For Seeking Heat Capacity
We can all tell when the cold season is upon us. The outside air temperatures drop, yet at the same time the ocean is much milder, even feeling like a warm spa depending on the discrepancy between the ambient air temperature and the water.
The difference can be so stark that, in some locations, ocean temperatures can be double that of the air outside.
While most surfers are familiar with this phenomena, you may not know why there is a difference in temperature. So today we ask the question: Why doesn't the water temperature drop at a similar rate to the air when transitioning from summer to winter.
Melbourne Mean Maximum Air Temperature vs Ocean Temperature in Port Phillip Bay and Bass Strait. Notice the lag in ocean temperatures and also the bigger variance in the shallower bay.
The answer comes down to the heat capacity of various substances and materials. Heat Capacity is the amount of energy needed to raise one gram of material 1° Celsius under constant pressure.
Water has the highest Heat Capacity of all common substances on Earth, meaning it requires more energy to change its temperature by 1° compared to anything else. Whether that be an increase in temperature or drop in temperature, the energy needed for this is greater than any other substance.
This makes water a great thermal buffer and herein lies the answer to the question. Air on the other hand has a much lower Heat Capacity requiring less energy to increase or decrease its temperature.
Looking at specifics, the Heat Capacity of air is 1,000 joules/kilogram, while water is 4,179 joules/kilogram. Adding salt (to make sea water) lowers the Heat Capacity, though its influence is minimal and only drops the specific Heat Capacity to ~ 4,000 joules/kilogram
So when comparing the Heat Capacity of air to sea water we see that it takes four times the amount of energy to heat the ocean 1° compared to air.
Coming out of winter the water has usually reached its low point in temperature and as the air and atmosphere start to warm due to increased solar energy, we see a lag in the heating of the ocean owing to this difference in Specific Heat Capacity. The solar energy is heating the air four times quicker.
Conversely, come late summer/early autumn a peak in ocean temperature is usually seen across most regions.
Once the solar energy reduces into autumn and winter we see the atmosphere dropping in temperature quicker than that of the ocean, owing the fact that the atmosphere will lose its heat four times faster than that of the ocean.
Now there are other nuances that come into play, such as global circulation of currents as well as upwelling 'hot spots' which can experience some of Australia's coldest waters in summer. The Mid North Coast under sustained north-easters and the South East region of South Australia under strong south-east winds are a couple of great examples.
Then there's the East Australian and Leeuwin Currents, which are both major transporters of heat from the northern climes. This southward flow of tropical water creates an even stronger lag between outside temperatures and the ocean temperatures with relatively warm water seen well into winter.
One final factor to consider is also the depth of the water - i.e the shallower the water basin, the quicker it will react to solar heating and cooling owing to there being less of it to heat/cool. This is why Port Phillip Bay gets colder and warmer than Bass Strait. Similarly the South Australian gulfs are cooler/warmer at their peaks compared to the nearby Southern Ocean.
// CRAIG BROKENSHA
Comments
Great article. As a vicco surfer there is no crueler temptress than the September - November window.....air temp rises, smell of wattle and coastal tea tree, sun is hot on your skin, you duck down for the first swim of the season and nearly die of shock when you dive in. So you end up shivering on the beach, staring at clear green water that would be 20 degrees were it the South coast of NSW....the price we pay hey!?
Steamers come out if the water gets to 20c in winter here.
Water might be 20c but air can be single digits and the offshore blowing out the creek at dawn. The best bit is feeling the sand get warm about 1m away from the water. I bust out the steamer for about 8 weeks of the year.
Gave it good rinse over the weekend, dried in the garage & put it away for next year.
Yes, love that warm sand feeling in autumn and winter. You can also feel it in the air once down near the ocean as well.
One more factor that comes into it is the thermal mass (and subsequently thermal inertia). Actual volume of the substance in question is a factor in that (as well as heat capacity etc).
Hence why a backyard swimming pool will be pretty icy in winter, but the ocean still pretty warm.
(edit: you kind of touch on that in the last paragraph).
