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"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
Laser’s 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 system’s 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 water’s 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:
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 15-20° 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, don’t
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. |
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