Thoughts on Winds
Overview
This will be a five part series broken down in a number of posts as follows:
Overview of ‘beyond basic” wind concepts
Wind data sources
Coastal and Inland wind elements
Oceanic and Offshore winds
Strategic analysis of wind factors
Wind Concepts - Beyond Basics
Quick Recap of the Standard Fare
I am assuming the reader has heard the standard fare about wind, but to be clear I am referring to:
Wind moves from high pressure to low pressure and closer isobars indicate higher wind speeds flowing generally along the isobar
Wind moves counterclockwise around lows and clockwise around highs in the northern hemisphere, and in the opposite direction in the southern hemisphere
Wind blows towards the warmer of the land and the sea nearshore.
Convective storms form ahead of a low and have winds that flow out ahead of them at significantly higher velocities than surrounding winds.
Beyond basics - Wind is air moving in three dimensions
We generally consider wind to be two dimensional because that is how we experience it. We experience that wind can blow from a given cardinal direction, and has some velocity. But the consideration necessary to start really critically thinking about wind on the race ground is that it is air moving in three dimensions.
A high pressure system actually represents the descent of air, conversely a low represents ascending air. We will talk more about why they spin later, but the important thing to consider now is why would air rise, or descend.
The answer is as simple as fourth grade science class. Warm air rises and things that warm air cause the air to rise, conversely cold air descends and things that cool air cause it to descend.
Air flows from high pressure towards low pressure because as air rises into the low, the surface air mass has to be replaced, so wind flows from high pressure, where it is descending, to fill the vacuum of the rising air.
Finally, on the three dimensional wind concept. Areas in the lee of a hill or knoll with a cool land surface met by a cold surface water temp are natural inversions, which can “block” the wind and sends it over the inversion and interrupt sea breeze creating a “hole” in that location.
Courtesy NWCG. Windflow around a hill.
This means that as a rule of thumb, the lee of that hill or knoll will have a growing “hole” in that area in the warmth of the day. There is more to discuss on this topic in the coastal and inland post because the wind hole size and location can be roughly calculated.
In summary, consider which things on the race ground cause air at the surface to warm, what would cause cooling, are there any obstructions and the timing of the boat in relation to each. How will the wind will react to the heating of the day?
Beyond Basics - Surface winds may not be at the surface
In a past life I worked as a wildland firefighter, where I was introduced to fire weather classes that broke this down in great detail. The factors are all similar, we want to know which direction the wind is blowing, which side of the surface to be on strategically for the best advantage, so it’s a natural transfer of concepts between sailing and wildland fire mitigation.
Two interesting graphics from that training are very applicable to sailing strategy and can explain why “the HRRR is off”, as one of my very good friends likes to say. I have added them below.
Most forecasts and models are based upon winds above the “friction layer” that is about 20M above the surface. Other factors like prominences, gaps, points and temperature deltas between water and land surfaces all affect winds.
Courtesy NWCG. Mid-sail wind versus mid-flame height.
The other graphic from NWCG that is useful is the graphic showing the level of different wind phenomenon. Further this shows graphically what synoptic versus mesoscale versus microscale mean.
At the top are Rossby waves, climatological waves that steer cold fronts and the jet stream. These are covered in Chesneau and Lee’s book “Heavy Weather Avoidance”
Near the surface is a surface thermal, our most prevalent surface thermal in the western US is the persistent coastal trough in the summer along the northern California coast with a predominant surface counter thermal being the water temperature.
Courtesy NWCG. Factors affecting surface winds
Beyond Basics - Coriolis Force is everywhere
So we all know that there is this thing called Coriolis force that causes things to spin one way in the northern hemisphere and the opposite direction in the southern hemisphere. More specifically, fluids in the northern hemisphere are drawn to the right and in the southern hemisphere to the left along their direction of travel.
The reason for this at a high level is the linear velocity along the lines of latitude of a sphere are greater at the equator than the poles. The earth’s equator is about 21,600nm long, the 45th parallel is about 15,200 nm long. So if we think about this relative to rotation, a tree at the equator is actually moving continuously eastward at approximately 900 kts, but a tree in Astoria Oregon is moving closer to 635 kts.
In a simplified description of tangential velocity, a wind flowing due southward from 45ºN will have an east directional vector component of approximately 635 kts based on its surface velocity caused by the spin of the earth . As it progresses southward towards the 40ºN, it would need a to accelerate to approximately 690 kts to keep moving due south, in response as it flows southward, it moves to the right along the direction of travel.
If we reverse the direction and flow northward, the faster moving eastward flow of 690kts is going to move into an eastward flow moving at 635 kts, therefore moving faster but again curving to the right along the direction of travel. There are more complexities here, but we will look at those relative to inshore versus offshore considerations in later posts.
Consider one other simplified but reasoned assertion about angular velocity, when viewed with north to the top of the observation area(map view), a wind flowing down from the upper atmosphere in the northern hemisphere curves to the direction of spin as it approaches the surface due to fluid surface tension then surface friction causing it to move to the right along the descending direction of travel. A low curves to the right in the upward direction of travel as it sees reduced surface friction as it elevates.
In the northern hemisphere, all of this this leads to a clockwise rotating high, and the counterclockwise rotation of a low. We will discuss the three dimensional component by hemisphere in upcoming posts relative to the context of the wind analysis region(coastal vs offshore).
Finally Coriolis force has an equation
F_coriolis = 2 x volumetric mass x (angular velocity x tangential velocity)
Beyond the basics - Wind Compression in Gaps and Points
Wind is a fluid so when there is a point obstruction with wind flowing tangentially to it, or when a wind flows through a gap, it compresses and speeds up.
This is obvious when passing under the Golden Gate on a warm day. The compression effect defined by gap effect is readily apparent when you go from 8-10kts outside of the bridge to 15-20kts after passing into the bay. This also happens to a lesser degree in the Kingston/Edmonds gap in Puget Sound.
The image below is the Strait of Gibraltar gap wind, the western version of which I believe is called a Levante wind.
Gap winds affecting the Strait of Gibraltar.
Air flowing tangentially to a point, peninsula, or linear obstruction behaves based on a principle defined by Bernoulli. To visualize it, think about when a person sticks their thumb into the stream coming out of a hose, it causes the water to speed up as it exits the hose, the water also bends as it passes the obstruction of your thumb.
Point compression at Cape Blanco and Cape Mendicino
As a rule of thumb(pun alert), a result of Bernoulli effect causes wind to back when approaching the end of a point from windward and veers when approaching end of a point from leeward. On the other side of the point wind veers if departing into the lee, and backs if departing into the wind.
We will discuss these in much greater detail in the inland/coastal post.
Beyond the Basics - Warm/Cold Front and Hurricane movement
Warm fronts, Cold Fronts, and Hurricanes are driven primarily by the winds at the 700mb level.
Cold fronts on a synoptic scale move at roughly 85% of the perpendicular 700MB wind and they move generally equator-ward. They move at a speed of 10-15kts if an active front, and 25-30kts otherwise. They also cause winds to veer(Northern hemisphere) as they pass over.
Warm fronts move about 10-20kts(or about 70% of the 700mb wind), progress pole-ward, and generally cause backing in the northern hemisphere.
There are two situations where a warm front and a cold front are on the same boundary, an occluded front in a low and a stationary front are a combination of both a low and a high, but with significantly different behavior.
More about this later when talking about wind strategy along the coast and approaching storms.
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