Sail-World.com : Great time of the year at Airlie Beach
|
Great time of the year at Airlie Beach |
|
|
Bureau of Meteorology
|
What a great time of the year to be sailing in the tropics. It’s little wonder that the Meridien Marinas Airlie Beach Race Week has great patronage each year. Please elaborate you ask?
The belt of high pressure (sub-tropical ridge) that usually has a big bearing on weather outcomes over much of the Australian region is usually situated over southern Queensland during the colder months of the year. This means that to the north of this high-pressure area, a ridge of high pressure will extend along the Queensland coast and this will ensure a broad easterly onshore flow and mostly sunny skies.
A westerly land-breeze usually develops at night over the Whitsundays, particularly the Airlie Beach area. This will oppose or replace any prevailing wind, say a south-easterly or a sea breeze, that has been blowing during the day-light hours. This will normally form a zone of light winds, 0.5 to 1 mile across, some one to three nautical miles offshore. To seawards of this zone south-east to east flow will prevail. This situation will break up by mid to late morning so that south-east to east flow becomes established again at the coast.
At times when the belt of high pressure is situated over the Whitsundays, local winds will dominate. This will see the land breeze blowing overnight and the sea breeze (usually fairly weak) operating during the day, normally from late morning till about dusk.
West to south-west changes (or southerly) are most common during the winter and spring months over Queensland. These changes are basically associated with cold fronts, but these fronts are modified by the long passage across the Australian continent. The best development occurs over southern Qld but occasionally extends well into the tropical north. When these changes make it into the Whitsundays, they bring cooler conditions coupled with gusty winds and at times rain showers. It does pay to keep some warm clothes and wet-weather gear onboard to cater for these situations amongst others.
When the high pressure belt is situated to the south of Queensland and a firm ridge of high pressure is evident along the coast, a feature of note are surges in the trade winds. These arise from sudden strengthening of these high pressure systems, normally following the passage of a moderate or strong frontal system. Watch the weather maps regularly because these surges may arrive in the tropics a day or two after the cold front has passed through well to the south. There will seldom be a well defined change marked on the weather maps, but you should notice the isobar spacing tighten to the south of you before the surge arrives. These surges can bring about showers over the Whitsunday area. Showers, particularly the heavier ones, can lead to squally and erratic winds as they pass over or are close to your yacht.
On a more localised scale, wind-flow can be markedly affected by the presence of headlands and islands. Wind can be channelled or funnelled between islands or between the mainland and an island. Wind speed tends to be enhanced when blowing around headlands and the direction can be quite erratic. On the other hand, wind flow can be very much impeded in the lee of any land mass. The general rule of thumb is to stay roughly fifteen times the height of any obstacle downwind to be assured of a steady wind. This however will have to be fine-tuned by the user.
In the Whitsundays, tidal currents can play a big role in determining the state of the sea and can also have a marked affect on the surface wind flow. For example, the situation of tide opposing wind can see the set-up of a very nasty sea-state and this should be factored into any race strategy.
Well-informed local knowledge can play a big part in any successful race outcome in this part of paradise! Do your homework!
Some BIGGER weather problems at this time of year MAY be as follows:
East-coast-lows can be a problem over south-east Queensland at this time and their effect can be felt north into the Whitsundays. Normally dry and gusty west to southwest winds prevail in this situation.
Sea fog can occur over the Whitsundays at this time of the year.
Following the passage of a cold front, south to south-west winds over the Whitsundays can be stronger than indicated by surface pressure gradients (isobar spacing).
The odd thunderstorm associated with the passage of a cold front or upper trough could pose some problems. These events may produce very squally and erratic winds, lightning and heavy rain.
Tracking changes in the weather in the tropical regions requires a different approach to that used elsewhere in Australian waters. The synoptic weather map commonly used is an analysis of pressure fields. However, the pressure changes in tropical areas are mostly slight, making these charts of limited use. Note that for equal isobaric spacing, the wind will be stronger in the tropics than outside of the tropics (extra-tropics). An old rule of thumb used by tropical mariners goes like this- Take the last two digits of the central pressure of the high pressure system exerting the ridge along the tropical Queensland coast, and that will roughly be the average surface wind speed. For example, if the central pressure is 1030hPa, then the average wind speed should be around 30 knots. Wind field charts, called streamline charts, which show the prevailing wind flows, are normally used instead of isobaric charts. These are available from the Bureau of Meteorology’s web-site (www.bom.gov.au) and are broadcast as a part of the HF radio facsimile weather chart product set.
Good racing!
www.northsails.com.au
by Kenn Batt
8:34 PM Wed 13 Aug 2008 GMT
Click here for printer friendly version
Click here to send us feedback or comments about this story.
If you want to send this story to a friend, please use link below
http://www.sail-world.com/BoatingAndBoats/Great-time-of-the-year-at-Airlie-Beach/47663
Click on thumbnails to enlarge and find more photos:
Click for further information on
North Sails Australia
Related News Stories:
MORE STORIES ...
|
Based on your current download speed Sail-World has delivered large images (660 pixels long side.) If you would like to see only small images (300 pixels long side)
click here . For medium sized images (500 pixels long side)
click here .
| |
