Wind Resistance
Confusedboy
Posts: 287
Wind has been set in from the East/Northeast/Southeast for ages now and while it is gradually decreasing to a mere gale force, it has been a nightmare to ride into. Much of my riding through and around Cardiff is done on the old A48 main road, though it has been called something else for many years now. This road has its origins in the Roman road from Glevum to Nidivum (Gloucester to Neath) and is aligned Northeast/Southwest in my half of town, more East/West in the other half. A ride crosstown against the wind on Saturday was a bloody nightmare; an endless struggle spinning in the grannies in which I doubt if I achieved more than 12 mph. When I got home and caught sight of my face in the mirror, it was like a preview of what I will look like in 20 years time when I am properly old; the sunken, rheumy, eyes; deep etched crow's feet and furrowed brow, and veins standing out like purple river basin maps. I ride bikes cos I like to, but there was no fun in this!
Anyway, I have this theory about headwinds which I am inviting comments on. It goes that the way the wind actually feels subjectively to a cyclist is influenced by atmospheric pressure. The weather people measure this pressure in millibars (don't they run the Labour Party), with an average at sea level of 1000 mb. Now, (pay attention at the back!) this varies by about 50 mb either side of that, so the usual range is from 950 to 1050mb, a factor of about 10%. This must mean that if the pressure is at the top of that range, the air itself is 10% thicker than if it is at the bottom. Normally, in the British Isles, low pressue is associated with strong Southwesterly or Westerly winds which are accompanied by rain and unsettled weather generally, whereas high pressure is usually accompanied by settled weather, fine and sunny in summer and cold and misty in winter (what they call on the East coast a 'har'), and Easterly winds which are usually fairly light. The ones last week were pretty strong though, and the combination of strong winds with thick, heavy, high pressure air and the hyperbolic parabolical nature of trying to increase or even maintain speed accounted for my misery on Saturday. In short, it is less unpleasant to attempt to make headway into a low pressure Westerly gale than a high pressure Easterly one, even when the wind speeds are the same, because high pressure air offers more resistance.
This must affect racing cyclists, not to mention runners, as well as having an effect on the fuel consumption of vehicles, aircraft, and trains operating East-West routes, but one never hears of it. I cannot believe that I have actually formed a whole theory based on this, but the evidence seems to suggest that. Please, someone, disprove this nonsense for me or I'll start to think I've actually done something clever, which would be very unlikely given my record so far....
Anyway, I have this theory about headwinds which I am inviting comments on. It goes that the way the wind actually feels subjectively to a cyclist is influenced by atmospheric pressure. The weather people measure this pressure in millibars (don't they run the Labour Party), with an average at sea level of 1000 mb. Now, (pay attention at the back!) this varies by about 50 mb either side of that, so the usual range is from 950 to 1050mb, a factor of about 10%. This must mean that if the pressure is at the top of that range, the air itself is 10% thicker than if it is at the bottom. Normally, in the British Isles, low pressue is associated with strong Southwesterly or Westerly winds which are accompanied by rain and unsettled weather generally, whereas high pressure is usually accompanied by settled weather, fine and sunny in summer and cold and misty in winter (what they call on the East coast a 'har'), and Easterly winds which are usually fairly light. The ones last week were pretty strong though, and the combination of strong winds with thick, heavy, high pressure air and the hyperbolic parabolical nature of trying to increase or even maintain speed accounted for my misery on Saturday. In short, it is less unpleasant to attempt to make headway into a low pressure Westerly gale than a high pressure Easterly one, even when the wind speeds are the same, because high pressure air offers more resistance.
This must affect racing cyclists, not to mention runners, as well as having an effect on the fuel consumption of vehicles, aircraft, and trains operating East-West routes, but one never hears of it. I cannot believe that I have actually formed a whole theory based on this, but the evidence seems to suggest that. Please, someone, disprove this nonsense for me or I'll start to think I've actually done something clever, which would be very unlikely given my record so far....
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Comments
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Yes, but actually high humidity, which make air feel 'thick', usually accompanies very low pressure and often ends in a thunderstorm.0
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Confusedboy wrote:Yes, but actually high humidity, which make air feel 'thick', usually accompanies very low pressure and often ends in a thunderstorm.
I think the humidity is more a question of where the depression is. Lows tend to move in a north-easterly direction and often pass north-west of the UK. The effect of this is that warmish moist Atlantic air is brought up from the south-west, which tends to ''condense'' over us. This makes much of Wales look very green - if you can see it.
