Wind Shear & Gradient Effects on Trim & Strategy

by
Shevy Gunter


This document is partially based on the "OCKAM U. Seminar Manual" by Ockam Instruments Inc. and on various articles published in the media.

"Wind Shear" and "Wind Gradient" may effect how you sail your Laser. Wind shear is a change in true wind direction with altitude, and wind gradient is a change in true wind speed with altitude. Thus, combined together, they affect the wind seen by your sail aloft, and may considerably affect how you trim your sail on each tack. Wind shear and wind gradient most often go hand in hand. In the USA and Canada, they occur in most sailing areas. Furthermore, their presence can be felt even at small altitude differences, such as at heights below the height of a Laserís mast.

What causes wind shear and wind gradient

Within this height above the water level, there is a large amount of interaction between the wind, the water, and any nearby land. The wind at this level is influenced by the vertical stability of the air.

Vertical stability depends partially on temperatures. When cold air is above warm air, it is vertically unstable. In fact, because air expands as it rises, the case of colder air aloft and warmer air closer to the surface is the "normal" state of the air, in the absence of external effects. The warm air rises, and the cold air sinks, mixing the two layers together. This implies that when mixed, the two layers of air will tend to flow in the same direction and at the same speed. In such cases, there is not much wind gradient effects.

On the other hand, when warm air is above cold air (which is called a "temperature inversion"), the air is vertically stable: the colder and warmer air do not mix together as much. In vertically stable conditions, if there are also meterological changes creating any differences in the speed and direction of the winds at different heights above the water level, then these differences are not mixed. They are not reduced in magnitude due to mixture. The result is strong wind shear and wind gradient. That is, the higher the vertical stability of the air, the higher the likelihood of experiencing wind shear and gradient.

The degree of vertical stability (and hence the likelihood of shear and gradient effects) can be predicted based on the following factors:

The temperature difference between the water and the air above it:

The colder the water, and the warmer the air right above it, the more likely it is to have vertically stable air. The cold water will cool the air right at the surface level, and a colder air level will stay under a warmer higher air level. This implies that you are more likely to experience wind shear and gradient in the springtime when the air is warming up but the water is still quite cold, or early mornings during the colder seasons when the water is still cold but the air warms rapidly.

The weather systemís air characteristics:

Compared to the land, the sea has a relatively uniform temperature. It is not subject to the rapid heating and cooling by the sun the way the land is. In fact, the temperature of the water urcae remains constant both day and night. This is because of several factors. A part of the heat is used in evaporating water. The remaining heat is distributed over a deep layer of water. Although most of the sun's radiation is absorbed in a relatively shallow surface layer, the mixing coused by the wind and waves distributes this heat in a deep layer. The result is that there is little variation in the temperature of the sea surface.

Air masses that form over the seas are generally more stable vertically compared to air masses that form over the land. Wind shear and gradient are more likely to be encountered in these vertically stable marine air masses.

The wind speed:

A stronger wind helps vertical mixing of the air. Thus, light winds allow tempreature inversions to form, and wind shear and gradient are more likely to be encountered when the wind is not blowing hard.

Surface friction:

This is the primary cause of wind gradient. Friction reduces the wind speed at the waterís surface. On an average day, the wind 25 feet above the surface can be about 1.5-2 times the strength of the wind at the water level. You can expect more wind gradient in choppy or large wave conditions.

Symptoms of wind shear

The presence of wind shear can be detected by observing the differences in the way your Laser behaves on different tacks.

Telltale behavior differences:

When there is wind shear (a difference in direction of wind aloft), the telltales on your sail will respond differently on each tack. With identical settings of your sail controls on each tack, you will notice that on one tack, the top of your sail will be "lifted", and on the other tack, it will be "headed". That is, on one tack, the top of your sail will be stalling, with the leeward telltales sagging; and on the other tack, the top of your sail will be luffing. If you use a masthead wind indicator, you will also see that it is pointing noticibly wider on the "lifted" than on the other tack. If you use a gooseneck-level wind indicator, when you trim your sail to your liking (probably based on the average wind angle at the hight of your steering and trimming telltales on your sail), you will see that the burgee is pointing at a considerably narrower angle of attack on the "lifted" tack than on the "headed" tack.

Of course, you will try to compensate for this by trimming your sail correctly for each tack, as described below.

Boatspeed differences:

You will notice that your boatspeed (over the water) is higher on the lifted tack and slower, even terrible, on the headed tack! The Laser will "feel dead" on the headed tack. No matter how hard you try, you will not be able to get her going as fast as on the tack where the top of your sail was lifted. You need to mentally prepare yourself for this, and resist the temptation to tack back onto the lifted tack. (More on this below.)

