A fraction too much friction
When hitting the highway chasing waves, how often do you check the local Automatic Weather Station (AWS) for a real-time update on the wind?
If it's your habit - and it should be! - you may have noticed that sometimes the AWS reads offshore yet the water surface isn't as clean and groomed as you'd expect. Instead, the local wind isn't blowing from the exact same direction the AWS is reporting.
Your next question should be: Where is the AWS located? And if it's inland, the mystery is as good as solved.
The reason for the differing directions is friction, and the more inland you head, the more the wind is steered away from the direction experienced immediately on the coast.
Starting from first principles, air flows from high pressure to low pressure. Much like having two water tanks sitting next to each other but with different water levels. If you connect them, water will flow from the tank with the higher water level and pressure to the other tank until they become equal in level. Another example; a punctured bike tube will see the high pressure air from inside flow out of the hole, into an area of lower pressure outside the tube as it tries to equalise.
This is all well and good, but because the Earth rotates, air doesn't simply flow from a high to low in a straight line. The Earth's rotation introduces another force on moving air, known as the Coriolis Force. The Coriolis Force is sometimes called an apparent force, yet it's very real in terms of its applications in our frame of reference, that being the spinning Earth.
What the Coriolis Force does is steer the air parcel flowing from high to low pressure to the left in the Southern Hemisphere (and the right in the Northern Hemisphere). This is applied continuously over time, so the small deflections to the left continue until the magnitude of the Coriolis Force matches that of the Pressure Gradient Force and we achieve Geostrophic Flow. Geostrophic Flow follows the isobars drawn on a weather map, clockwise around low pressure systems in the Southern Hemisphere and anti-clockwise around high pressure systems.
As the Pressure Gradient Force between a high and low increase (think of a deeper low or a stronger high), the air parcels flowing between them accelerate and with all things being equal, the Coriolis Force also increases. This then results in stronger gradient winds flowing parallel with the isobars.
We then introduce friction.
As wind blows, it experiences friction and this differs depending on the Earth's surface. If blowing over an ocean or lake the friction is less than when blowing over land, and mountains produce even more friction again.
Adding this frictional component into the mix, we then get a slowing of the air parcel's movement and with this a reduction of the Coriolis Force. This then means the Pressure Gradient Force flowing from high to low is greater than the Coriolis Force, steering the wind across isobars and in towards the centre of low pressure systems, and accordingly out of highs.
The angle in general is about 30° off the parallel-running isobars, but even more when moving over rougher surface and land features.
And this is where things become interesting.
Visualise a low pressure system sitting just offshore from the East Coast, producing gale-force southerly winds along the coast. With the steering of the wind into the centre of the low, plus the added frictional effects of the land we'll see south-west winds at the coastal interface, but more inland they'll swing even more westerly in direction. This would see an inland AWS station reading westerly, yet on the coast winds they'll be more south-west, hence adding small bumps and ridges to spots that under a westerly would be clean.
Below is a classic example from last week.
Overlaying the wind forecasts (blue arrows) on top the Mean Sea Level Pressure charts, you can easily see the frictional effects of the land mass, swinging winds 45° from the synoptic and isobaric flow (white arrows), most evident on the southern NSW coast, while out to sea winds follow the isobars more closely.
Looking futher a little further inland, across the Great Dividing Range which provides the most friction, winds are 90° off angle to the the isobars.
During the mornings, terrestrial land breeze effects from overnight cooling will steer the wind more locally offshore, so this becomes less of an issue. Yet as the land breeze weakens through the morning, the effect will be exacerbated. Therefore, when checking AWS observations that aren't coastal it's worth keeping in mind that on the East Coast the winds at the beach are likely to be more southerly than what the inland station is showing. This effect decreases the closer the AWS is positioned to the coast.
For Victoria the same also applies, but when looking at Western Australia, where high pressure systems bring those favourable easterly winds, the wind is also likely to be a touch more south compared to the inland observations as frictional effects turn the wind out of the high. For example, an inland easterly breeze is likely to be more towards south-east at the coast.
The take home messages are thus. Firstly, try to source coastal wind observations for a more accurate representation of the actual winds on the coast, and secondly, if you can only access inland AWS readings then factor in friction.