Wind Theory For Sailors

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Wind Theory For Sailors

• US Sailing Team AlphaGraphics

Click here for audio narration
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Objectives

• What is the wind
• Why does the wind vary
• What affects the wind on the race course

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Wind

• Wind is the flow of air from areas of high
  pressure to areas of low pressure. The larger the
  difference between the high and the low, the
  pressure gradient, the stronger the wind.
• The coriolis force, a result of the earths
  rotation deflects the air flow until the coriolis
  force balance the pressure gradient force
  geostrophic balance.

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Pressure Gradient Force

• Wind in its simplest form is the movement of air
  from areas of high pressure to areas of low
  pressure.
• The difference in pressure that results in air
  motion is called the pressure gradient force.
• The higher the difference in pressure between the
  high and the low, the stronger the wind.

Images from WW2010 Project U of
Illinois http//ww2010.atmos.uiuc.edu/(Gh)/guides/
mtr/fw/home.rxml
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Coriolis Force

• But we know that the wind does not blow straight
  across the gradient from high to low, the wind
  flows along the isobars. This is due to the
  coriolis force.
• The coriolis force is a result of the earths
  rotation. In the northern hemisphere the coriolis
  force deflects the wind to the right.

Images from WW2010 Project U of
Illinois http//ww2010.atmos.uiuc.edu/(Gh)/guides/
mtr/fw/home.rxml
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Geostrophic Wind

• The coriolis force continues to act on the wind
  flow until it is balanced by the pressure
  gradient force, now we have geostrophic balance.

Images from WW2010 U of Illinois http//ww2010.at
mos.uiuc.edu/(Gh)/guides/mtr/fw/home.rxml
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Gradient Wind

• The geostrophic wind only applies where the
  pressure gradient force follows straight lines
  (Isobars).
• But in the real world the pressure field curves
  around the highs and lows creating centrifugal
  forces.
• The combination of the pressure gradient,
  coriolis, and centrifugal forces is the gradient
  wind. Finally.

Images from WW2010 Project U of
Illinois http//ww2010.atmos.uiuc.edu/(Gh)/guides/
mtr/fw/home.rxml
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• The net result is that the wind flows
  anti-clockwise about lows in the northern
  hemisphere (clockwise in the south) and clockwise
  about highs.

Note that the wind flows along the isobars aloft
and across the isobars at the surface. This is
because the friction of the earths surface
upsets the geostrophic balance resulting in the
wind flowing into the lower pressure.
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Buys-Ballots Law
You REALLY need to know this

• Stand with the wind to your back, the low
  pressure will be to your left (northern hem)

Images from WW2010 Project U of
Illinois http//ww2010.atmos.uiuc.edu/(Gh)/guides/
mtr/fw/home.rxml
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Wind Direction

• Wind direction is referred to in terms of where
  the wind blows FROM.
• A northeast wind (NE-045degrees) comes from
  that direction.
• If the wind direction trends right to higher
  values then it is said to VEER.
• If the wind trends left to lower values it is
  said to BACK.

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Wind Variations

• The wind is not always a simple flow of air in
  geostrophic balance because there are other
  factors such as
• Atmospheric stability
• Differential heating
• Barrier Effects

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Stability

• The first characteristic of the wind is how
  steady or unsteady it is.
• Laminar flow occurs when the atmosphere is
  stable, there is little or no vertical mixing and
  the so the air flows in nice orderly layers and
  the surface wind is steady in speed and
  direction.
• When the atmosphere is unstable the wind flow
  becomes turbulent as parcels of air rise and sink
  creating a shifty, puffy wind.

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Effect of Stability on Wind

• When the air is stable the wind flow is called
  LAMINAR, there is no mixing of the air from one
  vertical level to another.
• In a stable condition the stronger winds aloft
  remain above, resulting in wind shear.
• When the air is unstable the wind flow is
  TURBULENT. The wind above mixes to the surface.
  There is little or no wind shear.
• Unstable air is overturning, air is rising in
  thermals and sinking in adjacent downdrafts

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How to forecast stability

• Unstable conditions are present when
• cold air flows over warm water.
• Or cold air over warm/hot land surface blowing
  offshore creates a very unstable wind near shore
  becoming more stable further offshore.
• Isobars that bend around a low indicate an
  unstable atmosphere (curvature).
• Cumulus type clouds are a result of an unstable
  airmass. The clouds form in the rising thermals.
  They are an excellent indicator of instability.

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Stability on the weather map

• Areas of unstable are can be determined using the
  isobar analysis or forecast.

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Differential Heating

• Differential heating is part of the stability
  equation.
• Surfaces of different color, different land use,
  different current have different surface
  temperatures.
• These differences result in hot spots where
  thermals will be more active creating a turbulent
  flow.

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Barrier Effects

• Usually referred to as local effects on the wind
  by topography of coastline and nearby islands.
• Islands, hills, trees all act as barriers to the
  wind which must flow over and around the
  barriers.
• The rough surface over land compared to the sea
  increases friction which slows and shifts the
  winds direction.

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Friction
You REALLY need to know this

• Recall that friction of the ground surface
  results in the wind backing into the lower
  pressure. As the surface changes the amount of
  backing changes as well.
• An offshore wind will change direction as it
  moves off the land to the sea with less friction.

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Wind veers as it moves offshore due to reduction
in friction. The distance offshore for the change
depends on the stability of the air. In unstable
air the change is near 1km, in stable air up to
5km.
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The same frictional effect is important when the
wind blows along the shore
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Frictional Effects from an Island
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Funeling
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Combined Effects
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Cornering
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A change in the wind direction results in a big
change in where the Band of stronger wind lies
relative to the shore. A change in axis can mean
a big change in game plan.
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Cornering and Friction
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Summary of frictional Effects

• If wind blows within 20 degrees of shore line
  then consider convergence/divergence
• Offshore wind will veer 10-25 degrees from 1-5km
  offshore

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Coastal Cliffs
In this scenario sailing close to a high cliff
can result in big variations in the wind speed
and direction. An high sea wall can have similar
effects. Hopefully the race committee can keep
the course clear of this situation.
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On a Lake
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Before the Regatta
Before the Race

• Map out local effects for prevailing wind
  directions or where obvious effects will occur.
• The more often you do it the easier and quicker
  it becomes.
• The maps eliminate the need to rethink the local
  effects each time you hit the water.

• Using the forecast wind direction and course
  location
• Plot wind direction on chart
• Use maps of local effects to determine the days
  possibilities.
• Look for effects on course in pre-race warm up.
• Adjust local effects map for changes in course
  axis

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Remember

• Wind moves counter-clockwise about lows and
  clockwise around highs (in the northern
  hemisphere).
• The more closely packed the isobars on the map,
  the stronger the wind.
• Friction at the surface backs the wind, more left
  near shore in offshore wind and more right aloft.
• Atmospheric Stability greatly influences the
  steadiness of the wind stable steady wind
  with regular oscillations, unstable shifty,
  puffy irregular wind.
• The wind flows around and over land forms
  resulting in zones of stronger and lighter winds.

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Reference

• Most of this material is covered well in Wind
  Strategy by David Houghton and Fiona Campbell.
  HIGHLY RECOMMENDED especially on those long
  flights.