http://forum.weatherzone.com.au/ubbthreads.php?ubb=showflat&Number=494493#Post494493

KenHo said...

Gestalt said...

yeah that's the one, weather moves west to east and sometimes wwhen you get a big cyclone off WA it stops anything coming through and we get no wind for a couple of weeks.

not saying that that will happen though and models saying that it won't

there is currently some activity in the NE coral sea so we might get something in a fortnight. fingers crossed.....

around this time of year i start lurking in weatherzone forums in the coral sea thread. it goes all season and is dedicated to cyclone activity. i have learnt huge amounts about lows/cyclones reading the weatherzone stuff over the past 4 years or so.

they also talk about the different models available and which ones seem to be the most accurate.

interestingly, the european models seem to be the most reliable when it comes to lows. i beleive seabreeze uses said models also. or something very similar.... that's why seabreeze is so good at predicting wind through the SE's season and the Westerlies.

have you got a link to that forum ?

Swoosh,

When I went to Uni, density was a function of mass and volume.

So the real question here is "does a equivalent volume of air" have more or less mass when it is hotter or colder and if it does vary will it be significant - assuming the force we feel on a sail is a function of mass.

A wise man I met said one day the answer to any question is 'it depends'

Some preliminary calc's I've done show that for 'dry air' the difference really is very small. I got a 0.1% increase in force for warm air at higher pressure (1030 Hpa vs 995 HPa). For air moving at a constant velocity.

If we add temp into the equation and add humidity I wonder what we will get.?


- J

2 bobs worth... because I'm bored

A knot is a unit of measurement for speed whereby 1 knot = 1 nautical mile per hour. 1 nautical mile is also a fixed unit whereby 1 nautical mile = 1 minute of longitude on a map (therefor the circumference of the earth at the equator = 360 (deg) x 60 (min) or 21,600nm).

Since these are fixed units they cannot be affected by any other factor.

The old question "which is heavier? 1 tonne of lead or 1000kg of feathers. Of course, they are both the same. You could argue that lead is denser and therefor weighs more per unit mass than a feather but this is not the question.

As is with the question of 15 knots of wind speed and whether it is affected by temp, density, pressure etc. I would think not. 15 knots will always be 15 knots.

No, I think you would duck it up.

Richiefish said...

what if I have a red sail,spray it with teflon,and wear speedos(without a potato) and get a haircut??? will I be able to duck jibe ??

Not without potatoes. You need those.

dr J, yes density by definition is a function of mass per unit volume. but the mass of air in a given volume varies with temperature and pressure. you need to revisit ideal gas laws.

Here comes the calculator...

L=0.5*density*v^2*A*Cl

density from ideal gas laws is = pressure/ (R*T)

so say its 15kts, about 7.7m/s, 20deg, 101.3kPa outside, i'm on a 5.9m sail with an arbitrary lift coefficient of 1. Note because of the arbitrarily chosen lift coefficient, the magnitude of the result may be incorrect, but the relative difference between results is correct.

Lift force is 211.26N

if we maintain the pressure
@ 10deg outside, that number is 218.7N
@ 35deg outside, that number is 200.97N
, so a 25 degree temperature difference results in about 9% difference in lift.

if we keep temp the same,
@ 100kPa, lift = 208.50 N
@ 102.5kPa, lift = 215.71 N
, so the difference here is only about 2-3%.

If we vary both,
@10deg, 102.5Kpa, = 221.26 N
@35deg, 100Kpa = 198.35 N,
, so almost 12% difference, between a hot day under a low pressure system, and a cold day under a low.

Out of interest I calculated what would the affect of sail area be.
@ 10deg 102.5KPa, with a 5.3m sail, lift = 198.76kN
,so on a cold day under a high, a 5.3m generates as much lift as a 5.9 on a hot day under a low for the same wind velocity.

conclusion, temperature variations has a greater affect on available lift then pressure variations. So much so that there may be a sail size difference for the same amount of wind.

throw out the anemometers, you guys need to be packing a mobile weather station, and a calculator :P

Gestalt said...

http://forum.weatherzone.com.au/ubbthreads.php?ubb=showflat&Number=494493#Post494493

KenHo said...

