Adi,
To respond to your post.
From what I read of your post you are saying that the surface wind blows at 0° degrees then it can only blow +/-30° above the surface at 2000'.
More or less correct. To be specific +30° in the Northern Hemisphere and -30° in the Southern Hemisphere
So are you saying the wind can only change direction by 30° in a 2000' layer of space? You have lost me completely.
Yes , let me help your understanding. A fundamental and undeniable fact and proven principal of atmospheric physics is that the atmosphere consists of high and low pressure systems constantly on the move. Each system covers many hundreds of square miles of the Earth's surface. These pressure systems have pressure gradients across them. These gradients are represented by isobars on a weather chart in exactly the same way that we represent gradients (hills and valleys) across the landscape by contour lines. At heights at above 2000 feet and above the air moves around the pressure systems parallel to the isobars.
This movement of the air is of course the wind. At heights above 2000 feet other factors may or may not come into play and disturb the direction of that flow, Below 2000 feet friction generated by the Earth's surface causes this air movement to slowdown and in so doing change direction by 30°. So if you are in the northern hemisphere and the wind is blowing 0° at 2000 feet on the ground level it will be 330°. i.e backed by 30°.
This fundamental principle is taught on day one on any metrological course. Certainly to forecasters, met observers and anyone else who has an interest in the upper winds. It is one of the cornerstones of meteorology. If you want a good brief explanation of it then I recommend you to read the Air Pilots Weather Guide by Ingrid Holford. Chapter 3 deals with pressure winds.
Not many aircraft fly at 2000'.
Yes they do. Most light aircraft to fly at that height. When pilots are being taught to fly this is the altitude of choice for various reasons. Not least of which is that we tend to suffer low cloud bases in this country and the vast majority of light aircraft must maintain visual contact with the ground below. Because they just do not have the instrumentation required for them to fly in cloud and neither do the pilots have the appropriate training or qualification to do so.
So as you can well imagine there are many hundreds of flights per day at 2000 foot level or there abouts. Also many helicopters fly at these levels or lower. The vast North Sea offshore helicopter support operations running out of Aberdeen and Sumburgh fly at the 2000 to 3000 foot level. And on a good day there are more flights across the North Sea Oil fields by helicopters then there are out of Heathrow on a peak summer weekend.
If you ever get the chance to storm chase a supercell storm you can see vertical shear of greater than 30° in action.
Yes I am fully aware of the vertical shear that exists in a super cells or even lesser thunderstorms. The vertical winds traveling both up and down to devastating effect. On the down motion it can burst out of the bottom of the storm cloud hit the ground with considerable energy and disperse in all directions of the compass. Its effect being felt up to 20 miles away. This is what is known as a micro burst and is of course a localised atmospheric wind effect.
I have just found this video that clearly shows the low level cloud travelling in the opposite direction from the upper level cloud.
Yes the video demonstrate horizontal wind shear. Probably a very localised phenomena brought about by features of the landscape. If you know where to look on the Internet you can find all sorts of videos showing weird and wonderful weather effects. But they do not disprove the fundamental principles of air mass movement around an atmospheric pressure system. Surface winds are affected by all sorts of low-level effects such as hills, valleys, buildings and surface heating.
Which is something you are clearly very conversant with and is also discussed in the book reference given above. As I said in my very first post wind direction is notoriously fickle and difficult to predict; particularly at ground level for the reasons given above. However, on open ground such as the Essex Marshes, Exmoor the tops of the South Downs etc those effects don’t really exist other than perhaps on-shore breezes on a very hot day, so the wind backing by 30° at the surface will hold good. But there again for an on shore breeze to form to any great effect general surface winds have to be light. Which indicates the presnce of a high pressure system, clear skies and no likely hood of any weather.
And yes I appreciate that for your local area you like to predict convective weather (ultimately thunderstorms I assume ) from tepigrams. Building up a data base, if only a mental one, of what sort of patterns in a tepigram will give a probability of a thunderstorm in your local area is a neat skill indeed. However, what we have done I believe is both answered Hugh Westacott’s question from differing view points.
As he says he just wants to be able to tell his ladies if they need to keep their water proofs on or not. And I very much doubt if they will be venturing out when the weather is serious and thundery or in remote mountain areas. Interestingly MLTs Hill Walking Manual which discusses weather doesn’t go into any of this stuff about upper winds that you and I have been going on about. Unfortunately, we have done what so often happens in these forums. We have hijacked a simple question and set it off spinning in a different irrelevant direction. All Hugh required was a simple, practical, instantly usable answer to a simple question.
As Skills4Survival says in his posts: Yes, the prevailing wind I am clear on. Also, out of which direction the wind comes...I can see with my compass, I just do not understand the whole difficulty of determining the cloud thing
1. step one, feel wind
2. step two read compass
3. do not forget..and use it if needed.
So wind, understand, cloud direction...that part I do not see yet, why that would really add value to what we would want to achieve.
I just use the wet finger a few times before I pitch camp.
That neatly brings it all back on track and we should leave it there.
