Weather by Malcolm RileySea Breezes

Sea Breezes





The sea breeze is a big part of the Australian summer and the concept is a relatively simple one.
Sea breezes are mainly a feature of the warmer months but can occur at anytime of the year given the right conditions.
Sea breezes are caused by the temperature difference between the land and the sea. During the day the land surface heats, this warm land then in turn heats the air immediately above it. The now warm air then rises causing an area of low pressure over the land. The temperature of the sea surface remains cool; the air above it does not rise so the air pressure remains initially the same.
A wind is generated from the cooler (higher pressure) area over the sea toward the lower air pressure over the land.
On straight coastlines the sea breeze begins perpendicular to the coastline. As the breeze strengthens the wind will turn to the left of its flow. For example on an eastern coastline a sea breeze will start as an easterly but become north easterly as it strengthens. This is due to the coriolis effect (see Afloat September 2008).
Sea breezes can reach up to 25 knots in southern latitudes and strongest sea breezes generally occur during November and December when the land can get hot but the sea remains cold.
However, while the sea breeze follows the mechanism above, the existing wind of the day (gradient wind) and the topography adds complexity to the sea breeze regime.
I have often heard that the Sydney to Hobart can be won or lost coming up the Derwent. Marine areas that are surrounded by complex terrains often have their own wind and weather complexities and if you are lucky there is some local nautical folklore to help mariners.
I had this highlighted to me on a recent weekend away boating on the STS Lady Nelson.
The weather situation was a high in the Tasman and a light northwest to northeast flow over south eastern Australia.
We arrived near the Iron Pot light at the mouth of the Derwent around 1100hrs to sail back to Hobart on the sea breeze. We experienced a weak southerly sea breeze and we gradually made ground to the north. We caught up with the leading edge of the sea breeze that was an area of choppy water stretching across the river.
Sea BreezesThis area was between 50 and 100 metres wide. On the southern side was the weak southerly while on the northern side a very light north easterly. The area between the two was like a washing machine (albeit small wavelets) with a strange “hissing” sound from collisions of the opposing wavelets. This area was moving slowly to the north.
What was occurring in this situation was basically the sea breeze was in an arm wrestle with the gradient wind.
The area of the small hissing waves was the boundary between the two air masses and at the time they were evenly matched. However, as the land continued to warm, the land/sea temperature difference increased and the light southerly increased in strength.
This occurred very quickly once the sea breeze “started to win”. In about 15 minutes wind speeds were up to 15 knots and after 30 minutes were blowing 25 knots with some waves up to about 1.5 metres.
Sea BreezesWhile this was an effect in the Derwent, similar processes could be replicated in other complex waters such as Sydney Harbour, Port Phillip, Westernport and Moreton Bay.
One tip for offshore mariners; the mass of air that is the sea breeze has to come from somewhere. Taking mass away from an area will reduce the wind speed. While the sea breeze is blowing on the coast there will likely be a reduced wind speed or calm area off the coast (10 to 15nm).
For those who know Hobart and are tall ships tragics, we stormed home at over seven knots with nine sails set.

Sea BreezesSouthern Ocean Buoy Race

Pictured above are the tracks of the buoys that I launched last year from the Aurora Australis. Of the buoys, 56941 only transmitted periodically for the first few days and then settled down. 73651 failed on the 18 July.
Buoys send back air pressure and temperature data via satellite.New buoys will be launched from the Japanese research vessel Shirase in the next few days.
You can follow the buoys as they start transmitting by visiting the site www.sailwx.info
In all seven buoys will be launched with four along the 110°East longitude within a few days of each other.
Boating clubs might be interested in tracking these buoys as a fund raiser by getting bidders to ‘back’ their buoy for targets reached such as first past 150° East longitude.
To monitor the buoys progress go to www.sailwx.info and select the area south of WA down the 110° longitude.
The buoy deployment locations are as follows
45S 110E; 50S 110E; 55S 110E; 60S 110E; 60.5S 100E; 62S 88E and 62S 80E.
Numbers will be given to the buoys upon successful deployment.

*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.