www.bom.gov.au/australia/charts/viewer/?IDCODE=IDY00050


Can people fill me in to better understand seabreezes? Why somedays why not others... and today really confused my previous misconception as to a seabreeze?

It is always something other sailors and I that we have talked about for years... and spent many hours waiting for often to be disappointed!!

Is it the moving of air from cooler air to warmer land from ground level or does it come down from higher altitude to sea level then across?

In the bay is Dendy the best to catch it? is it time predictable? if so what time do we give it to?

Often the BOM never gets local seabreezes predicted right either so what hope do we have? Or is it just being in right location at right time?

I know that this site has a little on it... www.seabreeze.com.au/info/wind.asp but I think the bay is even wierder and local knowledge may hold some answers and bring on more time on water and less suduko.

Any sensible anwers welcome

don't be too hopeful

I think you need hot land and cool water...hot land air rises sucks in the cool ocean air.

I think too much heat in Melbourne i.e week long 40's heats water too much possibly increases humidity??? and kills the seabreeze and we sweat all night.

Cool front then cleans the air and resets everthing...guessing..

Read somewhere you need sustained 30+ degrees, low humidty, no clouds in the sky....and lots of luck.

I always found it funny that "Seabreeze" doesn't forecast seabreezes...not in Melbourne anyway.

A sea breeze is like most other winds that will travel from a high pressure to a low pressure. When the land heats up, the pressure decreases. Cooler air over the ocean then travels to the lower pressure. In the souther hemisphere the wind turns slightly left (coriois effect). The best weather for a sea breeze is generaly either side of a high pressure system on a day with high cumulus clouds (the fluffy ones). On Port Philip the northern edge of the bay is best situated for the biggest difference in temperature. Having said all that it can still be any ones guess.

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Glitch said...

In the souther hemisphere the wind turns slightly left (coriois effect).

That is true around a high pressure cell but the seabeeze actually bends the gradient direction to the right.

So, as i understand it a southerly or south-easterly morning breeze off WA turns into a south to south-westerly seabreeze.

Certainly here at Sandy Point, the NE high pressure winds on summer mornings back to the east and strengthen with the sea breeze effect in the afternoon. From January to March they can really kick in from 10-15 knots in the morning to up to 35 knots in the mid afternoon. If that coincides with a low tide in the late afternoon you can have really fast speed conditions.

I'm a bit of a spotter on this so prepare to be bored...

The key to a sea breeze is an unstable air mass over the land and hot ground relative to air temperature. By 'unstable' what you need is air that cools quickly with altitude. This means that air close to the ground will heat up then rise through the surrounding cold air. As it rises it expands and cools but remains warmer and less dense than the surrounding air and continues to accelerate upwards. This significantly lowers the pressure close to the ground and 'sucks' cool dense air off the sea which then warms over the land and the cycle continues. Seabreezes are actually fairly predictable but you need to look beyond simple air temperature. They are also affected by the prevailing wind direction, the stability of the air mass, the temperature of the air, the temperature of the sea and the temperature of the ground. Perfect conditions would be a gentle prevailing onshore breeze, unstable air, cold air, cold sea and hot ground. If you are a real spotter you can go to some of the victorian gliding / hang gliding sites and they will predict most of this lot and also give you thermal velocity which is a good indicator.

If you can't be bothered then look at he following - the difference between max and min air temp on the day, prevailing wind direction, and sea temp. As pointed out a change increases the seabreeze because you have cold air over hot ground.

You can also often see a sea breeze from the cloud formations. You will see no clouds over the sea and cumulus over the land. Fluffy cotton wool clouds are good, large inland storm clouds are great. Often their is a band of cloud parallel to the coast that indicates how far the sea breeze has pushed in land. This band of cloud can push as much as 80kms in land by 4 or 5pm on a summers day which can make it quite hard to spot. As the sea breeze pushes ashore it will tend to kill the cumulus clouds leaving a gap between the edge of the coast and the first cloud formations.

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Nick F said...



I'm a bit of a spotter on this so prepare to be bored...

The key to a sea breeze is an unstable air mass over the land and hot ground relative to air temperature. ...

