A truly fascinating question!

As  keen Motorcyclist for many years I am equally interested in all aspects of; "Weather" and its various effects!

Looking forward to some comments on the subject!

Paul... ...!

Quote:

I've lived near the coast in West Sussex all my life. I know nothing about sailing but used to do a lot of RC slope soaring which is dependent on wind speed & direction. No wind means you can't fly so we became very keen observers of wind conditions. The only effect on aircraft related to wind strength that I can think of is due to thermal activity. In some weather conditions it's possible to have a strong wind with no lift. What we called "dead air". A sudden drop in wind strength together with an increase in air temperature indicates a thermal & this is when you get the maximum lift. This doesn't last very long so we used to take the opportunity to gain as much altitude as possible. It might also explain the "gutless wind" that you describe.

Our beach is popular with wind & kitesurfers who I enjoy watching. Of course, they probably stay closer to the shore than you chaps & just like our slope soarers they're dependent on wind direction. A slight change of direction & they might have to pack it in for the day, even when a stiff breeze is blowing. Sometimes they move a little further along the coast to where the wind is working.

You don't say where your club is based but perhaps this is a local phenomenon caused by the terrain or when the wind is in a certain direction; for example, onshore or offshore. When you're out on the water what is the difference in altitude between your sails & the anemometer on the clubhouse?

Just spent a couple of hours googling. I found some answers.

It seems that a wind at sea level at a given speed (say 21 kts) can have a variation by as much as 10% density/weight.

The odd thing (for me anyway) was that increased humidity decreases the density of air Water vapour is less dense than air by quite a margin apparently.......well I'll be

G.K. wrote on Jan 23^rd, 2012 at 12:57pm:

Not quite true... although water vapor is lighter than the gases found in air, the amount of the gas, per unit of volume of atmosphere, is reduced when there's a lot of water vapor, and that's what matters to airplanes. Take an empty 1-gallon bucket (full of air, right?) and pour some water into it. There's now less air in that gallon of volume.
  And the hotter the air is, the more vapor it can hold. Hotter air, due to the reluctance of the agitated gas particles to stay close together, is less dense as well, so you can see how hot, humid weather can affect density altitude.
The problem for aircraft when it comes to high humidity is not so much with aerodynamics as it is with combustion. Non-turbocharged or supercharged piston engines in particular will have problems, because the engine needs a certain amount of oxygen per second to create the desired horsepower. It's getting the same volume of "air" all the time (for a given throttle setting), but there has to be a certain ratio of gas (specifically oxygen). Water is not useful at all to such an engine, regardless of its density. The fuel will only burn in the presence of oxygen. If the ratio of water vapor to gas starts to skew towards the water side, the problem is obvious. Leaning the fuel/air mixture only helps to a point, as many piston pilots have found when they tried to take off, leaned for as much power as they can, under density altitude conditions that had them starting out at an altitude equivalent to, or exceeding, the service ceiling of the airplane. 

G.K. wrote on Jan 24^th, 2012 at 10:52am:


From my experience of water vapour in the combustion engines of my Motor-Bikes, I observe that they run smoother, and generate more power, due to the added effect of generating steam in the combustion chambers, thereby increasing the compression ratio.
My normally aspirated (Carburettor) piston engines, love rain and damp weather conditions... ...! 

Paul....My Honda CB500RR loves the rain...(I don't!)... ...!

Many years ago, devices were marketed to inject small amounts of water into the carburettors of piston engines to increase power.
..the added possibility of introducing rust, and polluting the lubricating oil, was a problem...

http://en.wikipedia.org/wiki/Water_injection_%28engines%29

Fozzer wrote on Jan 24^th, 2012 at 12:15pm:


Water injection is another matter... definitely works, though. Many WWII fighters relied on such systems for brief spurts of extra power. The idea was to help keep the cylinder temps down to prevent detonation, allowing very high manifold pressures, but it's water droplets, not vapor (initially). It's been used with turbines, too, to increase pressure in the cylinders without adding more fuel to the mix. It does become, effectively, steam after combustion, so maybe there's something to your claim of added power on rainy days (although the generally cooler temps probably have more to do with it). And the water in such a system is a lot cooler than the water vapor found lurking in air on a hot, muggy day (which is when air-cooled engines can suffer miserably due to thinner air with too much water mixed in).
I never meant to suggest that it's the water alone that is the problem, just a factor... obviously, even on bone-dry days, as the temperature outside goes up, the air gets thinner, and the engine "thinks" it's at a higher altitude.

  On a side note (as if the thread creep isn't bad enough already):  a "full head" of steam in a gasoline engine can be disastrous! I once helped a friend tow and then start his car, which had stalled in flood conditions. We waited until the water had receded, but we did not know what we were doing...  He must have cranked it right after it died, when the tailpipe was still underwater, and sucked water into the cylinders, and possibly into the crankcase. So when we started it, it ran fine for a moment, then every single bolt holding the oil pan in place sheared off instantly, the pan smashed into the ground, and a red-hot froth of oil and water spewed everywhere. Oops. 
Good thing the oil pan bolts were weaker than the bolts holding the head on, or the hood would have ended up in the next county.