Hot air and choppy surf: Making sense of the sea breeze
The all day offshore and late arvo glass off (LAGO) are two of those special phenomenas that make for magic post lunch or after work sessions. But why are they so rare in summer and only start to show more prominently into autumn and winter?
The mechanism behind this is quite easy to understand, and it comes back to a simple principle learnt in high school: hot air rises and cold air sinks.
Delving a little deeper, as the days become shorter in winter, the land doesn't receive nor retain as much heat from the sun during the day, with it being released back into the atmosphere once the sun starts setting.
At most coastal locations the air temperature directly above the land drops below the adjacent ocean temperature overnight and it's this temperature difference that drives the offshore land breeze the following morning. As the ocean is warmer than the land during the night and early morning, the air directly above the ocean is warmed and consequently rises. All things being equal it has to be replaced by another air mass. This air mass arrives from the land with a steady breeze blowing out towards the ocean, creating cool morning offshores and a clean smooth ocean surface.
As the sun rises and the day gets underway the land is slowly warmed and at some stage the temperature usually becomes higher than the adjacent ocean temperature. This is when we see the morning land breeze back off, turning variable for a short period before giving way to the sea breeze.
This is the reverse of the land breeze, with the air over the land heating and rising, being balanced/replaced by the cooler ocean air. In summer this effect is amplified due to the larger heat differential between the land and the ocean, with the sea breeze developing a lot earlier and gaining a lot more strength. This effect may persist for days if the land never cools down enough overnight to allow the land breeze to develop as is the case in the drier desert states of Southern and Western Australia.
This is all quite simple to understand when taking temperature differences between the ocean and land as a single feedback loop, but when you add in synoptic weather patterns it makes it a little trickier. Sometimes an onshore pressure gradient will be strong enough to override and negate the morning land breeze effect, while at other times the offshore land breeze can be enhanced by an overarching weather system.
One other factor is the amount of moisture and humidity of the air over the land. The greater the moisture content in the air, the more energy and longer it takes to cool down or heat up. So if we have exactly the same coastal region that one day is dry, and the next day more humid, the land breeze effect during the humid day will be weaker than the dry day. While this isn't as effective in producing a land breeze during the morning, it also suppresses the strength of the afternoon sea breeze due to the humid air taking longer to warm up, reducing the temperature difference compared to a drier afternoon.
Late afternoon glass-offs occur when the sea breeze weakens earlier than usual cleaning up the waves and allowing a few hours of surfing before dark. Combine that with mid-summer twlight and you can understand why the LAGO is a revered phenomena.
Ending on a small anecdote, I remember many a summers day surfing around South Australia where a hot northerly offshore would gust during the morning with the help of a strong northerly pressure gradient. Come late morning you could tell the sea breeze was about to kick in as the wind died and all types of insects started to fly in from the Southern Ocean back towards the land. These insects had unwittingly been blown out to sea during the morning, and were then returning to the land helped by the incoming sea breeze. Within minutes of bees and dragonflies coming in from the ocean across a glassy ocean, the ruffle of the sea breeze would move in, bringing an end to the day's surfing. //CRAIG BROKENSHA
Comments
Could you shed some light on this phenomenon,please.Over the last 6 months there have been 3,possibly 4 instances where the following occured (June 24th being the latest);
morning begins with offshore winds which turn NE around 10:00-10:30 all the way from Southport Spit to the Pass at Byron Bay (confirmed by checking cameras).Inexplicably,the wind down at Lennox/Ballina remains offshore all day (confirmed by surfing clean waves on the point in the afternoon after having just checked blown-out NE conditions at Brunswick Heads)!
To quote "The Redhead You Can Trust".... "Please explain"?
Hi Della,
I looked at the synoptic chart for that day and there was a surface trough sitting just off the Northern NSW Coast. Winds vary depending on where you are on the coast in relation to the troughs axis and this is why there were NE winds on the Goldy but calmer winds from the southern quadrant south of Ballina.
You may be interested in this article for further reading regarding how easily winds and conditions can change over a small stretch of coastline either side of a trough/lows axis: www.swellnet.com.au/news/3711-axis-of-evil
I was wondering whether the topography of Cape Byron would influence the formation/position of the surface troughs and the difference in wind directions,as it appears to be the "border" between NE winds north of it and Westerly winds south of it,or is it just a coincidence?
Are there any other known landmarks that result in this effect,on a relatively consistent basis? Seal Rocks for example,sticks out into the ocean a fair way and lighthouse is 79 m above sea-level(Byron is 118 m).