Disagree! It's a bit more complicated. Stable air from a high pressure system is most conducive to seabreeze formation, particularly with a SW to SE synoptic flow in Melbourne. Basically, if you see a big high sitting over Melbourne and the temp is 20 or more, you should get a seabreeze. Subsiding air (i.e. descending air) from the high pressure system presses down against a layer of low level instability to about 1500m high. When land temperature becomes warmer than sea temperature, a seabreeze circulation sets up within this lower layer of instability, with the cool sea air moving onshore to replace the low pressure from rising land air. If you think vertically, the seabreeze circulation will be an unstable layer sitting beneath a stable layer. You will sometimes see cloud form at the edge of the seabreeze (say out near Melbourne Airport or beyond) and if you look carefully, the bottom of the cloud will be moving with the southerly seabreeze while the top of the cloud will turn back towards the south, indicating a circulation is present. The same thing can be seen in a SE synoptic flow down at frankston where clouds will move from the SE, then all of a sudden the seabreeze will start up as a NW wind and bend around into a SW as it strengthens and due to coriolis effect.
If the overall atmosphere is too unstable and too humid, the seabreeze circulation cannot set up properly, however in such conditions, a trough will often exist which can hold back the seabreeze like a slingshot. The seabreeze flow will sit in the stable air west of the trough. Thursday evening this week was a classic example of this with a hot humid airmass and no seabreeze, but then in the evening, the trough moved through over Melbourne and a 25 knot seabreeze/cool change combination pushed up the bay like a slingshot in the more stable air behind it.
Today is also a good pattern with a very warm airmass and a trough flicking through bass strait which should push a good seabreeze up in the stable air to the west of it.

Clouds

Great detailed information but seems to be way too many factors. I htink I would be happy if someone or some site can just say "at 5pm tomorrow there will be a rocking seabreeze of 25knots"

My understanding of seabreezes is that they appear to have been cancelled until further notice

Could we just get a few strong indicators that it's going to be good?

From above

1 Wind is light but at least Southerly in the morning
2 Difference between high and low (at 5am ish on day in question) is significant
3 The day is going to be hot with no cloud to stop the sun heating up the land
4 Wind is 15knots by 4pm
5 H20 cannot sail because of some commitment or I am stuck in a meeting that over runs by three hours
..?

I know it's really complicated but four or five acid test conditions would help. Or even the site that most accurately calls them.

Here's how I do it:

1. If there is a slight southerly in the morning &
2. Its +23 degrees &
3. After November & before March,
4. Not too humid,
5. Seabreeze doesn't start too early in the day,

=> maybe a seabreeze strong enough for slalom sailing.

Or just watch this graph: earthsci.unimelb.edu.au/engage/dynamic-earth-updates/bay-winds

so today scores a one - number 3 is true

I'll add some of my comments from daydreaming out the window watching the bay for the last 20 years (sad, isn't it).

Matthew and GPom's factors are about right. Seabreeze usually kicks in between 3 and 4pm.

Clear skies inland are important - as far over the Divide as Seymour.

An underlying southerly airstream is important, particularly for a few days after a low has passed through or as a new high is approaching. By the way Acker, I think that if a high is sitting over Melbourne you won't get as good a seabreeze as an approaching one. High over Melbourne, or as it moves across and away is more likely to result in calm or a northerly.

Cool morning- warm to hot day, especially inland.

Oh yeah, and DRY paddocks inland (read November onwards). Paddocks covered with green grass won't create the thermals as readily as dry or bare earth, that then turn into the fluffy cumulus. My paragliding mates really knew what drove the thermal activity.

By the way, there's an absolute cranker on at the moment.

Are you saying today was a seabreeze?

I'm flummoxed. It didn't seem that warm, it was a bit damp this morning, the sun was bright but not searing..

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ginger pom said...

Are you saying today was a seabreeze?

4.5 at Bonbeach is not a seabreeze, and how warm is the water

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Jman said...

4.5 at Bonbeach is not a seabreeze, and how warm is the water

Sailed at Carrum, some rather big bay swell rolling in yesterday..I am still smiling!

You get better at forecasting seabreezes the further you live from the coast. In Canberra they start the drive before it kicks in. On a straight bit of coastline in the southern hemisphere the best seabreezes come in from the the left at 45 deg to the beach. The synoptic wind, or the wind measured at a station inland will be from 45 degrees behind your left ear. In Canberra a moderate N to NW wind is associated with a NE seabreeze. In WA the SW seabreeze is associated with a SE wind inland.

The wind comes in from 45 to the left rises and then goes out overhead in the synoptic wind 45 to the right. A big corkscrew down the coast sort of idea.