Yep, mentioned in the final paragraph :)
Serves me right for skim-reading ;)
Thermal inertia though, great word.
Terrible combo, IMO.
And this my friend is exactly why Spring is shit in Nth NSW/SE Qld when it comes to morning offshores and clean conditions. Coldest ocean temps and warmer overnight land temps don't permit the land/sea temp differential to kick in and hence usually left with wobbly shit in Spring dawnies.
It’s with a view to this oncoming onslaught of dross that I’ve been keeping an eye out for alternative spring time activities. Luckily enough, I’ve recently learned that trout season in the hinterlands commences in the first week of October . Sounds like a great excuse to explore more of our fine land and perhaps catch a feed at the same time.
Sounds good.
If I can add to that suggestion: Ride a mountain bike to the trout hole.
It's all downhill to the hinterlands isn't it?
get your flathead game on.
walking the sand flats with the wind at your back chasing lizards is as good as it gets.
and nothing tastes better with a cold beer.
Can second that. Don't do it often enough...
Explains offshore winds in the morning and wind swings as it warms up. Water stays warmer over night, due to its heat capacity, thus the air above it is warmer (warmer air = less dense air = lower pressure). The land cools down over night at a faster rate and the air above it therefore also cools down (cooler air = more dense air = higher pressure).
Then its simple, air flows from high to low causing offshore winds in the morning. As the sun rises and the land heats up (at a faster rate due to its heat capacity) the pressure changes and you get the dreaded wind shift to onshores.
Physics is beautiful aint it
"ocean temperatures can be double that of the air outside."
The science teacher in me insists I get pedantic on that point.
Nice article, but "ocean temperatures can be double that of the air outside" made me close my eyes and think of Lord Kelvin.
blowfly will be pleased to know that more than 40 years after the fact, -273 is still said inside my head in the voice of my science teacher.
Ah yes, yes, good point. Thanks and also blowfly.
Recently almost had a pumping surf ruined in the SE south Australia region over summer thinking that caus it was 35° outside and January that I’d only bring my 3/2 (luckily not my short arm 2/2) caus within minutes of being out the back I was shivering and had numb feet. This surf was after the SE winds too as you mentioned
I was told this was because the continental shelf was nearby to this particular spot does this sound right?
Yep, the closer the shelf the more pronounced the upwelling is as there's less of the mixed-layer available on the shelf compared to if it were wider. So it starts upwelling water from below the shelf earlier.
Another good article regarding this here.. https://www.swellnet.com/news/swellnet-analysis/2019/01/03/analysis-cold-water-and-green-seas
Its been freezing up here. Springy in the early morning and even having to pull out the vest with my boardies for the lunch surf.
Talking to a chap from Seth Africa in the water the other day and he said Cape Town suffers from up welling in summer and that the water is warmer in winter. It seems the biggest effect on our ocean temp down here (MP) is current, would that be correct Craig?
Yep, because Bass Strait is sitting all on the shelf, it'd be a well mixed layer and the availability for colder water to be upwelled would be very scarce.
Are you sure about that, Craig? With water depths in Bass Strait being greater then 30m in most places, isn't there potential for at least localised upwelling in offshore wind conditions independent of cross-shores and Coriolis forcing?
It appears that the water column is generally well mixed through Bass Strait though there is stratification through the spring and summer months. The persistent strong tidal mixing and wind mixing through autumn and winter keep it well mixed, while tidal mixing through spring and summer maintain this through those months, though to a lesser extent resulting in some stratification.
Researching the upwelling/downwelling systems in and around Bass Strait it appears the maximum chlorophyll a concentrations (from upwelling) are through winter due to the wind mixing and movement of coastal trapped waves eastward, upwelling off the shelf to the west. Off the east there's also a downwelling system.
Here you can see the maximum chlorophyll a (black contours) from April through August (left chart).. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016GL072096
Also this quote from this research paper in reference to upwelling off the western entrance to Bass Strait.. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016GL072096
"Unlike Ekman upwelling, the upwelling described here is essentially a subsurface phenomena. Little or no signature may be present in surface temperature, and such upwelling may exist in other regions but remain undetected."
