Weather

Generally Queensland's winter weather is controlled by high pressure systems which travel across the Australian Bight causing our south east trade winds. A high of over 1030 hectopascals (HP) will generally give 15 to 20 knot winds, and over 1035 hp, wind speed will rise to 20 to 25 knots. Highs with concentric patterns tend to create more wind than the "wobbly" ones.

As winter progresses the centres of the highs tend to move north over the continent and so we get less wind than the Bight highs in Autumn. Highs on the waxing moon tend to build up stronger than on the waning moon... These highs drift eastward and weaken, the wind becomes more easterly and picks up moisture and cloudy periods can ensue. This allows the low pressure troughs to move further north and reduce the pressure gradient and hence the wind. Some of these troughs however keep moving north and east over the top of the surface high. These then cause an unstable upper atmosphere which creates rain and wind (more than just the high seen on the chart). But you can expect these conditions to only last a day or two.

Sometimes a surface low trough forms off the coast following this which then dramatically increases the gradient and can produce a lot of wind from even a small high!

Tides

Tides are probably the most important factor affecting reef trips. Simply put...Low tides in the middle of the day are good!

The reef flats actually begin to become exposed on about a 0.7 metre tide. With less than 1.5 metres of tide even in 25 knots of wind the snorkeling conditions are still good as the swell can't cross a reef with less than a metre of water over it. Conversely, a high tide of over 2.5 metres can be a bit lumpy in only 20 knots of wind as there is up to 2 metres of water over the reef.

Fortunately in winter the lowest tides come at about midday (even zero tides and you can see this when you refer to your tide chart).

The extreme low tides have another advantage. All the fish that normally feed on the reef flats suddenly have no water and swim to the edge where all the snorkeled are. The result is masses of fish in the shallow gullies waiting for the tide to come back. Also snorkeling with the coral right in front of your face and without metres of water to filter out the colour makes the experience the best the reef can offer.

Visibility

Contrary to what some believe, the wind strength hardly effects visibility on the Outer Reef except after a long calm spell. Then the silt and detritus that has settled on the reef gets stirred up for a day or two before being washed away by wind and currents.

In winter you can expect good visibility as the trade winds flush the reef with clean oceanic water.

In summer there is a lot of plankton in the water reducing visibility.

The big tides in spring (peak range) can affect visibility for 3 or 4 days, because it stirs up the bottom ...but you'll find wind currents soon clear this sediment away... Expect an average of 15 to 20 metres visibility year round with up to 30 metres visibility in winter.

Temperature

In June the water is about 23 deg C. This will drop to 21 or 22 in mid to late August, then the water steadily warms to 26 or 27 in November (coral spawning temperature...) This might appear warm but the wind chill can be quite cold, so wetsuits and warm jumpers are advisable from June to September to allow longer snorkeling time.

Wind and waves

The larger 16 metre (55 ft) monohulls commonly used as diveboats are designed for heavy weather, which is why they are used in Australia's commercial fishing industry all along the coast. The hull design provides a soft entry with no slamming even on the rough days. And while they have slightly more lateral movement than a catamaran, the motion is predictable.

Up to 15 knots winds on the way out to the reef only bothers passengers with the very weakest stomach... but expect 25 knots to give you quite a rough ride (depending on the tide again) but it only takes an hour to get behind the reef... so check the tides.

When it's blowing all boats present some degree of discomfort so if prone to sickness we recommend you take AVIL tablets an hour before you travel. They really do work and will make your day much more enjoyable... Don't have a nauseous day because you waited to "see how you felt first". Prevention is better than cure!

Sun and rain

As a general rule, in winter, as the southern easterlies pick up moisture off the warm ocean and then interact with the land , clouds and some rain will form along the coast. There are many days when you will be out on the reef 40 kilometres offshore in full sunshine only to return to a cloudy coast... As the ocean water cools, expect less cloud... the exception being the upper level troughs that can cause a little rain... So cloudy days on shore do not necessarily mean cloud on the reef.

The reef is quite a fantastic ecosystem, and while it's great if the sun is shining, in our opinion it makes little difference to the diving/snorkeling experience. In fact, the fish are far more active on an overcast day, and because the light is so intense in Australia's northern tropics, very little perceived colour is lost, particularly when snorkeling near the surface or on a low tide.