Victoria has a convoluted coastline so it'd be hard to get a handle on it. But the 90 mile beach is straight. The best seabreezes there should come from the east when Mt Hotham has a moderate northerly.

This doesn't work in the northern hemisphere, you'll just have to learn to jump on the other tack.

Hot, not cloudy, summertime all help.

Yesterday was not a Seabreeze but a gradient wind generated by a low in Bass straight. Maybe a small local seabreeze influence as it appeared stronger on the Northern bay than it was on the coast.

I aggree with Ian K. Over all my years of sailing I see the strongest seabreezes on Port Phillip occuring when the gradient breeze has a bit of east in it. They arrive later but pack more punch. You also still get seabreezes when the gradient has some north in it as long as the gradient breeze is weak. These days are usually warmer as well.

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MikeyS said...

I think that if a high is sitting over Melbourne you won't get as good a seabreeze as an approaching one. High over Melbourne, or as it moves across and away is more likely to result in calm or a northerly.

No, you need a high over Melbourne or a bit to the west, with clear skies and a calm flow or light S or SE flow for the best ones. In this situation I find the max seabreeze will generally be whatever Melbourne's observed max temp is (this has worked for me for about 15years). If the high moves out to the east at all the NE or N flow prevents good seabreeze formation. The seabreeze is separate from synoptic flow but can occur on top of it to enhance the synoptic flow. Yesterday was a good example of this.
Today the base synoptic flow is SSE about 10 knots with clear skies and Melbourne temp currently 20. The seabreeze will build on top of this to about 20 knots later but higher gusts if it gets warmer than this.

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Acker said...
Today the base synoptic flow is SSE about 10 knots with clear skies and Melbourne temp currently 20. The seabreeze will build on top of this to about 20 knots later but higher gusts if it gets warmer than this.

It certainly looks like its going to be on again today

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Acker said...

No, you need a high over (Do you mean NORTH OF?) Melbourne or a bit to the west (AGREED) , with clear skies and a calm flow or light S or SE flow for the best ones. In this situation I find the max seabreeze will generally be whatever Melbourne's observed max temp is (this has worked for me for about 15years). If the high moves out to the east at all the NE or N flow prevents good seabreeze formation(AGREED) . The seabreeze is separate from synoptic flow but can occur on top of it to enhance the synoptic flow(AGREED). Yesterday was a good example of this. (YUP)
Today the base synoptic flow is SSE about 10 knots with clear skies and Melbourne temp currently 20. The seabreeze will build on top of this to about 20 knots later but higher gusts if it gets warmer than this.

I still think that if a high is plonked right over Melbourne it's usually a good day to get the boat out in the arvo because there is usually not going to be a seabreeze but light winds, tending light notherly or calm. One of those balmy summer evenings when everyone hits St Kilda. I think we're only disagreeing on the position of a high to create a "background" onshore airflow, but I won't claim to be an expert.

And no GPom, if yesterday was just a seabreeze, there'd be thousands of WA sailors moving to Vic. I think the extra 5-7kts was the seabreeze cream on the cake.

I'm not sure that the 90 mile beach gets as good a seabreeze as Melbourne (stand to be corrected). I wouldn't be surprised if the mountains behind aren't as good as big flat paddocks north of the Divide to get the thermals happening.

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MikeyS said...
I'm not sure that the 90 mile beach gets as good a seabreeze as Melbourne (stand to be corrected). I wouldn't be surprised if the mountains behind aren't as good as big flat paddocks north of the Divide to get the thermals happening.

Sandy Point (the end of the 90 mile) gets very good seabreezes, sometime in excess of 35kn.

easy just move to west aust 25 to 30 mmmmm

Ref: Dendy Street.

I've often found that the seabreezes are stronger the closer you get to the city. I prefer to sail Dawson Street in Elwood, but several times last summer I was sat on the beach watching 16 knots float around, packed up and went up to St. Kilda on my way home to find that it had been 18 knots for an hour and all the guys were out playing.

Ok, Boxing day was a great seabreeze

1. If there is a slight southerly in the morning &
2. Its +23 degrees &
3. After November & before March,
4. Not too humid,
5. Seabreeze doesn't start too early in the day,

1 Tick
2 Cross- high at Moorabbin airport was 21.3
3 Tick
4 ? Between 48 and 52 around lunch time at Moorabbin
5 ? Beacon went above 20 at 2:30pm

Other points

Low point during day 9.9C