What is a wind shadow?

August 2, 2022 | Windsurfing
Wind shadow: a place or zone where the wind is not blowing or doesn't reach | Photo: Shutterstock

Wind shadows are a common phenomenon observed in all types of water bodies and inland.

A wind shadow, also known as dirty air and wind wash, is a place or zone where the wind is not blowing or doesn't reach.

It is frequently spotted in oceans, seas, rivers, lakes, lagoons, estuaries, basins, harbors, and even outdoor swimming pools and puddles.

However, it can also occur inland and far away from bodies of water.

In other words, it's a sheltered area protected from the effects of wind - a place where the breeze cannot be felt or is minimal.

The sudden, abrupt and unexpected absence of wind in a restricted area is not rare and happens all the time.

Disturbed airflow zones are more frequent downwind of obstacles.

The wind shadow results from the objects obstructing or creating turbulence upwind of the wind flow, including:

  • Watercraft;
  • Coastal features like headlands, mountains, cliffs, or hills;
  • Shoreline buildings;
  • Landscape;
  • Breakwaters, piers, pontoons, groins, and jetties;
  • Waves and swell;

Wind shadow: waves act as wind barriers creating a no-sailing zone in their troughs | Photo: Creative Commons

Avoiding and Sailing Through Wind Shadows

They are particularly relevant in sailing, as they affect the normal course of dinghies, yachts, and sailboats.

"When you're sailing close to land, be aware of the effects of wind shadows - areas of reduced wind speed," notes Steve Sleight, author of "The Complete Sailing Manual."

"An offshore sailor will notice the large wind shadows often found in the lee of large islands, and the coastal sailor will be affected by smaller features in harbors and rivers."

"Trees, buildings, ships, and high ground can all disrupt the passage of the wind and cause lulls, gusts, and wind shifts," adds Sleight.

"Sailors who race on inland water need to be particularly aware of wind shadows and microclimate disturbances caused by very localized heating and small convection currents."

However, it's essential to always pay attention to geomorphological features and the wind forecast for the region you're sailing on.

Here's why.

"Sailing in the lee of a headland, the wind may seem light close inshore," explains Jeremy Evans, author of the book "Sailing."

"It is easy to assume the wind will stay light around the corner, but a sea breeze may be building to Force 6 on the other side."

Wind shadows represent a challenge before, during, and after you go through them. But there are tricks to overcome the spell that suddenly cut off the wind from your sails.

"If your dinghy capsizes to windward, don't panic," stresses Evans.

"Just slide back into the water in the space between boom and boat so that you don't end up under the mainsail."

Sailing: in competition, sailors create wind shadows to keep their opponents moving at slow speeds | Photo: Creative Commons

Tactics and Challenges

In competition, sailors create wind shadows to keep their opponents moving at slow speeds.

The tactic in which one boat keeps a rival boat in its wind shadow is called "covering."

Every time the other boat tacks to break cover and find clean wind, the leading boat tacks to re-establish cover and stay ahead.

So, whether you're sailing on or off competition mode, it makes sense to stay out of the main channel when there are large boats around.

And remember that you can also get stuck in the wind shadow of a ferry.

Last but not least, wind shadows affect water starts, especially in windsurfing.

Onshore wind creates wind shadows on waves, blocking the breeze, especially on the trough, and hindering sailors from moving forward.

As a result, windsurfers and other sail-driven watercraft struggle to get past the surf zone due to the absence of wind, especially in large swell conditions.

The sudden interruption in smooth, steady airflow can also affect kiteboarders, especially while launching and landing kites.

Wind shadows: an object can create a no-wind zone that is 7 to 15 times the obstruction's height

How to Spot a Wind Shadow

You may predict a wind shadow before actually spotting it - experienced sailors know when they're about to enter a no-wind zone.

Nevertheless, on some occasions, it just appears in front of them.

The best way to identify and observe a wind shadow is from a higher level - the higher you are, the easier it is to recognize it.

You can be standing on a boat, windsurfer, stand-up paddleboard, or checking from the top of a lighthouse or cliff.

So, how do you know there's a wind shadow in a particular area?

It's relatively easy to detect.

The wind fetch is the length of water over which a given wind blows without obstruction. These more or less extended wind-affected zones display rugged water surfaces with small uneven ripples.

You will notice a wind shadow where the wind fetch is suddenly interrupted by a small, clean, silk-like water surface area.

It resembles a hole.

The color or contrast of the wind shadow is also different from the surrounding wind-affected zone.

It's still water with no wind blowing over it.

One can also spot wind shadows on the edges of rivers and closed water bodies and on wave troughs, where the wind can't reach.

And as we move inland, we observe a curious event.

A wind shadow forms downwind of barriers, but it can also occur on the upwind side of obstacles such as houses, trees, trucks, and other objects.

Why?

Because when it encounters a barrier, the wind will be channeled upward, leaving a restricted windless area below where it bends up.

On the sheltered downwind side, an object can create a wind shadow that is 7 to 15 times the obstruction's height.

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