hlb
RIP
A F 4/5 or six is great and fun if it's sunny. Crap if it's raining and cold. Why do the waves go bigger when the clouds come down. Then dispear when the sun comes out. /forums/images/graemlins/confused.gif
Beaufort Scale definition.
- Thread starter AndrewB
- Start date
jimi
Well-Known Member
I think its cos the clouds are heavy and lie on top of the air thus increasing the pressure.
Seriously the clouds often have stronfhger wind underneath them. I recall one channel crossing at night when generally it was clear but when a cloud came over the wind would pipe up from about 28 to 34 knots .. quite significant from fast and comfortable to being a bit uncomfortable. Apparently the wind force is logarithmic rather than arithnmetical to wind speed?
Seriously the clouds often have stronfhger wind underneath them. I recall one channel crossing at night when generally it was clear but when a cloud came over the wind would pipe up from about 28 to 34 knots .. quite significant from fast and comfortable to being a bit uncomfortable. Apparently the wind force is logarithmic rather than arithnmetical to wind speed?
Sgeir
Well-Known Member
Thank you Haydn, I've also wondered about that, but was too shy to ask.
Sui Generis
Well-Known Member
Its a bit late but if memory serves it's a square relationship.
However, what I think is significant is the moisture content of the air that is travelling at you at a say F4. The higher the moisture content - the heavier it feels (OK density). Hence the relatively harsher the winds in the southern ocean, and hence the fact that it feels stronger when it's raining than on a sunny day.
That's my theory and I'm sticking to it. . .
However, what I think is significant is the moisture content of the air that is travelling at you at a say F4. The higher the moisture content - the heavier it feels (OK density). Hence the relatively harsher the winds in the southern ocean, and hence the fact that it feels stronger when it's raining than on a sunny day.
That's my theory and I'm sticking to it. . .
Twister_Ken
Well-Known Member
Except moist air is lighter. That's why clouds rise.
Tisme
Well-Known Member
Is it?
Doesn't air rise because it is warm and therefore less dense? (Like a hot air balloon). It also happens to be damp because it is warm.
If two lumps of air are the same temperature, but one is moist the other dry surely the moist one will be heavier.
A bit like 2 sponges; same size but the wet one is heavier.
I think? /forums/images/graemlins/crazy.gif Well maybe /forums/images/graemlins/crazy.gif Dunno really /forums/images/graemlins/crazy.gif
I suppose it boils down to what the density of steam is.
Doesn't air rise because it is warm and therefore less dense? (Like a hot air balloon). It also happens to be damp because it is warm.
If two lumps of air are the same temperature, but one is moist the other dry surely the moist one will be heavier.
A bit like 2 sponges; same size but the wet one is heavier.
I think? /forums/images/graemlins/crazy.gif Well maybe /forums/images/graemlins/crazy.gif Dunno really /forums/images/graemlins/crazy.gif
I suppose it boils down to what the density of steam is.
tcm
...
Tempwise use kelvin scale. So 0 v 30 degree centrigade is actually only 273 v 300 degree, 10 at same pressure.
Much more important i reckon is that if higher winds causes spume to hit boat. So 70 knots could bust things big time at sea that wouldn't onshore, praps.
Much more important i reckon is that if higher winds causes spume to hit boat. So 70 knots could bust things big time at sea that wouldn't onshore, praps.
Ships_Cat
Well-Known Member
Went to the trouble of working it out on the forum once but not about to do it again. Essentially both temperature and pressure differences as we experience them make insignificant difference to the force of the wind.
People sometimes get confused by the expression "the weight of the wind" but that is an unrelated matter.
John
People sometimes get confused by the expression "the weight of the wind" but that is an unrelated matter.
John
alant
Well-Known Member
"Except moist air is lighter. That's why clouds rise."
Do they? & most of my life when living in God's Country (Wales of course) I was told that the regular rain showers were caused by Tropical Maritime clouds being unable to rise above the Brecon Beacons. These mountains causing the bottom of the clouds to rip & deluge to result.
Do they? & most of my life when living in God's Country (Wales of course) I was told that the regular rain showers were caused by Tropical Maritime clouds being unable to rise above the Brecon Beacons. These mountains causing the bottom of the clouds to rip & deluge to result.
guernseyman
Well-Known Member
Are the waves bigger, or do they just appear bigger in the different light?
gandy
Well-Known Member
Moist air is lighter, because water vapour is lighter than air. That's not exactly why clouds rise, though. Many types of clouds form as air rises enough for its temperature to drop low enough for condensation. The air may rise due to convection, or due to other factors such terrain.
On the original point, the Beaufort scale was intended to allow people to compare the effects of different wind strengths. So there would be no inconsistency in a given Force not always equating to the same number of knots of wind speed, so long as it gives the same effect.
Someone who's read the book can confirm. Wasn't the scale originally based on the amount of sail you could carry, and at that time the ships wouldn't have wind-speed instruments.
On the original point, the Beaufort scale was intended to allow people to compare the effects of different wind strengths. So there would be no inconsistency in a given Force not always equating to the same number of knots of wind speed, so long as it gives the same effect.
Someone who's read the book can confirm. Wasn't the scale originally based on the amount of sail you could carry, and at that time the ships wouldn't have wind-speed instruments.
oldharry
Well-Known Member
[ QUOTE ]
Are the waves bigger, or do they just appear bigger in the different light?
[/ QUOTE ]
Appearances are all: A fresh breeze on a sparkling day raises a 'pleasant chop'. The same breeze in worsening condiitons, with rain sweeping in is infinitely menacing with a promise of worse to come... and ALWAY on the nose!
