Understanding the sea breeze: Part 1

Stu Nettle
Swellnet Analysis

It’s the beginning of October. Winter ended a month ago and summer is two months away, yet already there’s a feeling of transition. Aside from the rising temperature the most obvious change is the wind: the persistent south-westerly regime of winter is relenting and in its place is the coastal seabreeze - warmer, gentler, more localised. Outside of the tropics, seabreezes characterise the Australian coast during the warmer months.

But what causes the seabreeze and how does it affect surfers?

Seabreezes, as their name implies, only affect the coastal margins of the continent. Rarely will they travel more than 100 kms inland, although Canberra, 120 kms from the coast and 600 metres elevation, will feel the occasional sea breeze during summer.

The genesis of the seabreeze is a temperature difference between coastal land and the adjacent ocean. In spring and summer, the sun heats the land during the morning while the ocean remains the same temperature. As the land heats up, the air above it rises, as all hot air does, leaving a void underneath that needs to be filled, and this comes in the way of the relatively cooler air from over the ocean - the sea breeze.

Though they're common features of the spring and summer seasons, sea breezes are generally stronger during late spring and early summer when the difference between land and sea temperature is at its greatest. A big disparity between hot land and cool water amplifies the effect. In Sydney, by Christmas time sea surface temperatures have usually risen enough to temper the strongest sea breezes though they'll still blow till autumn.

When a seabreeze is fully established it moves in a cycle: Hot air over land rises and cooler air from the ocean moves in to replace it, while in turn the hot air cools and moves out to sea where it descends and then once more blows toward land with the seabreeze.

The seabreeze will often only be felt up to approx. 300 m above sea level, while the air blowing back out to sea - called the return flow - is above 600m. The strength of the return flow is always less than the seabreeze below it.

A fully established sea breeze will move in a cycle with a return flow back out to sea

Though the dynamics are the same, seabreezes come from varying directions depending upon the alignment of the coastline. When the seabreeze first begins it blows directly from sea towards land, however due to the Coriolis Force the wind moves counter-clockwise as it strengthens, so what we have in Sydney is a nor-easter, in Perth and south-west WA a sou-wester (the Fremantle Doctor) with each coastline in between having varying angles of seabreeze.

The Coriolis Force comes from the spinning of the Earth. It’s the same force that makes low pressure systems spin clockwise and highs anti-clockwise. In theory, wind should blow directly from an area of high pressure to low pressure as the atmosphere tries to find equilibrium. This would mean wind blows directly across isobars. However, the Coriolis Force intervenes and ‘knocks’ the wind to the left (in the southern hemisphere) so that synoptic wind roughly follows the isobars.

The same force is applied to seabreezes though on a much smaller scale.

With that in mind, it’s worth thinking about the area over which a seabreeze works. As stated earlier, a strong sea breeze can push over 100 kms inland. It’s easy to record this from the numerous weather stations on land. Harder to record is how far out to sea the sea breeze extends, and this is important as it has a direct effect on surfers.

You see, the further out to sea a sea breeze extends, the longer it takes for the swell it creates to reach the coast. And as most seabreezes slow down during the evening and stop before midnight, much of the swell - albeit small windswell - is hitting the coast during the unsurfable hours before sunrise.

Yet if we know a seabreeze has developed further out to sea then it means its influence will remain for longer during the surfable daylight hours.

Anecdotal evidence from yachtsman suggests the nor-easter seabreeze at Sydney can extend up to 60 kms out to sea. In Perth it may extend further, the reason suggested is that the Great Dividing Range limits the scope of the seabreeze on the East Coast while in WA there are no topographical barriers. It’s felt further inland, and it starts further out. Again, that remains anecdotal.

But however far out to sea a sea breeze extends there are ways for surfers to maximise its effects. The most important is to understand that while it may only extend 60 kms out to sea, that distance is measured in a straight line while the wind blows at an angle to the coast. Some parts of the NSW coast can have fetches over 100 kms, meaning that a jumpy little windswell at home acquires new dimensions with a drive to somewhere with favourable alignment.

