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Winter Series Session 3: Recognizing the Persistent Shift

Rob DeWitte Updates

This week we focused a lot of our attention on trying to understand what makes the wind shift.  We started off with a look at the gradient wind, as shown on www.windy.com, and then focused on how the gradient wind makes it way to the surface under different conditions.  This led us to unpack Walker’s ODSSSIC factors (explained in Ch 3 of his book), and then we built on that to try to understand what to look for in order to anticipate that a persistent shift might develop during the race.  The notes are copied below, but the fun part was putting it together on the ping pong table — our makeshift Burlington Bay!

ODSSSIC Explained

Sources:

  1. Walker’s “Advanced Race Tactics”, Ch3 p29
  2. “Wind Strategy” (Houghton & Campbell)
  3. “Annapolis book of seamanship” (Rousmaniere)

Contributors to wind variation:

  • Predictive of oscillating wind shifts for the race: O
  • Predictive of potential persistent wind shifts: DSSSI
  • C stands for Current: ignored in this discussion.

O: Oscillating.

Air flow is unstable and oscillating

Stable air is characterized by haze or flat, low clouds.  Unstable air is characterized by cumulus clouds (popcorn clouds) and good visibility.  Unstable air allows the exchange of gradient wind and surface wind: a dynamic process that includes variation in wind speed and angle:  oscillations!  (remember that surface air is always backed relative to the gradient wind due to friction – in the Northern Hemisphere).

D: Dying.

Will there be a change in wind strength, either increasing or Dying?

Typically at the end of the day, the land begins to cool and the air near the surface becomes more stable and the surface wind dies down.  As a wind dies at the surface it will also likely change direction persistently as the air becomes more stable.  Look for the wind to back as it dies (less gradient wind mixing in).

S: Sea Breeze.

Is a sea breeze likely to be an influence in the race?

First thing in the morning, a light gradient wind is likely, but as the land warms, a sea breeze will begin.  The effect is particularly strong when the gradient wind is from the land toward the sea.  The sea breeze blows in the opposite direction at the surface – from the sea to the land.  As such, it begins right near the shore and builds outward.  As it builds it expands further into the lake and gains strength.  The direction of the sea breeze begins as blowing directly onto shore, and as it builds the resultant wind will be a mixture of the sea breeze direction and the gradient wind direction.  Eventually, if the sea breeze overcomes the gradient wind, it will become strong and veer significantly until blowing at an angle 20 degrees to the shore.

As the sea breeze dies down in the day, the process is reversed.  It diminishes in strength, retreats to a smaller zone near the shore, and backs in direction (to blow directly onto the land) before dying out and being replaced by the gradient wind.

Sea Breeze Notes for Burlington Bay (based on theory):

  • The sea breeze effect needs kilometers of shoreline to take effect
  • As a result, the direction of “shore” on the bay and western end of Lake Ontario is essentially due west. This means a sea breeze can appear as an Easterly breeze in the bay.
  • Since a mature sea breeze is veered, we should be on the lookout for sea breeze effects when:
    • The gradient wind is out of the west
    • The surface wind in the middle of the day is out of the south-east (ie: a veered Easterly)
  • In the evening:
    • This SE wind will begin to lose strength and collapse toward shore
    • As it loses strength it will back toward the East
    • As it collapses into the bay, it will bring a dead zone with it. As the dead zone passes us, we will have a new breeze from the West (the gradient breeze)
    • The dead zone will spread up the middle of the bay, with the last gasps of Easterly wind near the shore
    • The new wind will spread up the middle of the bay, and appear as 180 degree change in wind direction.
  • In the morning
    • A light westerly breeze will be flowing
    • The wind will die down as the sea breeze fights against the gradient breeze, creating glassy patches.
    • We’ll see boats near the lift bridge sailing in a Westerly, while boats near western end of the bay will be sailing in a light Easterly breeze. Boats near the Burlington shore will have a light southerly breeze and boats near Hamilton will have a light northerly breeze – each edge of the bay will have a different wind direction!
    • The new breeze will spread toward out from shore and toward the bridge, strengthening as the new Easterly breeze replaces the gradient wind
    • Once it is formed it will start to veer quickly toward the South East.

S: Squall.

Could a squall appear?