So it appears any noticeable upwelling would be seen through spring and summer when there's the stratified layers of water temperatures but the effect appears minimal.
Gee, thanks for that, Craig.
I thought there might be some influence from winds/swell/tide and the bathymetry which shows the canyon off the Otway coast and large depression in the central area of the strait.
I'm reading between the lines that there's been relatively little work done on the physical or chemical movement of water in Bass Strait? In my experience, the lion's share of marine research in Vic has been undertaken for the Bays, though this has started to shift over recent years.
Yeah papers on the water movement and mixing through Bass Strait is minimal and not many papers came up. This one is a good one to read, but otherwise they look at the interactions with the shelf at the western and eastern entrances/exits.
Also it looks like the upwelling mostly arrives up from the shelf from between Tasmania and King Island.
1983!
Annual Cycle of the Density Field in Bass Strait
Cmon, that's only 37 years ago...and at least they're still there doing research and haven't been scuttled because they can't commercialise it. Goodness.
This paper has me a bit confuddled in that it seems to be an exercise to validate a model with actual observations and use that to infer things rather than an actual study of water movement through Bass Strait. Interesting that they don't seem to consider swell as a factor in the winter mixing when the longer-period swells predominate (OK, except in 2020). They factor in wind-driven mixing which only affects the uppermost layers of the water column.
Another great article Craig. The Cape Town upwelling thing is pretty fascinating.
Cape Town is probably a bit more complex due to warmer Indian Ocean to east and colder Atlantic Ocean to west.
Yep, the cold Benguela Current socks them from the south, while the much warmer Agulhas flows down from the NE and E.
Complex upwelling too, though. I summer, they get very strong SE winds, as the ridge of high pressure digs in under the continent. That drives very strong Ekman Transport (to the left in S Hemi), making for significant upwelling on the west coast. It can be 30C+ on the beach and 8C in the water.
Meanwhile, across False Bay (also in Cape Town) you can be surfing in boardies and a vest.
The sub-antarctic current that flows up the east coast of the South Island clearly has a huge impact on water temperatures in NZ. It's counter-intuitive but in my experience the water temperatures around Wellington aren't much warmer than the breaks around Bluff (bottom of South Island), while Dunedin is seriously cold.
Currently 13 on The Wellington Southcoast. Invercargill is 11 and Dunedin 10.
been an incredibly mild winter here for water temps with the profusion of E swells which helps kick along the EAC through winter.
Byron buoy has been consistently 21-23.
if we get a La Nina/ -IOD spring with attenuated northerlies it could be a mild spring.
Great article, Craig.
I try and walk around in a t-shirt all year round at work. (open workshop. no a/c here)
Getting use to the cold makes it easier when the briney gets chilli I believe.
My opinion only. May be so far from the truth, I don't know.
But if I can train the brain to think that and it works for me.
Who am I to question facts :)
I do this regarding wearing thongs year round in Sydney. This year however it caught up and I got chilblains on some of my toes on the left foot.
This resulted in the toes going numb out in the surf even when the water wasn't that cool so I've had to wear booties to keep them warm, which I actually love. I'm used to booties and that extra grip on the deck with late or white/water drops and hanging onto reo's you'd usually not make is a plus.
Its totally legit 69 LB. Exposure to cold enables your body to ramp up its own inner processes for heating and circulation. It's actually as much mental as physiological though, learning to just deal with it does more for managing it than you would imagine, then there are Wim Hof breathing training ramps it up even more.
It was in reading about him that I changed my mental approach, trying to just stay calm and not overreact to cold, just accept it, even resist shivering to the extent you can. Significant difference in response. Nassim Taleb expounds on a much broader level in his book "Antifragile".
Having said that, I'm no Stunet!!
Disappointed no-one got the Swerverdriver reference.
Ah, kids these days. I feel like the middle aged bloke in the 90's pining for Creedence.
Went straight over my head. I'm 48, but then again I am from NZ and we're a bit behind the times over here.