Mid-January 2015, Whippy surfs a jumped up nor-east windswell (Tim Bonython)

The only downside, however, is timing. Seabreezes are notoriously short lived. As soon as the breeze stops blowing, the waves start dropping, so you don’t want to be driving when you should be surfing. Fortunately there’s an (almost) surefire way to predict when a sea breeze will blow. If during spring and summer you wake to find light dew on the grass then the sea breeze will blow that afternoon. It’s not a guaranteed method but then that’s what the BOM’s for. We live in the Information Age after all.

All of the above applies to periods when the seabreeze isn’t unduly affected by synoptic winds. At times, the seabreeze can either be nullified or amplified by the prevailing synoptic wind. This topic is covered in Part 2.

Read Understanding the Sea Breeze: Part 2

Comments

Terminal's picture
Terminal's picture
Terminal commented Friday, 6 Oct 2017 at 3:06pm

Whip sighting!

Legrope's picture
Legrope's picture
Legrope commented Friday, 6 Oct 2017 at 4:08pm

Yeah. I’m in WA and I can’t see the ocean from home( too dark on surfcams real early) and I know If there’s swell and I go to put the board in the car but if there’s dew on the roof of the car, it’ll be a light onshore already. Go back to bed.

pigdog's picture
pigdog's picture
pigdog commented Friday, 6 Oct 2017 at 4:14pm

"Though they're common features of the spring and summer seasons, sea breezes are generally stronger during late spring and early summer when the difference between land and sea temperature is at its greatest. A big disparity between hot land and cool water amplifies the effect. In Sydney, by Christmas time sea surface temperatures have usually risen enough to temper the strongest sea breezes though they'll still blow till autumn."

you forgot vicco brother:(
for port phillip bay in victoria the sea breeze is not at its strongest until jan feb...the land temp just doesnt get high enough in spring and early summer
as a surfer who does not have blinkers on and loves windsurfing and kitesurfing i have a sharp eye on the sometimes unpredictable seabreeze in melb.
#1 its at its strongest when the centre of the high is over melb.
#2 the more north the centre of the high is over melb the stronger the sea breeze will be
#3 if the sea breeze starts in bass straight then funnels though the bay this will make it a very strong sea breeze

goofyfoot's picture
goofyfoot's picture
goofyfoot commented Friday, 6 Oct 2017 at 4:43pm

This is interesting, thanks

Lanky Dean's picture
Lanky Dean's picture
Lanky Dean commented Sunday, 8 Oct 2017 at 4:03pm

# 3 - I have seen and swam in messy Surfable waves in Port Melbourne. Did not bother surfing them. I think there is a pretty devoted crew of Port Phillip surfers.

frog's picture
frog's picture
frog commented Friday, 6 Oct 2017 at 5:21pm

The modern surf skis can turn tiny sea breeze windswells into fun runners. A proper blow will whip up chunky chop that way out off the beach is fun and even scary at time to ride on a ski within a half hour of the blow starting.. Ride one peak to the next for hundreds of metres with no crowds and no need to find anything more complex than a place to pleasant place to launch. If a bit of swell in under the seabreeze it actually gets pretty full on with big drops into the holes in the ocean.

Getting a a surf fix from a seabreeze on a board is a struggle - on a ski it is easy.

Frogg

campbell's picture
campbell's picture
campbell commented Saturday, 7 Oct 2017 at 11:45am

What about when a sea mist or fog rolls in?Is it the same cause/effect? Ive only seen it in Victoria and also far up Northern California, both on warm / sunny days then out of the blue a total blanket of fog came in off the ocean to the point where you can't see anything out to sea (even meters away). Both have much colder ocean temps than land at the time, it was summer in Vicco and Autumn in California (Nor Cal the surf was huge also that day so would have been a pretty dangerous to have been out in the surf and that happened). In winter the fog in California is a pretty regular event. Can if be forecast these days?