But for a while now there's been a high planted somewhere to our north and this is dragging colder and drier air ''anticlockwise'' from the north. It hasn't had a decent rain in ages and along with cloudless skies we've had a very fresh easterly wind.
And from what I understand about wind speed, the crucial factor is not the pressure itself, it's how close together the isobars are - that is, a big difference in air pressure to either side of you will mean you're either going to be cycling out into a nasty headwind or cycling back into a nasty headwind. Best to head off into the wind and when you're knackered let the wind carry you home.0 -
Yes, the isobars are like contour lines showing how steep the slope is that the wind is falling down if you want to use that analogy; the closer they are together the higher the speed of the wind, But I still maintain that, while speed is the most crucial factor, denser high pressure air feels more resistant to me trying to cycle through it than thinner, low pressure air going the same speed and direction, and that this factor is less crucial but definitely noticeable.0
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The wind is approximately geostrophic (parallel to the isobars) assuming the pressure gradient approximately matches the coriolis force. I'd be inclined to agree with the previous posts that allude to the fact there's lots to think about including partial pressure of water vapour. Not to mention how diffilcult it may be to cycle in very humid or very dry conditions, which would influence how difficult you find things.
I'm no engineer, but a quick google suggests that force per unit area is proportional to the square of wind speed, assuming constant air pressure. So a change in wind speed will be more significant.
I might be wrong but would'nt you only be able to determine the effect of air density in near zero relative velocity of the air/wind, so to take velocity out of the equation. So there may be an effect of going from 950hPa to 1050hPa, but the minute you go from 0mph to 10mph and higher, your equation is dwarfed by windspeed^2.
I am sure there's other factors too e.g. pressure drop with altitude (if pressure really is significant), but i like the theory - very elegant.
edit: It would be interesting to know what transatlantic aircraft do when flying east->west i.e. would they fly above or below the jet stream?0 -
jawooga wrote:edit: It would be interesting to know what transatlantic aircraft do when flying east->west i.e. would they fly above or below the jet stream?
When we flew to LA a few years ago we went over Iceland, so maybe they just 'sidestep' the jetstream. On the way back I think we took a straighter route.
EDIT: Or was it Greenland....0 -
The shortest route from the UK to LA is over Iceland, and Greenland. That looks wrong if you draw a straight line between the 2 on a map, but the world is round and not flat like a map. If you have access to a globe of the world, lay a piece of string across it between Londan and LA and you will see that the shortest route, the nearest to a straight line, would look on a flat map like a arc going thousands of miles out of the way towards the North Pole. This is what is known as a 'Great Circle' route. The 'Titanic' was taking one between Queenstown and New York when it hit the iceberg.
Stephen Fry tells a yarn in which a bloke goes into a shop in Manchester to buy a globe, but is unable to find what he wants. As the shop has a large and high quality selection of globes, the assistant asks if he can help. 'These are all globes of the world' says the bloke, 'I am looking for a globe of Lancashire!'
The height that transatlantic aircraft fly at varies according to weather, air traffic control and all sorts of things, but as a generalisation, westbound flights are at around 38k feet, just below the jetstream, and eastbound ones at 42k inside the jetstream to take advantage of the tailwind opportunity to make speed and save fuel, as well as to have 4k feet vertical distance separating them from the westbound traffic.
Thank you for your explanation Jawooga, and your kind comments. It looks like I have not quite discoverd an amazing new scientific breakthrough, which is a bit of a relief, really.0 -
temperature affects air density too
http://en.wikipedia.org/wiki/Density_of_air
The warmer it gets, the less dense the air is so there will be greater air resistance on colder days!0 -
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The increase in air resistance on cold days is, to a greater or lesser degree, compensated for by an increase in engine power. Cold air, as you say, is denser, so more oxygen gets into the combustion chamber which can burn more fuel. This also works for lungs - colder, denser air diffuses more oxygen into the blood. In humans, cool temperatures also help cool the body - marathon runners are faster when it's cool.
Cooling the air is what an intercooler does in a turbocharged engine, it increases power and efficiency.0 -
I'm sure that's true to a point (definitely true for an engine, no idea for the human body)
However below a certain temperature, I'd think any efficiency from the uptake of oxygen through denser air would be cancelled out by the fact the body is working (shivering) to keep itself warm over and above the heat generated through activity.
Also, I remember being in Canada several years ago in mid winter. It was -30°C and the air was so cold it hurt to breathe. Again, not good for efficiency if you can't get a good lung full of air, no matter how dense it is!0 -
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Yeah, I meant "cold" in the context of the rather temperate British weather
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