Wind Speed vs Gust line Speed

Wind Speed vs Gust line Speed

Post by Breeze Hea » Mon, 01 Jun 1998 04:00:00


Watching gust lines come towards you on the water, it seems to me that the
gust line moves over the water much slower than the speed of wind you get
in your sail when it arrives.
Is this true or is it my imagination ?
If it is true, can you explain it ?

Thanks

Steve Joures

 
 
 

Wind Speed vs Gust line Speed

Post by Richard. R. Thomas. (Ri » Tue, 02 Jun 1998 04:00:00


Quote:

>Newsgroups: rec.windsurfing
>Subject: Wind Speed vs Gust line Speed
>Date: Sun, 31 May 1998 21:19:14 GMT

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>Watching gust lines come towards you on the water, it seems to me that the
>gust line moves over the water much slower than the speed of wind you get
>in your sail when it arrives.
>Is this true or is it my imagination ?
>If it is true, can you explain it ?
>Thanks
>Steve Joures

Yes Steve.

You see a lag effect caused by the frictional interface between the air and
water.

The squall- line has a vertical componet. The higher strata moves at the True
Wind Speed.

Another reason for useing a wind indicator?

Rick.

 
 
 

Wind Speed vs Gust line Speed

Post by NLW TFW » Tue, 02 Jun 1998 04:00:00

RE:"You see a lag effect caused by the frictional interface between the air and

water. The squall- line has a vertical componet. The higher strata moves at the
True
Wind Speed."

While I surely don't have the right answer (this has puzzled me for years,
too), I'm not convinced the frictional lag explains the disparity.

1. If the windline on the ground/water moved slower than the squall, before
long the squall would be scores or hundreds of miles ahead of the surface
windline.
2. The wind below a squall has little to do with squall (upper wind) speed.
Consider a hurricane, tornado, or T-storm -- they can move very slowly but
generate tremendous surface wind speeds. The first two do it with rotational
winds, and the T-storm (or any little squall) does it by shoving huge
quantities of air straight down until the ground forces it to move sideways.
Thus the windline can be barely moving, yet its windspeed can be great. I think
that's what the above post was referring to with "The squall- line has a
vertical component".
3. There is often no squall within hundreds of miles, or, as has been he case
here in NM for a couple of days -- no precip (hardly any clouds at all) --
within a thousand-mile radius. Yet the wind is gusty and fluky. That's both
terrain turbulence and variations in vertical mixing. As a bubble of hotter air
rises over a dark patch of ground, it sucks in surrounding surface air (a
breeze) and displaces faster-moving upper-layer winds. Both of these effects
can increase surface the wind speed, yet the lateral speed of the rising bubble
-- and thus its windline advance-- can be much slower than the wind thus
generated.

Or it could just be Jason, amblin' along at a snail's pace, shootin' his foul
mouth off.

Mike \m/
Never Leave Wind To Find Wind

 
 
 

Wind Speed vs Gust line Speed

Post by NLW TFW » Tue, 02 Jun 1998 04:00:00

Re:"You obviously haven't read his posts with the appropriate seriousness"

Oh, I suspect we've ALL read -- or ignored -- his posts with appropriate
seriousness. 8<)

Mike \m/
Never Leave Wind To Find Wind

 
 
 

Wind Speed vs Gust line Speed

Post by Nick Ki » Wed, 03 Jun 1998 04:00:00

Friction and waves. The wind approaching you is subject to the effects
of the water it is coming over. That means if there are waves ( always
some) it will  be disturbed so to speak by the waves. This causes
little eddies like watching water go over and around rocks (in fact
wind responds very similarly to water when it meets obstacles).  As
the gust comes it adjusts its pattern to the water (this is actually
always changing but the first thrust has more to overcome) As the
waves build and go faster the wind will adjust to that, too. Since the
wind is a primary factor in the waves eventually they will find an
equilibrium of sorts.

Watch a stream sometime and then think of the wind coming over land
and around cliff faces and such. Evey wonder why it swirls in some
places?

On Sun, 31 May 1998 21:19:14 GMT, "Breeze Head"

Quote:

>Watching gust lines come towards you on the water, it seems to me that the
>gust line moves over the water much slower than the speed of wind you get
>in your sail when it arrives.
>Is this true or is it my imagination ?
>If it is true, can you explain it ?