...

thermalben's picture
thermalben's picture
thermalben commented Saturday, 7 Oct 2017 at 11:59am

Unrelated, and worth a seperate article. Sea fog forms when cold air moves over warm water. Requires a few ingredients and is hard to predict (I can only recall once or twice where sea fog was forecast in advance) but if the synoptics are right sometimes we'll see multiple occurrences over consecutive days.

the_b's picture
the_b's picture
the_b commented Saturday, 7 Oct 2017 at 12:22pm

I'm sure it's the opposite... warm air over cold water. Spent some time in San Fran and the fog was best/worst in spring/summer with the hot land air but water still cold... also seen the same locally at 1770 in August with warm air but the water got down to 18-19deg, and on the cenny when the inshore water down to 17deg and high humidity air some decembers/jans

thermalben's picture
thermalben's picture
thermalben commented Saturday, 7 Oct 2017 at 1:38pm

Argh! Yep, mistake at my end, edited my sentence as I went (via iPhone) and got it mixed up.

Lanky Dean's picture
Lanky Dean's picture
Lanky Dean commented Sunday, 8 Oct 2017 at 12:03pm

@ The b , I call it the fogmonster usually starts appearing in March/April with the ever familiar North West wind in NorCal, OR , Washington. Some times I will arrive at the beach, clear skies offshore and look towards the horizon ( over ocean ) and see the fog monster waiting.
Sometimes it takes five minutes sometimes it takes an hour and some times it will take all day!

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Saturday, 7 Oct 2017 at 12:36pm

Love these articles.

I like a light sea breeze after a good offshore, gives great little frothy lips to hit

Ada gula, ada semut!

groundswell's picture
groundswell's picture
groundswell commented Saturday, 7 Oct 2017 at 2:44pm

Once i stopped to check the surf at sandon point from up on bulli lookout. the wind up there was going straight up i kid you not. The seabreeze had started i figured.

GuySmiley's picture
GuySmiley's picture
GuySmiley commented Saturday, 7 Oct 2017 at 4:06pm

Love the days the sea breeze goes to calm an hour or two before sunset. lots of empty waves that earlier were packed. Absolutely love surfing in a fog, the best seeing swelling emerge out of the mist, happens a few times each summer here.

desoutc's picture
desoutc's picture
desoutc commented Saturday, 7 Oct 2017 at 11:28pm

What about the effect of local topographic features i.e headlands, mountains.
Watched the breeze come in at Red Bluff the other day. It came in all the way to the break, then just stopped. Two almost opposite win directions blowing within a few feet of each other. Quite bizarre.

truebluebasher's picture
truebluebasher's picture
truebluebasher commented Sunday, 8 Oct 2017 at 2:05am

Swellnet blows my mind. Stu that's precious, pride of place in this top shelf set.
Deserves a reward of some sort of complimentary noteworthy curios for Stu's Gurus.

(Obscure inshore variants will do just nicely).

Bigger picture shows Sun has reverse effect across the equator as do wind & currents.
Oz naturally follows time on sundials east to west we see that as counter clockwise.
So why must we slavishly obey empire clocks recording lost time reverse to our nature?

Northern NSW rivers once flowed inland in opposite direction. (This keeps getting weirder)

Today's man made coastline interferes with both water flow and wind patterns.
Gold Coast sky scrapers detour winds, no longer shaping up an A frame as good as age old grom's wind tunneled well trodden sand finger groynes.

Cyclone Debbie produced somewhat stranger results again.
Rivers stirred whirlpools in both clockwise/counterclockwise direction.
This upper river anomaly can deviate the flow of downstream floods.

River mouths again now rotate clockwise/counterclockwise due to excess water volume.
Rising water can enter the ocean in reverse effect due to any mix of mod cons.
This challenges the very nature of inshore currents.
(Swellnet) Debbie's Browntown a man made anomaly? None said it looked normal?
The containment line possibly due to a reverse river mouth effect.
Similar to that of a ten pin spin bowler. Spins itself out then instantly cuts a zip line.

Natural/man made currents then feed Waterspouts again not always Clockwise.
On land or water - Dust Devils/Waterspouts will mostly (85%) follow the Coriolis effect.
This was recently noted on US Twisters as well. This mystery throws out weather data.
(Factcheck) Two Dust Devils can dance then cross in opposite directions even change rotation at random ... As can any of the above. Nothing is set in motion.
Not one person has a clue.. Weather's dark matter? Be my guest ...