Some areas do see local funnelling and steering of certain wind directions, ie a southerly around Cooly is usually steered SW during the morning and a west-southwester across Cape Otway will tend west-northwest along the Surf Coast.
In your example though a north-east wind is unobstructed as it approaches both Byron and Ballina. Tallows will see the NE wind blocked but down stream influences towards Ballina wouldn't exist.
It seems to be a coincidence, as it would have to occur more regularly if there was a topography/local effect taking place.
Offshore island such as Moreton and Straddie also come into play with potential wind sub-directional changes/influences.
Craig, further to DB's comments, do you know as to what height (in the atmosphere) local wind effects such as sea breezes and land breezes usually travel?
Hi Don, the sea breeze under moderate heating reaches around 1km, but under intense heating can get up 2.5km so much higher than the coastal topography along the East Coast.
I'm guessing the land breeze would be much shallower.
Hi Craig/Don, I live in Perth, the home of strong summer seabreezes. My basic understanding of this phenomena matches what you have described in your article. However, I have never understood why the west coast seabreezes are almost always from due south to south westerly, rather than westerly (which would seem logical given your article). Could you explain this? Also the land breezes that we get seem to be due east - just confuse things a little more...
Monk, our land breezes up here in Qld are typically NE'ly. So I'm guessing it's possibly a combination of the coastline shape/direction and possibly coriolis effect?
Thanks for that Craig. I have what seems like a dumb question.
What is it about the onshore exactly that makes the surf choppy? Is it the distance over which the wave of energy is affected by wind (presumably the offshore breeze only travels so far)? Or is it something about the greater depth of the onshore wind having greater force on the water?
If it is distance travelled, how far out to sea do you have to go before the offshore breeze creates chop in the other direction?
I guess I'm trying to figure out the specific mechanism that makes chop rather than grooms lines.
Monk, it's got to do with the Coriolis Effect as Don touched on.
If the Earth didn't spin the sea breeze would be straight from the east all day but because it does the Coriolis Force kicks in, swinging the breeze to the left (anti-clockwise) in the Southern Hemisphere and right (clockwise) in the Northern Hemisphere.
Generally you'll see the sea breeze start from straight onshore if the morning wind was straight offshore and then swing more towards the north on the East Coast and south on the West Coast into the afternoon as the Coriolis Effect needs time to make it's biggest impact.
Benski great question and I don't know if I can answer that easily or correctly.
Basically the offshore wind blows against the incoming lines of swell holding them up slightly, ironing out the small imperfections and lumps while not adding any energy. But further offshore away from the coast, this offshore wind will be generating chop going the other way and if you've ever been in a boat offshore when a stiff offshore is blowing it will be all choppy and messy.
So strong offshore winds will be making chop going the other way out to sea that gets larger the further you head offshore.
The onshore wind is adding energy to the already generated swells in the form of small capillary waves which travel at different speeds and add imperfections to the swell. The stronger the wind the bigger the imperfections and unorganised the swell is once it hits the shore.
Nice one thanks Craig!
Benski .
In WA ( place of the Strongest and furthest reaching Land / Sea breezes ) many days of the year the Sea Breezes will still be blowing ( or originating up to 80-100Kms ) offshore . So beyond that and in the middle of the Summer & Winter High's ( up north in winter DS in summer ) it'll be dead calm ..... But also during summer a regionalised prevalent Trough tends to set in . Which I'm sure Craig will take great pleasure in explaining it to you !!?
Pity Rotto wasn't further out offshore . !! Oh the Possibilities ...
And Della ..... Large bodies of inland water ( Bays , rivers etc ) can bend or even lessen a Sea Breezes direction and effect ...... ! Food for thought ....?
Great thread Craig, these bits of information are great and it is why I like Swellnet.
As Don said "Offshore island such as Moreton and Straddie also come into play with potential wind sub-directional changes/influences", which is interesting as well.
As southey mentioned, "Pity Rotto wasn't further out offshore . !! Oh the Possibilities ..."
If Rotto was further out offshore, would that mean Rotto would get more off shores due to the sea breezes originating up to 80-100kms offshore? What direction is the sea breeze off Perth?
WM ,
I meant that it would be out beyond the Sea Breeze influence . ie glass all day for alot of the year .
Perth's land Breezes range from E-NE depending where the ridge is sitting in Autumn /Spring ( but mainly Autumn ) . And Sea breezes predominate nearly 9 -10 mths of the year which 95 % of the time are SW . Very little topography to influence over there . Also the breezes are so strong that they could almost be categorised as Trades winds ( by definition as opposed to literal ) . Bonus ofrotto out so far would also result in Perth actually having decent surf .....
