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Does Height Effect How Strong The Wind Is?


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Posted
  • Location: Brongest,Wales
  • Weather Preferences: Stormy autumn, hot and sunny summer and thunderstorms all year round.
  • Location: Brongest,Wales

I ask this question because I have noticed that when on top of a hill the wind seems to be blowing much faster and also more constant compared to when I am on ground level or in the garden.

Also I have noticed that the wind readings on observation stations seems to be much higher compared to readings on my weather station. The anomometer is up at 21ft above the ground in the garden but according to the Met Office an anomometer should be at a standard height of at least 30ft. This would explain why the tops of 70ft Ash trees move a lot more in the wind compared to trees which are only 20ft up.

Basically I would just like some help on this matter because I am worrying that my wind gauge may be broken or inaccurate.

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Posted
  • Location: Carlisle, Cumbria
  • Weather Preferences: Atlantic storms, severe gales, blowing snow and frost :)
  • Location: Carlisle, Cumbria

Wind speeds are slower at the surface due to friction on the ground, as height increases there is less friction and so the wind speed will be stronger.

Land, trees, vegetation and buildings etc create friction near the surface, a reason why wind speeds are usually recorded stronger on the coasts and hills. Water has very little if any friction for the travelling wind and also at elevation on the hills and mountains the wind has encountered little friction.

Say for example last week I recorded gusts to 75mph on low ground and Great Dun Fell which is 848m ASL was recording gusts to 90mph+

Edited by Liam J
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Posted
  • Location: Camborne
  • Location: Camborne

Also this from the link.

Basic requirement: The wind speed and direction (see ** below) in the first 30 metres or so of the atmosphere varies rapidly with height, due to the varying frictional effect of the general 'surface roughness'. It is greatly affected by undulating ground, and by adjacent obstacles such as trees, bushes, buildings etc. This is a common experience - noted for example within built up areas, major shopping centres etc.

For synoptic and climatological work therefore, a 'standard' exposure is required. That standard is for the wind speed and direction over a level surface to be measured at a height of 10 m above ground level (agl). When these conditions cannot be met, it is permissible to raise the anemometer to give an <a name="effecwind" title="effecwind">effective height of 10 m, provided the obstructions are not large, and are distributed uniformly around the instrument site.

It will be immediately apparent, that in the common 'back-garden'/urban development situation, a considerable mast is needed to carry the anemometer clear of these 'ground effect' generating obstacles. For example, consider an outer-suburban garden with houses/trees of approx. height 6m in height, the recommended exposure height would be 6 m (obstruction) + 10 m (standard height)=16 metres. (That's around 50 feet!) This provides problems in maintenance of the sensor, and also there would possibly be planning and structural constraints. To be stable, such a structure would need to be well braced which will not be easy. When considering larger obstructions, such as large blocks of flats, or office blocks, then the sensors would need to be raised even more. An example: For an obstruction of some 15 m in height (a typical large building), which is about 75 m from the site of the intended anemometer site, then the wind vane/cups would need to be about 25 m above ground level.

If such conditions are beyond the scope of your pocket or what the neighbours will allow, then the best compromise would be a fitting a short height above the ridge of a house, provided always that adjacent buildings do not unduly affect the airflow at the sensor level.

[** Wind direction, when quoted in standard meteorological reports such as SYNOP and METAR, are given in terms of deviation from TRUE north (degT). This should be remembered when setting up a weather station that includes an anemometer: align the directional head such that the read-out gives degT - to do this you will need to allow for the local magnetic deviation of your compass from true north: many websites are available that will help with this subject.]

http://weatherfaqs.org.uk/node/124

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Posted
  • Location: Brongest,Wales
  • Weather Preferences: Stormy autumn, hot and sunny summer and thunderstorms all year round.
  • Location: Brongest,Wales

Wind speeds are slower at the surface due to friction on the ground, as height increases there is less friction and so the wind speed will be stronger.

Land, trees, vegetation and buildings etc create friction near the surface, a reason why wind speeds are usually recorded stronger on the coasts and hills. Water has very little if any friction for the travelling wind and also at elevation on the hills and mountains the wind has encountered little friction.

Say for example last week I recorded gusts to 75mph on low ground and Great Dun Fell which is 848m ASL was recording gusts to 90mph+

Does this mean then that there would be a big difference in wind speeds from my 21ft high anomometer and the 70ft or more Ash tree by the river in the front garden?

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Posted
  • Location: Camborne
  • Location: Camborne

There would certainly be a difference, I don't have a weather station but I would hazzard a guess that most people who do find it very difficult to get an ideal exposure for the anemometer and rain gauge. Much will depend on the exposure of your anemometer in relation to the surrondings. I assume it's not stuck near the Ash tree.ohmy.png

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Posted
  • Location: Whitkirk, Leeds 86m asl
  • Weather Preferences: Anything but mild south-westeries in winter
  • Location: Whitkirk, Leeds 86m asl

Usually the higher and more exposed you are, the windier and gustier it gets, which is why you see the likes of Cairngorm and Great Dun Fell recording gusts in excess of 100 mph every so often.