But Admiral Beaufort set up his scale to give the Sailing Navy a scale for assessing wind strength, because they had no means of actually measuring Knots or metres per second. Once accurate measuring instruments were available, the Beaufort scale was pinned to specific wind velocities - probably in a way the good Admiral did not intend. But if you develop a new means of measuring something accurately, then you have to graduate it to a standard unit, and you can not make allowance for variables that the device cannot measure.
An anemometer only measures the velocity of the wind, without taking in to account other variables such as temperature, and humidity, which go up to make the 'weight' of the wind. So far these variables are not readily measurable to make up a composite wind speed. It is arguable anyway whether there is a measurable difference: as Guernseyman points out, even the light makes a difference to the perceived size of the waves!
So come on you Boffins - design for us a 'subjective anemometer' which can diffferentiate between the Beaufort 5 which gives a decent sail one day, and a hard wet slog the next! /forums/images/graemlins/crazy.gif
Are the waves bigger, or do they just appear bigger in the different light?
[/ QUOTE ]
Appearances are all: A fresh breeze on a sparkling day raises a 'pleasant chop'. The same breeze in worsening condiitons, with rain sweeping in is infinitely menacing with a promise of worse to come... and ALWAY on the nose!
But Admiral Beaufort set up his scale to give the Sailing Navy a scale for assessing wind strength, because they had no means of actually measuring Knots or metres per second. Once accurate measuring instruments were available, the Beaufort scale was pinned to specific wind velocities - probably in a way the good Admiral did not intend. But if you develop a new means of measuring something accurately, then you have to graduate it to a standard unit, and you can not make allowance for variables that the device cannot measure.
An anemometer only measures the velocity of the wind, without taking in to account other variables such as temperature, and humidity, which go up to make the 'weight' of the wind. So far these variables are not readily measurable to make up a composite wind speed. It is arguable anyway whether there is a measurable difference: as Guernseyman points out, even the light makes a difference to the perceived size of the waves!
So come on you Boffins - design for us a 'subjective anemometer' which can diffferentiate between the Beaufort 5 which gives a decent sail one day, and a hard wet slog the next! /forums/images/graemlins/crazy.gif
Twister_Ken
Well-Known Member
The amount of water vapour in the air effects the density. Water vapour is a relatively light gas when compared to diatomic Oxygen and diatomic Nitrogen - the dominant components in air.
When water vapour content increases, the amount of Oxygen and Nitrogen decreases per unit volume and the density will decrease because the mass is decreasing. Dry air is more dense that humid air!
(from engineeringtoolbox.com)
When water vapour content increases, the amount of Oxygen and Nitrogen decreases per unit volume and the density will decrease because the mass is decreasing. Dry air is more dense that humid air!
(from engineeringtoolbox.com)
capt_courageous
Well-Known Member
Harry
As you say Beaufort was trying find some sort of standard way of describing the effect of wind using the equipment he had at the time. Andrew - The 'force' a wind puts on an object is defined by:
1/2 * air density * velocity squared * the area of the object * its drag coefficient.
We can't control some of these but one control is obvious - put a reef in.
As you say Beaufort was trying find some sort of standard way of describing the effect of wind using the equipment he had at the time. Andrew - The 'force' a wind puts on an object is defined by:
1/2 * air density * velocity squared * the area of the object * its drag coefficient.
We can't control some of these but one control is obvious - put a reef in.
kengill
Well-Known Member
[ QUOTE ]
whether its hot. cold or indifferent makes no difference
[/ QUOTE ]
Erm Spheroidal objects!
With a variation of up to 10% from the standard 1000 millibars and the differences between hot dry winds and cold wet ones IMHO anyone who has been out in those extremes will have noted that the forces on the boat are much greater when its cold, wet and high pressure than when its hot dry and low pressure. This has to do with the greater mass of the incident wind.
whether its hot. cold or indifferent makes no difference
[/ QUOTE ]
Erm Spheroidal objects!
With a variation of up to 10% from the standard 1000 millibars and the differences between hot dry winds and cold wet ones IMHO anyone who has been out in those extremes will have noted that the forces on the boat are much greater when its cold, wet and high pressure than when its hot dry and low pressure. This has to do with the greater mass of the incident wind.
DJE
Well-Known Member
We've been here before. See this thread.
Ships_Cat
Well-Known Member
And looking at your posts in that thread DJE, best I recall I got something similar along the lines of your half knot increase in wind speed from 15 knots for a 20C change in temp too.
I have subsequently found that the real /forums/images/graemlins/smile.gif experts on the internet are in general agreement on that too.
John
I have subsequently found that the real /forums/images/graemlins/smile.gif experts on the internet are in general agreement on that too.
John
Pye_End
Well-Known Member
I suppose the interesting question is whether wind measuring equipment really measure wind speed or force?
Cup ones probably wind speed - but are they really completely unaffected by air density?
Vetus type - with no moving parts - force?
The old really simple schoolboy made lever with a guage - has to be force, as it is comparing gravity and wind force as a ratio, and g is reasonably consistent.
Cup ones probably wind speed - but are they really completely unaffected by air density?
Vetus type - with no moving parts - force?
The old really simple schoolboy made lever with a guage - has to be force, as it is comparing gravity and wind force as a ratio, and g is reasonably consistent.
LeonF
Well-Known Member
Isn't it because the moist air does rise.....like steam from a kettle...it then begins to clump..perhaps because of the ceiling of cold air it eventually meets, until it's sheer weight begins to drag it down, or it just cannot contain itself and bursts ?????
LeonF
Well-Known Member
Still lost Jimi.....if the high pressure causes it to change speed then it has become a Force 5, and is no longer a Force 4?? It cannot be a force 4 with a force 5 windspeed which is my understanding of what you are saying ?? Or is it that a force 4 can have the same effect as a force 5 depending on the fetch etc etc....but the wind speed stays the same....