Cheers Southey,
I don't live in Perth and have never surfed Rotto, when flying into Perth I see Rotto out there and don't understand how it blocks so much swell, it faces west to east and is longer that way? If it faced North to South wouldn't that have way more influence? Food for thought.
Not exactly Southey regarding glassy all day a lot of the year. It would be quite the opposite!
The synoptic wind would actually create windy days most of the time relative to the strength of the pressure gradient. Sure if the pressure gradient was real weak there'd be no wind, but that is less frequent than winder days at Rotto's latitude.
The land actually helps create glassier cleaner conditions than if it weren't there due to the land breeze overriding the pressure gradient wind when weak.
And Wellymon, it's not entirely Rotto that blocks the swells coming into Perth but a barrier reef system sitting offshore the WA Coast.
@Craig I thought so.
No1 Rotto does not block all the swell, due to offshore reefs.
No2 Rotto being further out would experience different winds due to the synoptic winds thru various pressure gradients.
Sometimes I work in the Timor sea and often there is no wind out there for days on end and hot, which is halfway between Timor and Darwin.
I couldn't imagine Rotto being further out to sea to be similar and that this would have the same effect as the Timor sea,due to latitude! Correct me if I'm wrong.
No disrespect southey,I was trying to understand "Pity Rotto wasn't further out offshore . !! Oh the Possibilities ...". Was trying to find out if sea breezes don't travel that far off shore?
mmmmm....
All good
Cheers
Exactly Wellymon, because Timor is closer to the equator you enter the doldrums where there is generally no wind and glassy conditions. But down at more southern latitudes there's nearly always some kind of wind.
does the reef system offshore of southern w.a. coastline have a name ?
Not quite sure Udo, maybe Southey can help on that one?
Craig, speaking from experience the trades blow like a gale in Timor!
Ah yes you're right Blindboy, Timor is smack in the middle of the trades.
Wellymon, more around the equator or more correctly, where the Intertropical Convergence Zone sits is where the doldrums and light variable winds occur.
I know this is probably off the subject! but maybe not?.....
I live 280m above sea level in the hinterland Gold Coast (up Bonogin Valley).
I have lived here now 9 years and at the moment, being nearly August we are experiencincing rainfall at night similar to summer patterns? Altho I should note the rainfall here for the last 4 years have been intense.
This arvo we had rain and the first time in a winter senario, that we have lived here, mist rose off the ground and thru the house, it was spectacular due to the setting of the sun in the west. Usually this occurs during summer periods and not at all in winter, since we have lived here?
To top it off, I went down stairs on dark to fill the fire wood basket up and saw a few fire flys, which we only really see thru the summer periods?
mmmm....
Can someone explain that!
Land temperature, Air temperature, Evapouration?
I know this is not an insect site but.....
@ BB
Where i work is have way between Timor and Darwin, along way by chopper. (2hrs flight at 130-140 knots)
There are so many days out there, where it is so still and the deck you walk on is 50 degrees plus.
In the middle of the ocean far away from any land at all?
Not a sea breeze or trade wind in sight.
This is why southey got me thinking if Rotto was further out to sea etc.
But due to latitudes , I knew this was not the senario.
So wellymon were you on the rig that had the big spill?
No that was the "Montara"
Horrible stuff I think.
You would be amazed how many rigs are up there and down the NW Australia.
Montara has been cut up know and they will put an FPSO over the well.
As again BB all the spill and different things environmentally get brushed under the carpet.
I get so disgusted with that.
Cheers
Thanks Craig! I thought that may be the case: amazing how corollas can cause so much energy shift...
@udo there are a few reef systems off Perth the swell energy. Main one is 5 fathom reef which extends between Rotto and garden island. Also 3 mile reef in northern Perth that ruins waves up there. A few in between: the fishos know them all. They are the bane of my existence!
Ps: don't mind the spelling/ gramma, stupid autocorrect. Corollas effect!
Haha, Corollas and steering, I can see the connection ;)
Look at the hot air blowing into the "Wave of the Day" Sublime Autumn Memories!
That is gold , some bloke or girl fully slotted on a shoulder height gem and the other people looking around to take off and drop in or something!
Classic Goldie material for sure.
What a great photo tho.
Monk . get a boat and explore ....
Craig , in Perth in from mid spring to late Autumn , a constant Trough sets up alot more than you think . And it waivers between being centred offshore around 50-100 k's of the coast or 50-100 K's inland . And obviously its NE ( before the Seabreeze kicks in ) when the trough is centred offshore . And SE - ESE when its centred over the land .... Anyway almost a topic in itself . I interested in the zone where the Sea Breezes and Synoptic influences cancel each other out .
