Effect of Air Temperature on Wind and Reefing?

[Wotname]

What is the effect of temperature on wind strength?

Put another way, does 25 kts of wind at say 25 C (77 F) heel the boat more than 25 kts of wind at say 5 C (41 F) (for a given sail area)? Is there a noticeable difference?

Clearly the lower temperature increases the density of the air and for a given sail area, the mass of the colder air hitting the sail must be higher than that of warmer air, thus the momentum of the colder air must be greater than the warmer air for a given windspeed.

But is the difference noticeable?

[UNCIVILIZED]

Most who've put in a lot of miles in places where it's predominantly colder seem to think so. And if you want the numbers as to how much difference it might make, plug the numbers into any program (software) which calculates Density Altitude.

Given the numbers you posted, the ballpark difference is about 7.6% in terms of air density. Surprisingly. Which would make a difference I'd imagine.

[El Pinguino]

Reeds https://books.google.com.au/books?id...effect&f=false say the difference in force can be 10% which is considerable but while I think it does occur much of it may be in the eye of the observer due to everything being cold wet and horrible....

Cold dense air certainly makes a difference to aircraft as you know, thats why there are so many abandoned piston airliners at La Paz, Bolivia, airport... one assumes they got in but couldn't get out in the thin warm air at that altitude.

[Wotname]

Originally Posted by El Pinguino

Reeds https://books.google.com.au/books?id...effect&f=false say the difference in force can be 10% which is considerable but while I think it does occur much of it may be in the eye of the observer due to everything being cold wet and horrible....

Cold dense air certainly makes a difference to aircraft as you know, thats why there are so many abandoned piston airliners at La Paz, Bolivia, airport... one assumes they got in but couldn't get out in the thin warm air at that altitude.

Well, landing is mandatory, take off is optional

Interesting to note your link mentions cold air gusts and presumably meaning that gusts are more likely and/or stronger in cold air...not ever considered that before

[UNCIVILIZED]

In some of the higher places on earth, such as cities way up in the mountains, it's not uncommon to have to push or tow start vehicles with others that are already running. As the engines simply have trouble getting enough air for proper combustion, especially when they're cold. So if you're building/buying an expedition vehicle, make it a stick shift.

As to the air density numbers, by playing with some of the values, you can have a 20% difference in the air's density between the tropics & places where it's below freezing. So the concept's certainly genuine. See for yourself Density Altitude Calculator

[Seabeau]

Many years ago on a cool spring day, we on a short hop outside from St. Augustine Florida north to Savannah GA.. High pressure had brought us fine sailing weather, wind from the west at 25 to 35 knots and mild temps. in the low 60's. We were about 5 miles off the beach in 30 feet of water when all of a sudden we were struck suddenly, by what I can only assume was a microburst of cold, dense air from the upper atmosphere. Apparently we were in the exact center of the column of cold descending air because the dense air stopped my boat in its tracks and proceeded to radiate in an approx. 360 degree field, where it totally overcame the previously existing west wind and even flattened the short chop we were sailing in for about a half a mile in all direction. The air temperature dropped about 30 degrees and absolute silence reigned. We were astounded. So totally bizarre! After a minute or so, the microburst ceased, the west wind filled back in and we proceeded on to Savannah. What assume power on display!

[El Pinguino]

I don't think its so much a case of comparing trade wind sailing with 'high latitude'.... plenty of warm(ish) N'lies in the far south frinstance.

I think that were people would notice it would be in flat water like Hobart.... racing this weekend in Northerlies compared with racing last weekend in SWlies frinstance.

[rgleason]

See this discussion please.
http://www.cruisersforum.com/forums/...d.php?t=170724

[Wotname]

Originally Posted by rgleason

See this discussion please.
Polars and Density of Air - Cruisers & Sailing Forums

Thanks, I had missed that discussion.

[Steadman Uhlich]

Originally Posted by Seabeau

Many years ago on a cool spring day, we on a short hop outside from St. Augustine Florida north to Savannah GA.. High pressure had brought us fine sailing weather, wind from the west at 25 to 35 knots and mild temps. in the low 60's. We were about 5 miles off the beach in 30 feet of water when all of a sudden we were struck suddenly, by what I can only assume was a microburst of cold, dense air from the upper atmosphere. Apparently we were in the exact center of the column of cold descending air because the dense air stopped my boat in its tracks and proceeded to radiate in an approx. 360 degree field, where it totally overcame the previously existing west wind and even flattened the short chop we were sailing in for about a half a mile in all direction. The air temperature dropped about 30 degrees and absolute silence reigned. We were astounded. So totally bizarre! After a minute or so, the microburst ceased, the west wind filled back in and we proceeded on to Savannah. What assume power on display!

COOL! Literally.

Thanks for sharing that chilling anecdote.

Natural phenomena like that can be amazing.

