I'm reading a book called Modern Marine Weather, I'm only a few pages in, and reading an explanation on wind speed in relation to actual wind pressure, set off a light bulb for me!
An excerpt to set the scene!
"wind force does not increase in direct proportion to wind speed"
"but rather in proportion to the square of the wind speed"
Example:
a 10 knot wind has twice the force of a 7 knot wind,
(since 10x10 is about twice as large 7x7)
a wind speed of 21 knots does not push the boat 3-times harder than a 7 knot breeze; it pushes, or puts 9 times the force on the sails and rig!
if you refer to my rather mundane looking post on Storm Clouds I experienced sailing back to Port Hacking a week or two back, from Lake Macquarrie; I was at the time the picture was taken in about 5 knots of wind, and motoring with no sails, as I was heading down wind. In the shake of a lambs tail, soon after that picture was taken, I saw the wind gauge hitting 46 knots, coming directly off the bow, IE a Southerly. I didn't take any pictures, I was, ahem busy! But using the rule from above;
5x5 = 25
46x46 = 2116
2116 divided into 25 = 84.64
so I experienced a rapid increase in wind pressure 80 odd times what I was basking in 5 minutes earlier.

Im glad I had stowed there sails away earlier, due to them flapping and flogging in the downwind light conditions with a rolling sea state, my good luck I guess?

This dovetails nicely into Bundeena's post on sail trim, it's not hard for me to understand how sails can quickly get out of trim in what seems like small increases in wind speed! When you understand the actually pressures in play!

sorry for the long winded post, pun intended!

Richard

Another question would be if the density of the air and temperature makes a difference to the effect on sails.

So its not true when it comes to sails that red ones go faster?

One of many things to remember when reading weather maps / isobars is that the lines drawn represent pressure and not exactly wind direction. Allow about 20 degrees Inward for a low and same outward for a high, off the pressure lines themselves. Winds travel in a spiral in or out of a system. Highs will generally dominate and push away a low and weather will do whatever it likes

and weather will do whatever it likes


Went out on Friday.
Predicted weather, 4 to 12 knots SSE turning to E
Reality, 0 knots raising to 10 knots from the N turning to 15 from the west.

Of course, I planned a sailing route according to forecast!
Gary

I recall a friend talking about dinghy racers coming down to Tasmania from Sydney and getting into trouble with their rig tunings because of the greater force in the wind here at any given wind speed. Here's an interesting explanation of the relationship between temperature, humidity, air density and the resulting force. My take away is that a drier colder airstream down here in Tasmania (given the same wind speed) might pack upwards of 5-10% more punch compared to the hotter, more humid parts of the country. What's your reading of this?

weather.mailasail.com/Franks-Weather/Weight-Of-Wind

lydia said..

saltiest1 said..
Another question would be if the density of the air and temperature makes a difference to the effect on sails.

Temperature does not make any real difference, but in colder areas it feels gusty not because of denser air but the boundary layer is much higher.
Summer sea breeze maybe 500 metres while westerly at 45 south might be over 5000 metres.
more an acceleration issue not a weight issue

Temperature can make a difference with wind shear and that does affect your sail trim.

I get that the air mass is far greater in altitude, but not how that affects gusts on the ground, can you explain how that works? I'd always assumed that the wonky westerlies down here were due to the turbulence caused across the mountains and hills to the west.

lydia said..

2bish said..

lydia said..

saltiest1 said..
Another question would be if the density of the air and temperature makes a difference to the effect on sails.

Temperature does not make any real difference, but in colder areas it feels gusty not because of denser air but the boundary layer is much higher.
Summer sea breeze maybe 500 metres while westerly at 45 south might be over 5000 metres.
more an acceleration issue not a weight issue

Temperature can make a difference with wind shear and that does affect your sail trim.

I get that the air mass is far greater in altitude, but not how that affects gusts on the ground, can you explain how that works? I'd always assumed that the wonky westerlies down here were due to the turbulence caused across the mountains and hills to the west.

The breeze you sail in does not simply flow parallel to the water surface but flow downwards and from the boundary layer.
So in a sea breeze the gust so to speak will fall from 500m while in the channel, 3000 metres.
Greater hieght and greater accelerations
So the channel the breeze is coming down from say 3000m and then on top you have the disturbance caused by the hills which cause further acceleration and deacceleration of that air flow.
The stretch from Gordon to Oyster Cove in a westerly is a good example.

It is like the old Tasmanian cruising saying, never anchor behind the big hill as it speeds up the breeze.

Another to consider is that for every metre high obstacle, 10 metres horizontal is needed to attain clean air again.