Exposed coastal locations are fairly windy too, and places on the eastern slopes of the Pennines, particularly in valleys, get pretty windy, due to 'Lee wave winds', when the wind travels down the Pennine hills, gaining speed.. Leeds city centre is in a narrow valley, and in 1990, recorded a gust of 98 mph.

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Posted
  • Location: Camborne
  • Location: Camborne

Usually the higher and more exposed you are, the windier and gustier it gets, which is why you see the likes of Cairngorm and Great Dun Fell recording gusts in excess of 100 mph every so often.

Exposed coastal locations are fairly windy too, and places on the eastern slopes of the Pennines, particularly in valleys, get pretty windy, due to 'Lee wave winds', when the wind travels down the Pennine hills, gaining speed.. Leeds city centre is in a narrow valley, and in 1990, recorded a gust of 98 mph.

That of course is not always true. Think of the windiest place on earth. Although the principle has some comparison. I suggest the thread is getting a bit repetative.

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Posted
  • Location: Derbyshire Peak District. 290 mts a.s.l.
  • Weather Preferences: Anything extreme
  • Location: Derbyshire Peak District. 290 mts a.s.l.

Usually the higher and more exposed you are, the windier and gustier it gets, which is why you see the likes of Cairngorm and Great Dun Fell recording gusts in excess of 100 mph every so often.

Not strictly true. It certainly gets windier the higher you go, as a general rule, but not gustier. Gustiness is a direct factor of the roughness of the terrain ( ie trees, buildings etc ) and is a measure of the highest wind speed over a short duration relative to the mean wind speed at the time. The gustiest places tend to be town and city centres where the character of the prevailing wind is completely altered by the configuration of streets and buildings.

In a city centre it's quite possible to have a mean wind speed of 15 mph and to record gusts of 40 mph whereas a mean speed of 15 mph on an open moor at, say, 400 mts a.s.l would produce gusts around 23 mph.

Does this mean then that there would be a big difference in wind speeds from my 21ft high anomometer and the 70ft or more Ash tree by the river in the front garden?

Assuming your anemometer at 21 feet is equally as exposed as the 70 ft Ash tree you'd expect the mean wind speed at the top of the Ash tree to be around 25% higher according to Met' Office guidance for the correction of wind speed readings to standard height.

However, if you anemometer is surrounded by other objects also around 21feet, or higher, but the Ash tree is considerably taller than any of the surrounding obstacles then the mean speed at the top of the tree could easily be 30 or 40% higher than that recorded by your anemometer.

How far away from your anemometer is the tree and in what direction?

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Posted
  • Location: Brongest,Wales
  • Weather Preferences: Stormy autumn, hot and sunny summer and thunderstorms all year round.
  • Location: Brongest,Wales

Not strictly true. It certainly gets windier the higher you go, as a general rule, but not gustier. Gustiness is a direct factor of the roughness of the terrain ( ie trees, buildings etc ) and is a measure of the highest wind speed over a short duration relative to the mean wind speed at the time. The gustiest places tend to be town and city centres where the character of the prevailing wind is completely altered by the configuration of streets and buildings.

In a city centre it's quite possible to have a mean wind speed of 15 mph and to record gusts of 40 mph whereas a mean speed of 15 mph on an open moor at, say, 400 mts a.s.l would produce gusts around 23 mph.

Assuming your anemometer at 21 feet is equally as exposed as the 70 ft Ash tree you'd expect the mean wind speed at the top of the Ash tree to be around 25% higher according to Met' Office guidance for the correction of wind speed readings to standard height.

However, if you anemometer is surrounded by other objects also around 21feet, or higher, but the Ash tree is considerably taller than any of the surrounding obstacles then the mean speed at the top of the tree could easily be 30 or 40% higher than that recorded by your anemometer.

How far away from your anemometer is the tree and in what direction?

The distance is around 100 yards and the Ash tree is to the SW of the weather station.

Edited by wimblettben
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Posted
  • Location: Brongest,Wales
  • Weather Preferences: Stormy autumn, hot and sunny summer and thunderstorms all year round.
  • Location: Brongest,Wales

There would certainly be a difference, I don't have a weather station but I would hazzard a guess that most people who do find it very difficult to get an ideal exposure for the anemometer and rain gauge. Much will depend on the exposure of your anemometer in relation to the surrondings. I assume it's not stuck near the Ash tree.ohmy.png

No, it's a hundred yards away from it actually. So it probably wouldn't have much effect on the weather station.

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Posted
  • Location: Derbyshire Peak District. 290 mts a.s.l.
  • Weather Preferences: Anything extreme
  • Location: Derbyshire Peak District. 290 mts a.s.l.

The distance is around 100 yards and the Ash tree is to the SW of the weather station.

Ideally your anemometer would need to be a minimum of 200 yds from the tree for it to be classed as having little or no effect.

There will undoubtedly be some lessening of the wind speed from that direction because of the tree, more so when it's in full leaf.

How great the effect will be will depend on a variety of factors such as density of the crown, spread of the tree etc.

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