From “Annapolis book of seamanship” (Rousmaniere):

Squalls often occur on exceptionally hot, humid afternoons.  They may be at the cutting edge of a new weather front, or they may be purely local in origin.  The calling card is a steep pile of dark, thick cumulonimbus clouds and, below it, ragged swirls – the kind that in a Hollywood film announces the arrival of witches and extraterrestrial beings.  When you see a black cloud approaching, take a moment to analyze it.  The sharper, darker, and lower the front edge of the cloud, the more trouble you can expect, so sail away from it if you have the chance.  Another cause for alarm is vertical turbulence between layers in the cloud as indicated by ragged scud….

…the squall may pass after an exciting half hour or so, leaving in its wake sunshine and decks scrubbed clean by hard rainfall.  There may be a fresh northwest wind or, then again, there may be a flat calm.

S: Shoreline.

Will the shoreline affect the wind during the race?

This is always the case on the bay!

I: Inversion.

Do we have conditions wherein an Inversion could break through?

Normally, the higher you go up in altitude, the cooler the air becomes.  An inversion is a situation where warm air is trapped above colder air.  This can be a stable situation when there isn’t much sunlight, but the sun can warm some of the lower air, which can rise and trigger a break-through where the warmer air from higher up comes streaming down below.

To identify these inversions look for smog close to the surface, foggy conditions, or a chilly dense marine layer.

Breakthroughs can be violent, including thunderstorms, hail or freezing rain.

Notes from Walker’s “Advanced Racing Tactics”, Chapter 29

  • Before the race
    • Acquire a feel for the wind – variations in strength and direction
    • Measure its oscillations
      • Track the compass headings for 20-30 minutes before the start
        • Is the wind stable and subject to minimal variations?
        • Is the wind stable and subject to progressive variation in a single direction
        • Is the wind unstable and subject to periodic oscillations
      • Measure long enough to
        • tell the difference between
          • a prolonged oscillation (rarely lasts more than 15 minutes)
          • a persistent shift
        • detect the full range of oscillations
      • If less time, measure continuous readings on a single tack
        • Wait for a header or lift
        • Tack to get the complementary heading
        • Take note of the compass headings of median wind, lifted tack and headed tack and tacking angle
      • If a continuous shift in one direction is observed over about 20 minutes, then presume a progressive persistent shift has begun
    • Look around
      • Sailboats on the horizon
      • Smoke from smokestacks
      • Dark lines on the water
      • Change in texture on the surface of the water
      • Other boats racing in the fleets ahead
    • During a race
      • Keep looking around
      • Keep logging compass headings on each tack
        • If a header or lift is significantly bigger than the ones before the race, it may be a persistent shift: get onto the headed tack!
      • Distinguish between completed persistent shift and a progressive one
        • If the wind speed doesn’t change much, then it’s probably over.
        • Usual cause is a change in wind, and the progressive shift is experienced while the new wind is gradually replacing the old wind. Cause could be:
          • Start or stop in a sea breeze
            • First thing in the morning: gradient wind
            • During morning: persistent shift as sea breeze builds – ultimate direction is blowing onto shore
            • During afternoon: we would be sailing in sea breeze
            • During the evening: persistent shift as sea breeze subsides – ultimate direction is gradient wind.
            • Note: the direction of “shore” on the bay and western end of Lake Ontario is essentially due west, until the last breaths of the sea breeze when the specific contours of shore in the Bay will matter. So, if an Easterly breeze is not due to the gradient wind direction, then expect it to subside (even reverse), as the evening progresses.  This new breeze will form first near the Eastern end of the bay, with some holes where the two breezes meet.  The sea breeze will linger longest near the shoreline.
          • Change in the degree of mixing of upper-level airflow (gradient wind) with surface wind (backed 15-40 degrees from gradient wind direction due to friction: 15 degrees on open sea, 40 degrees on land).
            • Triggered by an inversion breakthrough
          • A squall
          • A movement of the weather system itself
        • Most of these causes will initiate a progressive shift, but some can be short-lived and abrupt. It depends on how the new wind and old wind mix.
      • Assume it is progressive!
        • While it is progressing, act to take advantage of its further progression:
          • make sure we are on the headed tack
          • continue until near the layline,
          • then tack onto the progressively lifted tack to get to the mark
        • When it stops
          • Return to playing the oscillations, consider:
            • Where we are on the course
            • The direction of the new median wind
            • Most likely first oscillation will be in the opposite direction of the persistent shift.

Other sources of insight:

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