Air..... the creator of waves
I put this to a rather learned chap here..... His response was "temperature" is the most important factor.... The colder the air, the tighter the molecules......
So, here's one for the brainiacs.... Picture this...... We have in front of us , 6 identical swell producing fetches... TOTALLY identical on the map.... BUT!!!!!!!
Fetch 1 - the air is 90% humidity, temp is 25'c
Fetch 2 - the air is 45% humidity, temp is 25'c
fetch 3- the air is 90% humidity, temp is 35'c
fetch 4 - the air is 45% humidity, temp is 2'c
fetch 5 - the air is 90 humidity, temp is 2'c
fetch 6 - the air is 45% humidity, temp is 35'c...
Will the density or humidity levels in the air that is causing friction and therefore waves, create different wave sizes, even though the wind strength is exactly the same?
This would be surface wind Welly.
Interesting topic, will respond after a bit of thought.
It's all about "torque"...... My guess ( and it's just a guess), would be fetch 5.... The air molecules are compacted, plus so would the h2o molecules within the air.... So wouldn't the friction on the ocean surface be greater say compared to a warm but dry wind, which has less water in it, and "softer air"?
OR!!!!! It doesn't matter? If it does matter, could it be one of the reasons why the occasional swell is under or overcalled? Could it be the reason some swells have more grunt?
So, cold air is more dense, so could possibly create more friction on the ocean and bigger waves. Warm air is less dense so possibly less friction.
Humid air is denser than drier air and hence the same could apply as above.
But colder water is more dense and hence probably harder to get moving in the first place, so the colder air over the colder air could cancel out that extra frictional effect on the ocean.
This is probably a PhD project but I think you'll find any differences would be classified as insignificant in general wave creation equations and scaled out. Ie of much greater orders of magnitude less than other factors.
Interesting nonetheless.
Also the grunt of the swell is based on the period of the swell, local bathy of the incoming swell and then the water temperature. Colder water feels a lot more heavier and powerful due to its increased density than the same period swell hitting a tropical reef.
I noticed this when moving to Sydney that even good long-period groundswell seem to have less water/density and power than compared to the colder Southern Ocean swells of a similar period.
"But colder water is more dense and hence probably harder to get moving in the first place, so the colder air over the colder air could cancel out that extra frictional effect on the ocean"
Yeah, though about that.... So the "study area" would have to be in subtropical regions that can have "warm air swells" in summer, and "cold air swells" in winter, but the sub tropical ocean temp remains fairly constant.....
And for interests sake, that recent low ene of NZ..... On southern swell producing side of that low, alot of cold antarctic air was pumped up into that region before the fetch formed.... On the 15/5 thru to the 18/5, a deep polar low and cold front extended right up to about 24s... It was quickly and briefly followed by a 1037hpa high... Colder than normal air feeding into a humid subtropical fetch..... Check BOM loop pacific chart archive...... Punch in 16/4 to 21/4.... Very interesting.....
Sheepie the density of seawater at 20C is 1.0240 g/cc. At 0C it is 1.0273 g/cc. By my back of the, quite literal, envelope calculation that is a 0.3% difference. Good luck trying to make any observations with that degree of sensitivity!
BB, it's the density and thickness of the air we're talking about... The stuff that pushes the seawater....
Sorry about that. My take is that what matters is the total energy driving the wind which depends on the pressure difference. If we assume that this is the same for all fetches then the total, kinetic energy of the moving wind must also be the same. Less dense air would therefore travel faster but contain no more energy, friction with the slower more dense air would be greater, compensating for the greater speed of the less dense air. End result, equal amounts of energy converted to water waves........well that's my hypothesis!
Doing some quick calcs gives the following.... Using the most extreme of SD's fetches
Fetch 3 Density of Air = 1.12
Fetch 4 Density of Air = 1.277
Gives a difference of 13%.
So if we look at the potential power, P=0.5 x U^2 x A x p, using area of 1m^2 & 20knots we get.
1. 32.9kW
2. 37.5kW
This is purely looking at wind energy and not how it translates to the ocean surface or the environments influences.
Hmmmmm. I need to eat dinner and contemplate carpetmans numbers and BB's theory lol..... Very interesting stuff.....
Ps - smells like roast chicken :P
We really need Professor Pelinovsky to set us straight, but he is busy in Kamchatka.
While some have been focussed on the effects of temperature on water density, you should be looking at viscosity. Viscosity almost doubles from 30C Maldives to 10C Nazare. Isaac Newton solved this in 16something.
