"Heavy wind"

erbster

erbster

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I was talking to someone about sailing the other day (what else is there to talk about?) and he commented that the winds were heavier in his sailing area (N Wales) because the air carried a lot of humidity and so a wind of a certain strength had more force than dry air moving at the same speed. Thus sailing in such an area is harder work.

This only sounded partially plausible to me. Anyone know if this is true?
 
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IIRC from my flying days, humid air is lighter than dry air (that's whY clouds float in the sky!) and therefore humid wind should exert less pressure per square metre than dry air. Air temperature is also an important factor controlling how much water is in the air: warmer air can contain more water and is less dense.

I am sure someone with a better understanding will explain better

p
 
Puggy said:
IIRC from my flying days, humid air is lighter than dry air (that's whY clouds float in the sky!) and therefore humid wind should exert less pressure per square metre than dry air. Air temperature is also an important factor controlling how much water is in the air: warmer air can contain more water and is less dense.

I am sure someone with a better understanding will explain better

p
Click to expand...

Clouds "float in the sky", because the air is warmer.
The higher humidity, being a consequence of this warmer air, not a cause of them floating.
 
Yes, the water in clouds has condensed into droplets, so clouds are no longer behaving as an ideal gas. Water vapour as a gas has a lower molecular weight than oxygen, nitrogen etc, so humid air is less dense than dry air at the same temperature. It's all in GCSE Physics, or maybe Chemistry!
 
erbster said:
I was talking to someone about sailing the other day (what else is there to talk about?) and he commented that the winds were heavier in his sailing area (N Wales) because the air carried a lot of humidity and so a wind of a certain strength had more force than dry air moving at the same speed. Thus sailing in such an area is harder work.

This only sounded partially plausible to me. Anyone know if this is true?
Click to expand...

The conclusion here: http://www.engineeringtoolbox.com/density-air-d_680.html is that 'Dry air is more dense than moist air'.

Edit: this seems like quite an understandable explanation: http://wiki.answers.com/Q/What_is_the_density_of_saturated_air
 
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The sail-change tables carried and used on many round-the-world racers - which indicate at which wind speeds one should change to a heavier-cloth/smaller headsail - were calibrated for Temperate and for Very Cold ( i.e. the Southern Ocean ), 'cos the VC breezes exerted rather more load onto the sail than the more temperate ones..... and could 'blow out' the sails rather more readily.

As I seem to recall, the square-rigged grain and tea racers did much the same, with a heavy-duty suit of sails for the heavy/cold winds of the deep south.
 
erbster said:
So, does that suggest that COLD air exerts more force on be sails than warm wind of the same strength?

I.e. As I suspected, the person I was talking to was wrong....?
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Wasn't your mate talking about humidity though, not temperature ?
More water vapour in the air at a given temperature means lower density. So yes, he's still wrong.
But I wonder what happens at the dew point. Does the presence (the momentum really) of the water droplets add to the force exerted? Or would the extra weight cause the velocity to drop? Or what.....?
 
charles_reed said:
Definitely cold air, at an equivalent velocity, exerts much more force than warm air. Anyone who has experienced an Arctic flow will bear that out.
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Don't think you even need an arctic flow. I've always felt that the wind in early and late season seemed to have more effect for any given windspeed. Never really bothered to look it up but nice to know I wasn't imagining it.
 
Pye_End said:
What about fog?
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Advection or radiation?

Whichever, it is a result of warm moist air getting colder & the moisture condensing out.

Radiation fog usually burns off & is normally due to moist air & cold nights/clear skies (foggy bottoms), whilst advection fog is due to warm moist tropical maritime air meeting a cold surface such as the cold sea off UK in early spring (fog in channel, continent cut off) & can occur even in windy conditions.
 
Puggy said:
IIRC from my flying days, humid air is lighter than dry air (that's whY clouds float in the sky!) and therefore humid wind should exert less pressure per square metre than dry air.
Click to expand...

<pedant mode on>

Clouds don't float in the sky because clouds don't exist. That is, they are not cohesive units which "do" anything. What we think of as a cloud is simply a region of the atmosphere where the temperature is lower than the dew point -or, to put it another way, where the humidity is greater than 100%.

What happens for a cumulus cloud is that air near the ground is heated by the sun and rises. As it rises, it gets cooler. Eventually it gets to the dew point of the air that day and water vapour starts condensing out of it. The point at which this happens is constant for a given day and place, which is why cloudbase is locally the same and clouds formed that way have flat bottoms.

Above cloudbase, the condensing water vapour releases heat, which warms the air, which makes it rise faster. All glider pilots know that couds suck upwards.

Is damp air denser than dry air? No, it's very slightly lighter. Air is (approximately) 79% N2 and 21% O2. At 20C, say, the saturation pressure of water vapour is 0.024 bar, which is therefore also the partial pressure of water vapour in fully saturated air. So that make the composition 77% N2, 20.5% O2, 2.5%H20.

For one mol that gives a total mass or normal air of 0.79 * 28 + 0.21 * 32 = 28.84g. 1 mol occupies 22.4l at stp, giving air density of about (20C is not STP) 1.29 kg/m^3.

One mol of saturated air would have a mass of 0.77 * 28 + 0.205 * 32 + 0.025 * 18 = 28.57g, and a density of about 1.27kg/m^3, which is 1% lighter than dry air.

<pedant mode off>

I suspect that the idea of heavy air comes from the days of canvas sails (and possibly canvas wing coverings) when damp air would allow the fibres to swell and make the cloth less porous.
 
Cold air / wind is definitely more dense; in the old days of trading sailing boats the very experienced crews went by feel not an LCD readout.

Coastal trading boats used to send down their topsails each autumn and sail through the winter on mainsails and jibs.
 
Although water vapour is lighter than air, cold air contains generally less water vapour, as the vapour pressure of water increases with temperature.
100% RH air at 10degC contains less water than 50% RH air at 20degC.

The changes in density due to pressure and temperature are pretty small compared to the effects we feel as sailors.
I think what we feel on the sails is generally a lot more to do with how turbulent the air is.
 
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