Helm differences:

With all the extra twist and extra boatspeed on the lifted tack, the helm will feel great. On the headed tack, with the sail sheeted in very tight and lousy boatspeed, the helm will fell "mushy" and it will be hard to find the groove upwind.

Symptoms of wind gradient

On a Laser (any boat without electronics), detecting the presence of high wind gradient will be tougher than detecting the presence of wind shear. Basically, you will notice that you are heeling much more than you would expect based on the wind you feel at the water level on your cheeks and neck. If you capsize (intentionally or inadvertantly), you will note that as you right the boat, the noise the wind makes on your sail will get much louder than you would expect based on the wind you feel on the daggerboard. Another subtle sign is provided by the gusts: the gust will arrive sooner than you would expect based on the progress of the ripples on the water, and the strength of the gust will be stronger than you would expect based on the darkness of the ripples formed by the gust on the water surface. Finally, you will fell that you sail faster than you are used to based on the wind you feel at deck level.

How to deal with wind shear and gradient?

Once, and if, you decide that wind shear and wind gradient are present, you an take some measures to take advantage of them. To compensate for wind shear:

  • On the tack where the top of your sail is lifted, your sail will require more twist. Loosen the vang slightly, and tighten on the cunningham to open the leech.
  • On the tack where the top is headed, you will require less twist: tighten up on the vang, and loosen a bit on the cunningham to close the leech.

Under high wind gradient conditions, on a big boat with masthead electronics, normally the skipper would reduce the "target boatspeeds" to take into account the fact that the average wind speed is actually lower than measured at masthead. This prevents the big boat footing off upwind at wide angles to achieve the unattainable targets boatspeeds based on wind speed at masthead. Similarly, the big boat skipper would need to prevent the temptation to head up downwind to achieve a target boatspeed based on wind aloft.

On a dinghy without any instrumentation like the Laser, the situation will be the reverse. You will feel you are sailing really fast for the wind, and you will have a natural inclination to point while sailing upwind, and to foot off while sailing downwind. This is what you need to avoid: resist the temptation to continuously pinch to gain VMG upwind, and the temptation to continuously head off to gain VMG downwind. Watch the boats around you to see if you are fast, or if everybody else is fast, too. As always, base such course alteration decisions on the usual tactical factors and the changes in wind speed and direction observed. It is more likely that you "feel fast" because the wind is stronger that you feel at deck level rather than because you seem to be "hot" today.

Wind shear, wind gradient, and forecasting wind shifts

When the air is vertically stable, the cooler air at the surface level and the warmer air at higher levels behave more independently. The warmer air aloft responds to changes in atmospheric conditions faster than the cooler air below. It takes some time for the changes in the wind speed and direction observed at high levels to translate to the lower level. You can make use of this to predict wind shifts, and to set your racing strategy accordingly.

The best example of wind shear and wind gradient is a filling sea breeze on a warm spring day: The water is cold, and cools the air at the surface level, while the sunshine warms the air aloft, creating the vertically stable conditions. (We will base the discussion on the Northern hemisphere.) In the Northern hemisphere, a sea breeze moves clockwise to the right as it builds (i.e., it veers). The effect is more pronounced the futher Northh you are sailing. The fefect ceases at the equator and then reverses in the Southern hemisphere.

When a sea breeze starts filling in, the first effects will be seen at the very top of the sail where the air is warmer. If you are sailing on starboard tack, you will notice that the very top leeward telltales tha were drawing fine before will stat stalling. If you are sailing on port tack, you will note the top 1/4 of the sail starting to feel mushy and almost luffing. This is all happening because with the onset of the sea breeze, the wind at the top of the mast may be 15-20į to the right of the wind on the water.

Over time, the sea breeze will fil in downward as well as towards the land. So, you will soon start seeing the veering to the right at lower and lower portions of your Laser sail. How do you use this information for strategic purposes? The strategy is clear. For the period of time until the sea breeze settles in, it will continue veering. Thus, you need to treat it as a persistent shift for this duration, and sail to the right side of the course upwind.

The problem is that to sail to the right side, you need to get on port tack, but the port tack is the tack that is "headed" due to the wind shear aloft. The more you sail on port, the more severe the "header" aloft will become, and the more your boatspeed will suffer. Avoid the tendency to back onto starboard where everything feels good. Stick out on port tack and work hard to achieve whatever performance you can. Protecting the right will pay off in the long run.

This is all based on a wind shear to the right. You may also encounter wind shears to the left. In the Northern hemisphere, this may be associated with a dying breeze, or when sailing towards a shore. If you notice a shear to the left, this time you want to aggressively protect the left side of the course.

Finally, note that the only thing needed for large wind shears is a vertically stable air mass, cool air down, warm air on top. That is, donít forget the wind shear possibilities in the scorching summer days, or at early mornings when the rising sun warms the air much faster than the sea.