>Thanks

>Steve Joures

 
 
 

Wind Speed vs Gust line Speed

Post by Jonathan M Richards » Thu, 04 Jun 1998 04:00:00

: Watching gust lines come towards you on the water, it seems to me that the
: gust line moves over the water much slower than the speed of wind you get
: in your sail when it arrives.
: Is this true or is it my imagination ?
: If it is true, can you explain it ?

This issue is central to racers who look for the gusts and try to make
the most of each one. If you watch the water on a gusty/fluky day
(typical of lake sailing) you will see numerous "cats paws," as they
are called by sailors. The reason for this term is that the gust will
hit the water and then spread out. This can be explained by imaging
that somewhat above the water there is uniform air flow and as you
approach the surface of the ground there is turbulent flow. The size
of the turbulence is large, say 30-50 feet. The gust occurs when a
region of flow hits the water. About the velocity with which the gust
reaches you, consider that 20 knots does not appear that fast to an
observer.  If a cyclist rides toward you, his speed is not perceived
as being all that fast. 20 knots is still a fair gust strength,
though.

-Jonathan. (US233)

 
 
 

Wind Speed vs Gust line Speed

Post by Ian Knigh » Thu, 04 Jun 1998 04:00:00

Quote:

> Watching gust lines come towards you on the water, it seems to me that the
> gust line moves over the water much slower than the speed of wind you get
> in your sail when it arrives.
> Is this true or is it my imagination ?
> If it is true, can you explain it ?

> Thanks

> Steve Joures

I guess you sail on an inland lake.  One explanation is that eddies form
in the lee of a hill upwind of you and break away periodically. As the
eddies move downwind faster air from higher up comes down to the surface
in their wake. This faster air does not push the eddy but rises over
it's tail.  They're called cat's paws in meteorological books.

But then again the phenomena exists on our lake a fair way downwind of
any distinct hill. Maybe a gap appears in the slower moving lower layers
of air as part of the normal turbulence of a summers day, allowing the
faster upper layers to sweep past the surface for a while.

Whatever we spend a lot of time on marginal days waiting to see one on
the horizon, and then they take forever to cover those last 20 metres.

Ian

 
 
 

Wind Speed vs Gust line Speed

Post by Peter Som » Fri, 05 Jun 1998 04:00:00

Quote:

>Friction and waves. The wind approaching you is subject to the effects
>of the water it is coming over. That means if there are waves ( always
>some) it will  be disturbed so to speak by the waves. This causes
>little eddies like watching water go over and around rocks (in fact
>wind responds very similarly to water when it meets obstacles).  As
>the gust comes it adjusts its pattern to the water (this is actually
>always changing but the first thrust has more to overcome) As the
>waves build and go faster the wind will adjust to that, too. Since the
>wind is a primary factor in the waves eventually they will find an
>equilibrium of sorts.

>Watch a stream sometime and then think of the wind coming over land
>and around cliff faces and such. Evey wonder why it swirls in some
>places?

I think Nick hit the nail on the head: waves on the surface of water
are highly dispersive, i.e. velocity of wave propagation is not
constant, larger waves travel faster. When a gust hits and starts to
set up waves, these will be (initially) small so they travel slowly.
If the gust would persist (we wouldn't call it a gust then), then the
larger waves would correspond more to the windspeed.
Peter
_____________________________________________________________________
Dr.Peter I Somlo FIEEE | M1: "Every coin has 3 sides - at least"
Microwave Consultant   | M2: "The wind ain't gonna blow from where it
tel/fax: 61-2-9451-2478|      ought'a, it'l blow from where it can"
Mobile(AU):041-926-3168|      WWW:<http://www.zeta.org.au/~somlo>
 
 
 

Wind Speed vs Gust line Speed

Post by BDSincla » Mon, 08 Jun 1998 04:00:00

What??? Fluid dynamics/eddie currents...I hated this stuff 20 years ago.
Speculation:Wind is an energy wave moving through a fluid (air).  Propogation
velocity of the wave front is not the same as the individual waves in the
group, they drop out at the front of the pack and develop at the rear.  I'm
guessing the "gust line speed" represents the wave front velocity and the wind
speed represents the velocity of the individual waves in the group,
Terminology may be off-it's been a while.
Just guessing,
Brian