[S/V Alchemy]

Originally Posted by El Pinguino

Reeds https://books.google.com.au/books?id...effect&f=false say the difference in force can be 10% which is considerable but while I think it does occur much of it may be in the eye of the observer due to everything being cold wet and horrible....

Cold dense air certainly makes a difference to aircraft as you know, thats why there are so many abandoned piston airliners at La Paz, Bolivia, airport... one assumes they got in but couldn't get out in the thin warm air at that altitude.

Not just that. I was told that if I wanted to load up a rented Cessna 172 with three passengers plus pilot, it would go better on a cooler day at Toronto, because the lift characteristics change significantly on a warm day. We are at 75 metres/240 feet above sea level, not exactly a mountaintop.

[a64pilot]

Then whatever it's worth humidity lowers air density even further.
However I believe our anemometers compensate for air density as of course thinner air has to blow faster to spin the cups at the same speed.
Pilots think indicated as opposed to true airspeed. So maybe cold dense air at 17 kts will display as 20, but hot, less dense air will take 22 kts to display as 20?

I wonder though if the ultrasonic wind indicators indicate true wind speed or if they are affected the same by air density as a regular anemometer is?
Mine knows temp, humidity and barometric pressure, why would it want or need that if not to correct for true airspeed?

[SailingFan]

Originally Posted by Seabeau

Many years ago on a cool spring day, we on a short hop outside from St. Augustine Florida north to Savannah GA.. High pressure had brought us fine sailing weather, wind from the west at 25 to 35 knots and mild temps. in the low 60's. We were about 5 miles off the beach in 30 feet of water when all of a sudden we were struck suddenly, by what I can only assume was a microburst of cold, dense air from the upper atmosphere. Apparently we were in the exact center of the column of cold descending air because the dense air stopped my boat in its tracks and proceeded to radiate in an approx. 360 degree field, where it totally overcame the previously existing west wind and even flattened the short chop we were sailing in for about a half a mile in all direction. The air temperature dropped about 30 degrees and absolute silence reigned. We were astounded. So totally bizarre! After a minute or so, the microburst ceased, the west wind filled back in and we proceeded on to Savannah. What assume power on display!

I saw a documentary earlier this year about this sort of thing happening in the Caribbean, and they showed hexagonal cloud formations over the area it was happening. They said that it was likely the cause of many of the aircraft lost in the "Bermuda Triangle" as the downburst hits a plane and drives it to the surface before the pilot can do anything about it. Supposedly it also can generate significant sudden wave heights, and sink ships too. It is something to consider...

[Dockhead]

Originally Posted by a64pilot

Then whatever it's worth humidity lowers air density even further.
However I believe our anemometers compensate for air density as of course thinner air has to blow faster to spin the cups at the same speed.
Pilots think indicated as opposed to true airspeed. So maybe cold dense air at 17 kts will display as 20, but hot, less dense air will take 22 kts to display as 20?

I wonder though if the ultrasonic wind indicators indicate true wind speed or if they are affected the same by air density as a regular anemometer is?
Mine knows temp, humidity and barometric pressure, why would it want or need that if not to correct for true airspeed?

No, SPEED is not changed by density of the air, so neither a cup anemometer nor an ultrasonic anenometer will compensate for density. A tube anemometer (like an airplane pitot tube) DOES read differently as an almost linear function of the density of the air, but other types of anemometers do not.


Here's a handy calculator:

https://www.gribble.org/cycling/air_density.html

You can see that air at sea level and atmospheric pressure of 1018 weighs 1.2933 kg/m3 at 0C, whereas at 25C it weighs 1.1849, a difference of about 11%.

I can tell you from experience that it feels like more difference than that. Particular when the air is below freezing -- it feels really really "heavy". The sea seems kind of oily, and even a slight breath of wind seems to drive the boat hard. It is a remarkable sensation.

[a64pilot]

Speed is not changed of course, but indicated speed is with any device that mechanically measures airspeed.
The anemometer cups will be affected just like a prop would.
Think how fast the airflow would have to be to windmill your prop like it does when you sail, the difference of course is due to the density difference in air and water.
A standard day is 59F at sea level with a barometer setting of 29.92 " of Mercury. Assuming an air speed indicator is calibrated for a standard day, temps higher and or anything that reduces air density will cause a lower than actual indicated airspeed, lower will read higher than actual.

Now my just installed WSO 1000 measures temp, barometric pressure and even humidity. All the things required to take indicated airspeed and correct it to true airspeed. Now if I built the thing and had all the necessary data to correct indicated to true, I would, but I would have it software selectable.

Now our sails, just like the airfoils on an aircraft only " know" indicated airspeed, so displaying indicated airspeed would give you the most repeatable results for sailing.

I don't understand why the WSO measures temp, humidity and air pressure if it doesn't use that data?