Gestalt said...

yeah that's the one, weather moves west to east and sometimes wwhen you get a big cyclone off WA it stops anything coming through and we get no wind for a couple of weeks.

not saying that that will happen though and models saying that it won't

there is currently some activity in the NE coral sea so we might get something in a fortnight. fingers crossed.....

around this time of year i start lurking in weatherzone forums in the coral sea thread. it goes all season and is dedicated to cyclone activity. i have learnt huge amounts about lows/cyclones reading the weatherzone stuff over the past 4 years or so.

they also talk about the different models available and which ones seem to be the most accurate.

interestingly, the european models seem to be the most reliable when it comes to lows. i beleive seabreeze uses said models also. or something very similar.... that's why seabreeze is so good at predicting wind through the SE's season and the Westerlies.

have you got a link to that forum ?

Thank you for that sir.
Got a feeling we have a bit in common. When I move in a few weeks time, I might make a trip up to Wello and let you show me the ropes and meet a few of you fine folk. Presently I live 45 mins south of Currumbin, near Mt Warning, but we have a contract on our house, and I'm looking a moving to Currumbin or close by, so I'll be able to get more sailing days.
About the Fiji low, I've seen clonics just sit there for days without moving, so that might too.

Nice work Swoosh.

i think you guys need to get out more

I just had an hour fully powered on a 7.5 at Redcliffe. Does that count?

ikw777 said...

I just had an hour fully powered on a 7.5 at Redcliffe. Does that count?

I believe on this forum, that may be a trump card.
I bought a new Pryde 6.5 V8 though, so I get some windsurfing points. Just not enough wind on the coast here today to make it worth the drive. It's looking pretty sad all week actually.

NotWal said...

Gestalt said...

good stuff swoosh, not that i unsderstood it.

my questions.

does the wind gauge take into account density? if so wouldn't 15 knots on a cold day still read as 15 knots on a hot day. sure the real velocity on the cold day may be less but the additional density due to lower temps makes the effect on the gauge higher.

hope that made sense

I would guess that an impeller on a wind gauge would feel wind the same way a sail feels it. They are both foils. So the air speed on an impeller type gauge will be wrong but a very good indicator of available lift which is really just what we want to know

If I'm reading you right, you're saying a 15knot cold wind would read higher than a 15knot warm wind on the anenometer. Pardon my limited knowledge, but isn't this contrary to what Swoosh scientifically calculated above though i.e. the wind would read 15knots for both but the colder wind would pack more "punch" thereby requiring a smaller sail to get the equivalent lift force of a (slightly)larger sail in warm wind.

yes, that's my question sausage,

as swoosh has proven there is more lift associated with the lower pressure.

more lift and or lower pressure will also mean more velocity in his equation. so the wind gauge will read higher.

i think the limiter is the area. the gauge only has a small area so will not translate the percentage difference as much as a sail will.

Gestalt said...

Hi KenHo,

no worries!

next time you sail brissy just say high. those that know me know i am always up for a chat, before a sail, after a sail, during a sail.

p.s. yes the wind feels less powerfull to me on hot days.

So, do you always wear the green helmet, or is there another way I can recognise you ?

Gestalt said...



no helmet,

i use KA Kults usually orange but i also have a grey one.

what sails r u on?

Bit of a mix. Was mostly North, cos I lived in Bundaberg, and Russell used to get them at good prices. I scored a Pryde Alpha 6.2 recently, and that's pretty much all I use now. I'm very impressed with both teh low end power and how well I can hang on when it gets up.
I did grab a Pryde V8 on the weekend, to go with the JP slalom I got recently, and I'm looking forward to a session at teh Train or up at Wello with that.
I 've sailed very little in the last 5 yeras, but I have a van now, and I'm getting back into it, so I'm updating everything a bit at a time.
I got my old Delta wave board out last week, and truly laughed out loud when I put it next to my JP Freestyle wave. Man, what a difference !! So now I'm looking at a JP Wave, or a quad wave. New gear is so much fun !!