And to add something else to that ( VIC is way further south , and has far more Dead Still days than most places in Aust . )
Wellymon , the sea breezes can extend ( or more to the point lose strength ) 80-100 k's offshore but obviously depends on the day and the season . ( remember that it initiates at first on the coast then due to a flow on effect reasonates [ for want of a better word ] or grows out until the differential becomes negliable .
Craig many a day's are spent fishing by others , and occasionally ( in the past ) myself fishing way out west of Rotto in relatively light breezes . Its the coming back in late that gets interesting . But talking to people that regularly stay out overnight would shed more light . Crew from garden island would know heaps more .
But lets just say that similar " mid lattitude high pressure belts " create near calm days a plenty at places offshore during winter not that far north of perth .
As far as I'm aware the " barrier reef system " not that i'd call it that is far east of Rotto and more to the point without only one or two outlying reefs just to the Nth of it ( of which are probably prehistoric bluffs of the Swan River Basin ) that Rotto actually inconjunction with the Swan Canyon actually refract solid swell into Perth . If Rotto wasn't there AND more importantly the river then that Barrier Reef as such would block everything , including the Much anticipated W - WNW swells .
Now I'm getting where your coming from Southey. Yeah directly behind where the sea breeze is originating out to sea should be a calmer glassier seas under high pressure due to air descending from above.
As you said the semi-permanent trough sitting along the Western Australian Coast during the warmer months is nearly worth an article of its own. I'm not quite sure on the mechanisms that push and hold it offshore during certain periods, but I guess it's a large blocking high over Eastern Australia continuing to feed heat in from the north-east? I know it gets pushed east by frontal systems moving in from the Indian Ocean.
Hi Craig - Seeing how the gates have been opened on seabreezes, I have a quick question.
Quite a few times during past summers I have been in the Elliston region on 40 degree days with gusty northerlies. By mid to late morning as the seabreeze would normally kick in, the wind seems to drop off alltogether leaving prinstine all arvo glass sessions. Is it possible this is caused by the northerly and the southerly seabreeze negating eachother?
And one other Q which might be my imagination, but as a fellow Adelaide lad you may have an answer for. I've noticed that as dusk hits Christies during a stormy the waves turn to rubbish. It seems like the wind turns more south rather than the usual W thru to SW. I don't know, maybe it is just the lack of light that makes it appear different, but I'll be damned if the waves don't turn to crap. On a brighter note, Christies is such a jewel of a wave for what it is.
Hi Roubydouby, you've nearly answered that question yourself by mentioning the gusty northerlies.
Generally if the overlying pressure gradient is strong enough (a strong northerly in your case) it can hold the sea breeze at bay or further offshore, with the forces cancelling each other out as you suggested. There are cases outside of this though.
During heat waves this a common occurrence and one which exacerbates the heat wave by not allowing the cool relieving sea breeze back in.
Re Christies in a stormy on dark, that sounds like more of a myth, although with the temperatures cooling rapidly towards dark in such a setup a slight swing in wind direction to the south is more than plausible. Much like the late swing in winds to the S/SE on the Mid and other west facing coasts during summer.
Cheers for the answer Craig. I thought it was the case.
And I've never heard anyone talk about the Southerly at Christies before, it was just something I'd noticed. Or imagined.
Who knows.
"I've noticed that as dusk hits Christies during a stormy the waves turn to rubbish."
I know I'm being pedantic, but.... aren't the waves rubbish on a stormy anyway? And don't they improve with a southerly airstream, due to the protection from Witton Bluff? Unless you're surfing Y-Steps, that is.
Where I surf..in summer..when there is a sea-breeze.. I head over to T.O.S and quite often there is a northerly sea-breeze..for this spot is perfect offshore...but you only have to change direction 5-10 degrees and it is direct onshore..I think your pic 'illistrates' it..it can be onshore at one place..around the corner it is offshore..same 'seabreeze' scenario..different(not much) location?
The area you're talking about Barley is one of the funkiest and trickiest places to understand regarding sea breeze development.
Much like an island it has ocean surrounding it on three flanks which results in sea breezes coming in from all angles. At some stage one of them wins out and hopefully you're on the right side of the coast when it does.
I've come from 15kt southerlies at a well known a-frame reef facing west, driving a short distance around the corner to the southern facing breaks where it should be straight onshore to find clean glassy and pumping surf! Other times is southerly down the southern tip of the area and then further to the north around T.O.S. it's bloody northerly!
Yeah barley and craig, fully understand you both on that one,
As donweather said a few posts back.
