Weather by Malcolm Reilly - Bullets / Wind funnels into Shute Bay increasing the wind speed. Wind speed is further increased as the air is “forced” between the Conway Range (left) and Mount Mekara (right). The air is compressed below the inversion further increasing the speed of the outflow “bullet” that flows into Funnel Bay.
  There are many local wind phenomena that can affect mariners. Often these winds result as a combination of topography (the lie of the land) and atmospheric conditions. These winds are not included in the Bureau of Meteorology forecasts and affect too small an area to show up on the various computer model forecasts.
  During winter the southeast trade winds blow over much of the Queensland coastline. These winds are generally an east southeasterly between 15 and 25 knots. Vessels wanting to anchor on Queensland’s offshore islands go to the sheltered lee side (westerly side) of the islands.
  The vessel is at anchor, all is quiet and peaceful when a wind comes roaring from nowhere, violently moving the vessel at anchor and blowing unsecured items into the sea and occasionally causing damage. These are the notorious ‘bullets’ and they are very common over winter in the Whitsunday group of islands.
  To understand the ‘bullets’ there are aspects of the trade wind atmosphere to be considered. The first is the trade wind inversion; a layer of air where the air temperature gets warmer with height instead of the usual cooling.
  The height of this inversion can typically vary from 500 metres to 2,000 metres.
Movement of a parcel of air with an inversion layer and with a layer of stable air  The inversions act like a barrier between the trade winds and the atmosphere above the inversion. Another characteristic is that the air is stable, stable air is dense “heavy” air, if it is forced to rise by being pushed up one side of a hill when it gets to the top of the hill it will descend to the level it started at. Stability of the air can be complex.
  Back to our anchorage. The wind on the lee side of the island has ‘dropped out’ and become calm, this happens during the early evening. However, trade winds keep blowing and the air is forced up the windward side of the island. The air is then compressed between the inversion and the top of the island making the wind speed increase. This is the same principle as when you put your finger over the end of a running garden hose; the water flows faster through a narrower space. As the air is stable once the air/wind has cleared the ridge it moves towards sea level, picking up more speed.
Wind is compressed between the inversion and the top of the island increasing the wind speed. Wind speed on the lee of the island is further increased by the effect of gravity on stable air.  The wind reaches sea level (on the lee side) as a gusty downwards burst that is stronger than the trade winds on the windward side of the island. Sometimes these bullets can be heard by the noise they make passing through the vegetation on the lee side of the island. Other times there is no audible warning. The wind speed and the stability of the air determine where the ‘bullet’ reaches sea level.
  Bullets are not just night-time phenomena, they can occur at anytime given the right wind conditions. The bullets may be experienced up to a nautical mile out to sea from the obstruction.
  Funnel Bay near Airlie Beach is orientated north of a southeast to northwest valley. The southeastern end of the valley is wide and narrows as it as it moves over the land. This adds funnelling to the ‘bullet’ scenario increasing the potential southeast wind even further. Yachts have been dismasted at Funnel Bay as a result of ‘bullet’ winds.
  Bullets are well-known Queensland winds, however, similar wind regimes can occur anywhere around the Australian coastline where the right topography and atmospheric conditions combine.
*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of QLD and is a Master V. He gives education courses on Marine Meteorology.