As for CCR, I was never much of a fan. If we're talking about early 70's rock then I'm all about Neil Young.
Ripper article Crangle <3
first time this winter the water temp has gone below 20
One of the best feelings in winter at dawn on the MP is when there is no wind, frost in the low areas, fog settling in the low parts of the dunes, you walk down the tracks to the surf and as you come over the dunes you feel the air temperature rise and become more humid obviously being influenced by the warmer water.
Hey Craig how much does humidity and evaporation have an effect on ocean temperatures compared to air temp?
I know with pools down our way without any heating they struggle to get over 21 degrees in summer, where as QLD pools seem to get a lot warmer than our pools. Is it the average airtemp, or humidity and evaporation that has the biggest influence?
If true I'd say the warmer pool water in summer would be due to the air being more humid in Qld and hence not becoming as cool at night therefore the heat loss from the warmer pool into the atmosphere is less in Qld at night compared to that in Victoria.
Therefore the next day it's already warmer compared to the pool in Victoria and will then heat again on top of that. Just thinking off the top of my head.
Also humid air has a slightly greater Heat Capacity than dry air, but the difference is minimal due to the concentration of water vapour that has a greater Heat Capacity being very low in the air column.
For outdoor pools, energy loss is dominated by mass transfer (ie evaporation).
More humid air has less capacity to take on additional moisture, so evaporation is reduced - so pools will cool more slowly when the surrounding air is more humid*.
Not sure if the same holds for the ocean - it's a more complicated scenario with currents, upwelling, etc. But I'd imagine that all else being equal, the upper layer of water would cool less quickly if the air RH is higher.
*(this is pretty heavily simplified; the relative humidity of the "boundary layer"/free surface of the water will always approach 100%; but the RH of the air in the "far field" is what's important i.e. what a humidistat will measure a good distance from the pool. Also, the actual mass of moisture that a given RH corresponds to varies with temperature - air at 30 degC and 80% RH will hold a lot more moisture than air at 10 deg C and 80% RH...)
Yep, great post Pops.
Thanks Craig.
Since you've encouraged me to get my nerd on...
It get's really interesting when you start thinking about _why_ evaporation takes enery away from the water/why it even occurs.
That takes you into the realms of statistical mechanics...
In short, a body of water (or any other substance) with a "uniform" temperature (temperature being simply a measure of the mean vibration/kinetic energy of the atoms/molecules in the substance) will actually contain molecules with a pretty wide distribution of vibrational energies. Those with sufficiently high energies (ie enough energy to become a gas) and at the free surface* will evaporate off, which means the average vibrational/kinetic energy of the body of water will decrease i.e. the temperature will go down.
Again, this is a really simplified explanation that ignores a bunch of other factors at play (particularly pressure distributions). But the processes at play still interesting.
* (if they weren't at the free surface, they'd bump into other molecules and exchange (lose) energy first. Also; this assumes that there's somewhere for the "hot" molecules to go e.g. an adjacent body of air with moisture-carrying capacity)
Yeah right, never thought of it like that.
Get your Wim Hof on, and forget the wettie guys. With the whole Covid thing this year, I've been looking how to improve my immune system. Hyperoxygenating the system is good for cold tolerance and immunity it looks to me. So far I haven't worn a wettie all year, and people think I'm a bit crazy in my boardies. So far the ocean hasn't got much below 17 degrees in Sydney, even on the coldest mornings, so it's no great feat. Haven't caught Covid either, which is good.
Interesting read, Craig. I've always considered the wind and its affect on the EAC as the major influence on water temps (during summer and maybe early autumn, too) along the NSW coast? It's fascinating how water temp along the beach can drop (off the back of a strong NE) a few degrees overnight...
Thanks. Into autumn and early winter, for sure the EAC keeps temperatures warm as it pushes south down the NSW coast but it's also losing potency at the same time and slowly cooling itself.
Always struggle through Oct and Nov here when the air is 30 degrees and the water is 18/19. Absolutely cook in a steamer so end up surfing in boarders and 1.5mm rashie and out upo with numb toes and fingers.