Local Winds & influence of day and night
The weather basics of the big pattern you have just learned about can be found in every scale around the world. The simplest example of a local effect is the temperature differences between sea and land on a coast. During the night the land cools down much quicker than the sea and during the day it heats up much faster – especially with the influence of the sun when there are no clouds. The sailor experiences local thermals or a sea breeze.
Wind blowing off the shore
When there is wind blowing off the shore it normally turns to the right (veer). The reason for that is less drag as it moves out over the water. At the same time the wind hits the water the wind speed increases. This means if you are beating towards a mark you can generally expect port tack to pay. The closer you get to the coast the larger the bend in the wind. The length of the bend depends on the stability of the air. The more stable the air is the longer the bend. Very stable air describes a typical winter situation; less stable air is more likely in summer with high temperature variations. The length of the wind bend may variate between 1-6 kilometres.