The effect of air-sea temperature difference and air density on wave growth was covered in a paper by Resio and Vincent from 1977. The exam will be during the next good swell.
Thanks sluggoes I knew if we rattled on long enough someone who actually knew what they were talking about wouldn't be able to resist setting us straight!
It's a clever theoretical scenario but I think there's a slight flaw in the hypothesis..
I don't think it's possible to have 'identical' swell producing fetches for difference climate conditions.. the reason being the energy required to produce identical fetch swell will need to be difference due to the variances (as discussed above) of the air mass it is acting upon (newton's 3rd) ... thus if the base energy required is different (strength & duration & maybe even direction!) - the swell's caused are likely to be different..!
Bluemoon.... It's just a hypothetical... I could change the hypothetical from "fetch" to "wind strength"... We could for example give the 6 examples a smaller region of 50 square km containing a 50k wind with a water temp of 20'..... The region off central NSW can receive cold south winds, warm Ene winds, all with varying humidity.... And the water temp + viscosity remains fairly stable...
It's just a guideline, bluey....
yup, fairly nuff, that being the case - it stands to reason the climate conditions that contains the most and/or largest molecules will have the greater push effect.. as to the energy (kW) calculation above, I don't know enough about energy/wave height relationship to speculate if there would be tangible differences over a (relatively) small energy range.. anyone got any insight?
It's got me thinking though - what's the realistic ideal temp/humidity that, given a breeze, will create waves with max efficiency... (given that 90% humidity/2C air temp conditions may not be the most common of climate conditions! - if indeed this option is the winner)..
According to this, humid air is less dense..........................
It's a good question SD. I've had similar thoughts about whether density gradients could affect swell propagation, plus a wide range of other questions in the same field of research. It seems that the effects are minimal in some instances, but worthy of further investigation in others (ie swell/current interactions).
As for your particular question - a quick google shows that there are plenty of technical papers that discuss issues closely related to this, such as:
https://docs.lib.noaa.gov/noaa_documents/OAR/ERL_ETL/technical_memoranda…
https://images.remss.com/papers/gentemann/chelton_jclim_2001.pdf
Good god....... What have I done....... :)
A nice bit of light reading, Ben lol.....
What got me thinking about all of this was that current east swell in qld..... A colder than normal air mass.... Relatively warm water..... I wondered if that fetch had've been there in say Feb, with very warm air instead of the cold air that was delivered ( see post at 28/4 - 4.46pm), would the swell have been 1/2 to a foot smaller, therefore falling closer to the models......
My guess is that forecasted swell is closer to the mark than forecasted SURF. They are in a haze on the wind/ground swell borderline. If you graphed swell period vs surf height and kept swell height constant, you get a steep rise in surf size around 12sec wouldn't you?
I don't think it's measurable , to the point that it could be proven .
mitchvg wrote:
My guess is that forecasted swell is closer to the mark than forecasted SURF. They are in a haze on the wind/ground swell borderline. If you graphed swell period vs surf height and kept swell height constant, you get a steep rise in surf size around 12sec wouldn't you?
It was nowhere near close......
southey wrote:
I don't think it's measurable , to the point that it could be proven .
I'm sure there are Uni labs with their little ponds and fans and model ships that could do it..... You just need a sealed environment where humidity, viscosity, and temp' can be adjusted..
Anyone notice cyclone quang off exmouth this morn ? Forecast swell was 5-7 metres @ 10/14 sec approx depending on the site . I tried to get to get a real update from mate who lives there but he didnt venture out into the 130 kmh winds to see the beach . Im pretty sure there would have been big surf somewhere from that one becos winds were fresh east predominantly
caml wrote:
Anyone notice cyclone quang off exmouth this morn ? Forecast swell was 5-7 metres @ 10/14 sec approx depending on the site . I tried to get to get a real update from mate who lives there but he didnt venture out into the 130 kmh winds to see the beach . Im pretty sure there would have been big surf somewhere from that one becos winds were fresh east predominantly
Yep . But the swell still lining up is what im tryna find out . Predicted at 7m yesterday & no news about it ! Hopefully craig can pull out yesterdays forecast archive ?
Yep . But the swell still lining up is what im tryna find out . Predicted at 7m yesterday & no news about it ! Hopefully craig can pull out yesterdays forecast archive ?
ok sheepi I started a new topic ,first time at that . cheers to that, there will be many more hopefully
Air..... You're sitting in it.....