An introduction into wind shifts
The racing course normally starts with a leg straight into the wind, the beat. This is purposefully set to split and spread the fleet so that the sailing is not congested. This happens because the sailing boats can not sail directly into the wind, and need to sail a course approx 45deg (dependent on type of boat) off the true wind direction. There would be no advantage from sailing on port tack or starboard tack if the wind direction remained constant. Fortunately, nature adds the fun into sailing by continually changing its wind direction and strength.
Observe racers before the start of the race. The keen sail racers go off tacking to the first buoy mark to gain a feel for the changing direction of the wind. They understand that the wind oscillates about a mean direction, but take note of the size of oscillation (angles) and the frequency (time). For reading the changing wind shifts a frame of reference is required, to compare the boat heading with wind direction. Sailors can use external features on the reservoir like trees, pylons and smoke stacks. However, on different parts of the course these perspectives changes with distance. Tacking back and fore, the sailor can become dizzy and lose perspective. The best frame of reference is to focus on the buoy/mark that you are heading towards (remembering to lookout for other boats). During the alternating port and starboard tacks, turning your eyesight and head towards the mark enables the sailor to pick up a physical geometrical appreciation of the different angles.
Observe the course of the boat in diagram 1, it is “S” shaped. Initially on port tack (324deg), the boat heading is lifted (moves further to port) due to the change of wind direction (306 deg). Notice how the boat is headed after 306 deg. but still does not tack. Although the boat is heading away she continues her course to maximise the use of the “S” bend oscillation. On reaching the 324deg. the mean wind direction she realises that the wind has a further 18 degress of the same direction change. Now is the time to tack on to the starboard course. The judgement of reading the wind shifts enables sail racers to gain ground against competing boats. Notice how boat A has gained ground over boat B in diag. 2 due to correctly reading the wind shift “S” bends.
The difficulty in practice is to know how far along a wind shift bend the boat has travelled. This can be carried out by using other boats and buoy flags as wind indicators. Looking at beating boats to windward (further up the course), the sailor has a forecast of the wind approaching. This gives valuable hindsight information that the boats ahead do not have (sudden headers). Alternatively, boats in the lead can make a judgement of their course along a “S” bend wind shift by looking at how different their boat heading is different to the ones behind. This is an easy judgement with boat of the same class, but becomes more difficult in general handicap races due to the different pointing ability of boats (i.e. how close to the wind that they can sail).
Fortunately nature does not perform to a maths text book. Wind travels along the oceans and continents being hindered by obstacles; mountains, sand dunes, buildings, smoke stacks etc. These add all sorts of smaller oscillations on to the main wind shifting course (see diagram 3 below). There is a skill in identifying the smaller wind shift from the main oscillation.
Permanent Wind Shifts
During a time period of a day there are permanent shifts in the wind. The most obvious cause of these are the generation of gust cells in clouds. Winds are falling off the fronts of clouds while their rear are sucking up uplift. The clouds affect the surrounding environment and cause the wind direction to be permanently changed (see diagram 4).
A factor in summer coastal regions is also the battle between early morning land breeze, and the incoming sea-breeze due to the land warming more quickly than the sea.
Starboard for gusts and Port for lulls ?
If you stand on top of cliff during the summer you can watch the “cat’s paws” of gusts going along the sea floor. Watch carefully and you can see that the gusts are going in a different direction to the mean wind. The atmosphere is layered with anticyclone winds weather systems in the northern hemisphere. This is due to the earth’s rotation (Carioles effect). The lower layers of atmosphere are slowed with the friction of the earth. Gusts are formed in the higher speedier layers and propagate downwards towards the slower bottom atmosphere but retain their original direction. Therefore, the gust is veered from the direction of the mean wind and is stronger. On reaching the surface of the earth the gust cell is slowed and gains warmth through friction. Therefore, air rises at the back of the cell (lull). On not seeing the “cat’s paws“, sailors can prelude the main gust by a temporary disturbance of wind known as a gust tongue, see T labelled in diagram. Through the formation of these gust cells the wind oscillates. Therefore, sailing in the Northern hemisphere it’s advantageous to sail on starboard tack through the gust and on port through the lulls while sailing into the wind.
Wind direction changes during a weather system
Stand with your back to the wind in an anticyclone weather system, and as the weather system passes over you the wind will veer to your left.
The cross-winds rule will enable you to gauge your location in a weather system (not detailed here). It is a comparison of high wind direction of high cirrus clouds with the direction of the lower pressured controlled wind.