Straight Isobars & wind stength

[Simon S]

Tomasz schafernaker keeps stating that straight isobars are an indication of wind strength and not just how close together they are.
This is something I do not remeber hearing before?
What is the machanism for this to be the case?
I may have simply forgoten something very simple here.

Simon S (South Yorkshire)

[Jack Frost]

Is he just referring to the geostrophic wind? (https://en.wikipedia.org/wiki/Balanced_flow#Geostrophic_flow)

Liam

[Tudor Hughes]

        The wind strength depends on the curvature of the isobars, other things being equal.  If they are cyclonically curved the centrifugal force acts in the same direction as the Coriolis force and therefore the wind strength needed to balance the pressure gradient is less.   On the other hand if the curvature is anticyclonic the wind will be stronger than the pressure balance figure because the centrifugal force is subtracted from the pressure gradient force and the wind must be faster to balance.  With straight isobars the wind speed would be intermediate between these two extremes as there would be no centrifugal force.  Schaffernacker is therefore talking nonsense.  I get the feeling he is using the false analogy of a straight road on which it is possible to drive faster than on one which curves this way or that.  He should know better and may well do so but may have been told to keep it simple because you don't want to upset the viewers by talking difficult stuff.

Tudor Hughes 

[Paul Garvey]

I remember a long conversation with a physicist who explained in detail that centrifugal force doesn't exist. It doesn't, but its effect does. *>))

[Graham Easterling]

Drifting slightly, but still on the subjective of isobar curvature,  onto my favourite topic - ocean swells.

Hurricanes are clearly severe events, seas are extremely rough, storm surges create massive flooding, but actual swell size is often smaller than average winter conditions in the N Atlantic.

Looking at the swell charts, Hurricane Matthew resulted in a significant swell height of 10'-15', although individual waves would be bigger than this. You would have to double this to 20'-30' significant swell height for anything notable in the N Atlantic, and even on N Cornish, Irish & Portugese reefs, the swell can reach 50'. In fact in January, the highest average swell height anywhere in the world are just SW of Ireland.

This is because a long fetch & duration is required for a big swell, the tightly curved isobars of a hurricane are completely wrong for development of a big swell. You need a > 1,000 mile fetch, in a straightish line (great circle route ideally). The typical N Atlantic winter weather (not current conditions) is ideal for this. Strong to gale force winds can exist for a fetch of 3,000+miles, sometimes over a few days. This can build waves to >60' making reports of 17' waves recorded around Matthew seem pretty small. Also the distances involved allow the powerful long period waves to separate out and due huge damage. (There are lots of equations around this - but that's more Norman's territory!)

Size isn't everything of course. Also N. Atlantic coast beaches are built in typical N Atlantic weather. The beach sand levels somewhere like Florida are allowed to build to suit calm conditions, so when it is stormy they can soon vanish when conditions are so different.

Graham

Penzance

[Len Wood]

Yes, depends on the pressure gradient in the first instance.
For the same gradient (isobar spacing), air moves slower round a trough than in straight flow, but quicker round a ridge.
So his argument is invalid around a ridge (anticyclone), but there the gradient is usually less so the wind speed is less anyway.

You have got to hand it to him though for giving us a good laugh.

I would n't say the present easterlies are particularly strong compared with a good westerly gale with plenty of curvature.
But stronger than they would be if they had cyclonic curvature tis true.

Len
Wembury, SW Devon

[Tudor Hughes]

   An interesting point arises in the case of cyclonic westerlies.  If the wind speed is the same as that of the motion of the Low the actual path of an air particle may be nearly a straight line and not that indicated by tightly curved isobars.  ISTR this effect was invoked to explain the wind strength in the Burns Day storm of 25 Jan 1990.

Tudor Hughes.

[Tudor Hughes]

On Monday, 10 October 2016 12:48:21 UTC+1, Paul Garvey wrote:

I remember a long conversation with a physicist who explained in detail that centrifugal force doesn't exist. It doesn't, but its effect does. *>))

     Strictly speaking that is of course true but only in inertial coordinates.  From the point of view of an air particle travelling in a curve or a cyclist in a velodrome centrifugal force is a neat mathematical abstraction that explains quite a lot.  I think I'll stick with it and hope nobody notices.

Tudor Hughes. 

[Graham Easterling]

On Tuesday, October 11, 2016 at 6:54:43 AM UTC+1, Tudor Hughes wrote:

   An interesting point arises in the case of cyclonic westerlies.  If the wind speed is the same as that of the motion of the Low the actual path of an air particle may be nearly a straight line and not that indicated by tightly curved isobars.  ISTR this effect was invoked to explain the wind strength in the Burns Day storm of 25 Jan 1990.

Tudor Hughes.

I think this was a factor in the NW storm (associated with a depression slipping SE just to the NE)  which affected Cornwall in March 2008. Both the wind strength on sea height were way above forecasts.

and of course, all other things being equal, a hurricane moving will have winds stronger on one side were the wind coincides with the direction of movement.

All interesting stuff.

Graham

Penzance