Weather: When to Cross the Great Australian Bight

The Great Australian Bight, commonly known as The Bight, is a legendary stretch of water that tests even the most seasoned sailors. It is the enormous open bay along the southern coast of Australia, which spans 630 nautical miles (1,200 km). At a speed of six knots, a non-stop crossing takes five days, plus an extra day on each end to reach the nearest anchorage. A crossing takes you well offshore and it is the nearest thing to an ocean passage without leaving Australia’s search and rescue zone. Head south, and your next stop is Antarctica.

So when I met a delivery skipper in Albany who casually mentioned that he’d crossed The Bight 29 times, I was impressed. Despite The Bight's formidable reputation, he was surprisingly blasé about his track record, viewing it simply as a matter of picking the right weather window.

So what is the best time to cross The Bight?

The answer depends on which way you’re going and the time of year. It is critical to understand the weather systems in the southern hemisphere and, in particular, in southern Australia. Let’s dive into the key factors.

Prevailing Westerlies

Weather at middle latitudes, both north and south of the equator, travels in a generally west-to-east direction. This eastward movement applies to both high and low pressure systems, including cold fronts and troughs associated with the latter. The westerly movement is an indirect result of the Coriolis force, the force caused by Earth's rotation. Read the following explanation for more detail.

An explanation of the Coriolis Effect’s impact on prevailing winds Imagine a non-rotating Earth: warm air would rise at the equator, flow directly towards the poles, cool, sink, and then flow back along the surface towards the equator. This would create a simple, single circulation "cell" in each hemisphere, with air moving strictly north to south in the Southern Hemisphere (the red line in the diagram below). However, because the Earth rotates, this circulation becomes far more complex, breaking into multiple circulation cells. In the middle latitudes (30 to 60 degrees latitude), air at the surface is generally moving from the high-pressure areas around 30° latitude towards the low-pressure areas around 60° latitude. In other words, the air is attempting to move towards the pole, which is southerly in the Southern Hemisphere. As the air attempts to travel south, the Coriolis effect deflects it to the left, causing it to veer towards the east (the blue line). This creates the prevailing northwesterly/westerly winds in the Southern Hemisphere.

The Australian High: A Seasonal Influence

A prominent weather feature in Australia is the Australian High, a large, semi-permanent area of high pressure that sits over Australia, and moves northward or southward with the seasons. The technical term is the Subtropical Ridge as it is long and relatively narrow, reminiscent of a ridge. For example, the following Bureau of Meteorology (BoM) synoptic chart shows the Australian High over central Australia in mid-winter.

Summer Patterns: Ideal for Westward Crossings

In summer (December to February), the Australian High strengthens and moves southward, extending its influence over a larger part of southern Australia. This brings stable and dry conditions, with less chance of low-pressure systems and cold fronts reaching the south.

In the Southern Hemisphere, air circulates anti-clockwise around highs. Therefore, a high located south of Australia will produce southerly/southeasterly winds as it approaches followed by easterly winds. This is desirable for a westward crossing.

For example, here is a late spring synoptic map from the BoM with a High that is located well south of The Bight.

The corresponding winds, forecast by PredictWind, are perfect for sailing west.

Winter and Shoulder Season Patterns: Navigating Lows for Eastward Crossings

In winter (June to August), the Australian High weakens and moves northward. This opens the door for low-pressure systems and cold fronts (i.e., the boundaries between warm and cool air) to travel eastward across southern Australia. In the Southern Hemisphere, air circulates clockwise around low-pressure systems, a pattern that can be remembered with the mnemonic 'cyclonic clockwise’, recalling that cyclones are extreme low-pressure systems.

For example, here’s another BoM forecast for a cold front approaching The Bight in autumn.

PredictWind forecast corresponding winds as follows.

This could be a good time to sail from west to east, provided you kept to the north of the system, but it depends on the speed of the system. For a west-to-east crossing of The Bight, a slow-moving system is desirable. The downside is that it is likely to be associated with extended periods of rain. If a second cold front were immediately following the present system, the conditions would be less attractive.

Here’s another BoM synoptic chart, a couple of weeks later, showing a cold front that has already moved across The Bight.

The corresponding winds forecast by PredictWind were southerly ahead (east) of the front and westerly to its north.

This is the actual front that we used to sail from west to east in April 2025. We took advantage of winds backing to the south, i.e., changing from westerly to southerly, which propelled us east.

In our case, it was a fast-moving front and a few days later we were confronted with the following light conditions, necessitating that we motor part of the way. We anticipated this and deemed that lights winds were preferable to headwinds.

Finally, here is a mid-winter Low and its associated cold front moving across The Bight. Notice how far north the High is over eastern Australia and its elongated shape.

PredictWind forecasts strong trailing southerlies and southwesterlies for this Low.

Sea State

The final consideration is sea state. Winter brings huge seas that are best avoided, even if the winds are manageable. For example, PredictWind forecasts 9m seas to be associated with the above Low. As a rule, even if winds look favourable, it always pays to check wave and current forecasts.

Conclusion

In summary, the best time for crossing The Bight from east to west is summer (December to February), when highs are located well south. 

The best time for crossing from west to east is more nuanced. In general, any time you can harness a low-pressure system is good. In practice, you won’t want to do so in the middle of the winter, which brings cold weather, strong winds, and big seas. The preferred times are therefore late spring (October to November) or early autumn (March to mid-April).

By understanding these different weather patterns, you can harness the right conditions to cross The Bight. I advise anyone planning a crossing to start studying the weather patterns several weeks in advance.

OVER.