Two waves can reverse dance in a confined river on a reef but for how long in open ocean ?
Supersizing the sub structure of river/reef hydrodynamics possibly? Swellnet pic for sure!

*Stu's right in that even the Waterspouts do tend to mirror the coastline/temp' change.
(1) As a rule Wide Bays feed the OMFG ! We're all gonna die! Where did that ship go ?
(2) The Rivers feed that weird thing that's hoovering up your kids...clear the beach NOW.
(3) A run of creeks/estuary splits the difference into swell line triple spouts. (SN/1st)

To further explore man made hot & cold air extremes - Iirramirni (Double hunting fire)
DIY Ancient supercharged climate creation! or (Man made climate change on acid!)
2 man fire shapes Anvil cloud brings rain/lightning starting new fires bringing more rain. ...
(For this dreaming story) https://kurdijiapp.wordpress.com/2017/05/20/ ...

Part 2 You say! Can't wait.... Stu's Guru!

udo's picture
udo's picture
udo commented Sunday, 8 Oct 2017 at 12:40pm

Whats the deal when you have a Howling Nth west wind on west coast S.A. with land temps around 40-43 C...then around 2pm wind drops away to nothing...oil glass ocean ....land temps absolutely roasting...not a breath of wind....and no seabreeze ?

indo-dreaming's picture
indo-dreaming's picture
indo-dreaming commented Sunday, 8 Oct 2017 at 3:10pm

No idea but it sounds good...wish that happened here more nothing better than a LAGO

Ada gula, ada semut!

Craig's picture
Craig's picture
Craig commented Monday, 9 Oct 2017 at 3:55pm

Under those heatwave conditions you see the warmer air mass pushed further offshore and hence end up being in the doldrums between the sea breeze and offshore. Glorious! If you went further offshore you'd probably encounter the sea breeze, or the NW'ly was strong enough to overcome the sea breeze :)

Lanky Dean's picture
Lanky Dean's picture
Lanky Dean commented Sunday, 8 Oct 2017 at 3:29pm

LAGO, classic Indo! Took a while to figure it out. Pretty funny stuff.

Chipper's picture
Chipper's picture
Chipper commented Monday, 9 Oct 2017 at 10:07am

Spot on PIgDoggy

robbo's picture
robbo's picture
robbo commented Monday, 9 Oct 2017 at 1:27pm

Great article Stu, thanks for posting. There's a few things we've found on seabreezes in Vic for Port Phillip and Bass Strait coasts which are interesting. This includes local seabreezes on Port Phillip in light gradient winds which can have different wind directions because of the round shape of the bay - the baywinds obs webpage sometimes shows this. These local bay breezes are then often overtaken by a strong ocean seabreeze which moves in from Bass Strait in the afternoon from general S-SE direction that can be up to 20-25 knots. Along with early morning dew on the ground, later in the morning can also look for where small cumulus clouds start forming over land as areas where warm air is rising. For local breezes on Port Phillip, these clouds were generally first seen over the eastern suburbs, nowadays more common to see them first over the western suburbs Point Cook area due to the extensive housing developments now there that have replaced the grasslands.

For the Bass Strait ocean seabreeze, the cycle seems to setup with it extending around 2 miles offshore. As it grows in strength, the cycle extends further seaward to around 30-40 miles out by late afternoon early evening and a strength of up to 20-25 knots. As the land temp drops it then contracts back inward and the wind eases as the cycle dissipates, all the time Coriolis and land friction effects shifting it from S-SE-ESE sometimes as far as E, and ok for beachies sheltered by nearby eastern capes. If on a boat offshore, the sinking air part of the cycle can sometimes be found as a small zone of much lighter winds.

thermalben's picture
thermalben's picture
thermalben commented Monday, 9 Oct 2017 at 3:44pm

Right now we're seeing a very good real-time example of how fresh NW winds can keep the sea breeze out at sea. 

Today we've experienced NW winds in Newcastle around 15kts gusting up to 20kts since about 9:30am, and it's still offshore right now.

But to the south across the Cenny and Sydney coasts, the NW wind wasn't strong enough this morning, so winds have gradually swung N'ly and are mow moderate to fresh from the N/NE thru' E.