UKC

Hi All

I am doing my ML home paper and I am stuck with a question, I have tried to google it and have come up blank, also asked many ML holders and have had no conclusive answers. I am not asking anybody to do my work for me, but if anyone could point me in the right direction it would be a great help.

the question is:

If the wind speed is 15-20mph in Fort William, what wind speed would you expect on the summit of Aonoch Mor? How would this affect an average adult group?

Thanks

In reply to Nathancoward:

I always thought the rule of thumb was a factor of 3, but the Met Office website says windspeed at 900m is around double that at low-level: see http://www.metoffice.gov.uk/guide/weather/mountain-video

In reply to Nathancoward:

'Weather for Hillwalkers and Climbers' by Malcolm Thomas says:

'Low level wind can be assumed to be half to two thirds the speed of that at 2000ft. On that basis alone the wind even half way up some British mountains can be twice the speed at the bottom'

I've looked in a few other sources and cannot find anything better.

In reply to Mark Bull:

Thanks, got the answer i was looking for.

Aonoch Mor is 1200m high and 10 miles (by road from Fort William). Call it 10 km for the crow flies. If you do Pythagoras on a 10km x 1.2km triangle you get a hypotenuse of 10.1km. So air going up the slope at 20 mph would get to the top only 0.18 minutes after when air going the long way round the bottom was at the bottom of the other side of the hill, leaving it 1200m of ascent. In order to catch up (Kirchoff's law of wind) it has to rush up the side of Aonoch Mor at 385 km/h (what is called a ciabatic wind). This then mixes with the 20 mph wind (average 20 mph and 385 km/h) to give a final wind speed of 112 knots.

If you Google for "Group of X adults" you will find X =6 gives the largest number of hits (almost 1/2 million). And therefore this is your group size.

The average adult drinks too much, is slightly overweight and is in the early 50s. They do little exercise. But across the group there will be a spread.

However, it is unlikely that the average group will have anyone of a nautical bent, able to understanding the importance of 112 knot winds.

Especially as 3 of them will be nursing hangovers, one will be worrying about their teenager getting pregnant and one will be contemplating suicide due to their financial crisis. Two of them will drive a car made by the VW-Audi group (but that isn't relevant to the calculation).

However, if programmes like the BBC excellent castaway series and it's follow up Lost (crap), are to be believed, one of the members of the group will turn out to be Ben Fogle. He will lead 4 of the other members safely off the hill.

The remaining member will turn up years later in either a holiday photo on a marketing website for overseas villas or somewhere less public like Channel 5.

In reply to Nathancoward:

Have a look at the met office website - they have areas giving information to glider pilots etc, showing the wind speed at different altitudes. Not sure if this would help, but worth a look.

In reply to Nathancoward:

As a rule of thumb when guestimating windspeeds in the mountains i tend to work on 600m-900m double the speed at sealevel, 900m-1300m triple it.

given a windspeed in fortwilliam of 15-20mph i'd be anticipating a windspeed on aonoch mor (just over 1200m in height i think) of 45-60mph.With wind at these levels I'd be anticipating difficulties walking / maintaining balance and significant windchill. Your average adult group will probably not be enjoying the experience too much.

In reply to Nathancoward:

Doubling the surface wind speed plus a bit is often a useful guesstimate. However, it's worth bearing in mind that the stability of the air (i.e. temperature drop with height) will have a big effect. If your mountain is situated on the edge of an area of high pressure you could have a temperature inversion not far above the summit, squeezing the air as it passes over the top and giving considerably higher wind speeds.

In reply to Nathancoward:

First, I haven't got an ML, but I have assessed leaders for a walking holiday company: the required standard was equivalent to Summer ML. We asked the question about wind speed at height. There's no 'correct' answer - no magic formula for you to learn - we just wanted aspirant leaders to show they had a realistic appreciation of the conditions on the summits. So a written answer that said "twice" or "three times" the sea-level wind speed would have been perfectly ok especially if backed up with a bit of explanation about variability, gusting etc. Or cover yourself by saying "between two and three times the sea-level speed, maybe even more in certain conditions ..."

As for the effect on your group, that's more straightforward: you can find stuff in any book on mountain leadership. In discussion (but probably not in a written answer) quoting the "one pint" rule (once you've shown you have some sensible contributions too) can win a few brownie points. That's "the effect of wind speed on people is equivalent to one pint (of beer) per every 10mph of wind speed". So at 40 mph/4 pints you're swaying about and stumbling, at 60mph/6 pints you start falling over.

Good luck!

In reply to Rip van Winkle:

Love the 'pints' rule. Stolen.

In reply to Philip:

Best answer ever

Thanks it made me giggle

B

In reply to Nathancoward:

I do question the value of asking questions to which there's no answer.

If you assume that whatever it's doing in the valley it will be shite on the tops, if only for the 10 minutes while you're at the summit, you won't go far wrong.

In reply to Nathancoward:

1) If the wind speed was 15-20mph in Fort William you could expect the wind speed to be double/treble, 30-60mph on the summit of Aonach Mor. The effects on an average adult group would be walking in the wind will tire them out more, it will also upset balance and confidence on steep ground, thus slowing the group down. Also it will sap heat, the group will feel cold and if it rains and it is cold then hypothermia is a possibility.