As you say craig, "Much like an island it has ocean surrounding it on three flanks which results in sea breezes coming in from all angles. At some stage one of them winds out and hopefully you're on the right side of the coast when it does."
T.O.S is awesome like that, thats when there are only a handful of people there.
Come now Ben, there would have been a time when the sight of tightly packed isobars marching through the bight would have gotten your juices flowing.... for actual surfing. I think I even remember an article you wrote about stormys in the nascent days of Swellnet... though I think it was about the causation of the storms and how frothed Adelaide surfers got, rather than your particular fondness for them.
While Christies may not be ruler edged perfection, it is nice being able to get a few solid chunks during a storm. To be honest, I prefer surfing there on a stormy rather than clean mid. And besides, how many other opportunities do you get the bathe in the storm water run off of the Christies' beach locality?
But I take your point about Witton Bluff and the southerly. It has never made sense to me either. I'm just reporting my observation about dusk and 'greatly reduced' wave quality.
I'm sure it isn't going to rock the surfing world, but it might be worth 10 minutes discussion at the Portie's front bar.
The summer NE sea breeze on the east coast very often improves the surf. "Morning sickness" gets blown away and there are heaps of spots where it is straight offshore and many more which are improved as long as it stays light. Typical pattern is morning sickness to 8 or 9, light NNE for a few hours then shifting and strengthening slightly to the NE and, by about 3 gusty ENE.
SE sea breezes, on the other hand are pretty much the kiss of death for quality waves along the whole coast.
Thanks for the article Craig.
Being West Oz raised, I remember the sea breezes all too well. Back when the only information you got in order to predict them was yesterday's synoptic in The West, or TV's chart, I thought the sea breeze would originate north - up Carnarvon/Kalbarri way, and then peel south throughout the morning, a bit like peeling back a blanket on a bed going past all the coastal towns in succession. The hotter the land, the sooner it happened. On drives to the midwest I'd always try to get to Lanno before the breeze did coming the other way. If you were lucky in summer, it'd stay offshore Down South all day.
Also, in the Midwest the seabreeze is way more southerly in direction, always thought that was because there was a greater mass of 'continent' for want of a better word heating up immediately NE rather than to the east, and winds flowed up all the way into this mass. If sea breeze is a local <100km phenomenon, this is incorrect Craig?
Fondly remember 2 weeks solid of sea breze at times, it just didn't abate. And a morning getting into the water at 6AM being told "You're too late," as the wind came in!
As for the land breezes, sometimes they were so strong that it was better to wait for a couple of hours for it to calm down rather than get stuck in the lip!
I have to disagree Blindboy, it seems you are just taking just you area into account. Saturday as a classic example, every beach from Manly to Palmy was pumping during the morning with a mix of east and south swell under a moderate offshore westerly wind.
North-east sea breezes then limited options during the afternoon to protected northern ends of selected beaches and it was nowhere near as perfect as the morning.
On the contrary I could say SE winds in Manly are much more favourable than a NE'ly so taking into account the whole Northern Beaches stretch, I'd much rather a light to moderate offshore W'ly during the morning than a NE'ly into the afternoon.
And morning sickness isn't generally a problem during winter as winds more often than not swing offshore late evening and clean up the surf perfectly for the next morning.
Summer is a different story with that more variable breeze at dawn creating that morning sickness due to no sustained offshore to iron out the bumps and lumps.
And thanks VJ, the sea breezes in WA are generally from the S/SW and with strength usually helped by the eastern arm of a high pressure system lying along the coast. This setup magnifies the sea breeze due to the gradient wind being from a similar direction into which the sea breeze wants to develop from. It's also amplified/subdued to the positioning of the semi-permanent trough that Southey mentioned earlier.
Right you are regarding holding off until mid-late morning for the offshores to ease a bit, makes for much easier surfing, but it's all in the timing!
craig I did specify summer in my comment and morning sickness is common then. Westerly winds are pretty rare in summer and, counting the options from Palm Beach to Manly there are at leat twice as many NE spots as SE and the NE spots are more consistent and on average better quality. On Saturday the NE was so light many of the exposed beaches still had fun waves similar to summer mid-morning conditions.
Ah yes, missed that sorry Blindboy and agree on your second point as well.
VelocityJonno i couldnt agree more. In Gero summer surfing is an art in the timing between land and seebreezes. Often one golden hour of glass between 11am and noon surrounded by 20-30kt breezes for the remaining daylight hours - gotta get it right! And if the synoptics show a prevailing sth wind forget it - it will be 20-30kt Sw for weeks on end. Makes the option of having a backup windsport pretty viable - and kiting/windsurfing a good skill to have in the NW (NB. surfing should always be priority)