Glidemet

Better late than never  .

Thanks to Diana King, Gary Nuttall and Jim White for the excellent feedback in the comments to Saturday’s forecast.

Saturday itself went as forecast on Friday—-cloud spilling down south eastwards followed by drizzle. What was notable was that although the surface wind was fine, the wind gradient was extremely steep, reaching a good 40 knots at just 1,500’ (I flew a K21 backwards). There was also wave, which I’d mentioned in Thursday’s forecast. Here’s the analysis sounding for Saturday:

The static stability and the speed shear is clear (though so is the frontal cloud, where the ELR meets the dewpoint line).

In the forecasts leading up to Saturday it was notable that the forecast changed from bright and northeasterly on Wednesday to damp and southwesterly on Thursday. It only took a small change in the position of a low to bring about a larger change in the resulting weather, and as far as I can remember all the models were in agreement all along - it was just that what they agreed on, rapidly changed from Wednesday to Thursday.

Sunday looked for a long time earlier in the week that the wind would be close enough to the north for the South Downs to work, but by Thursday doubts were appearing and by Friday it was looking likely that the wind was too far to the west for Parham. Judging by the lack of flights on the ladder (Parham pilots are very good at listing their flights) I suspect this was the case in the end. At least I did realise Friday’s potential the night before on Thursday , and indeed Friday was a good day down there.

Elsewhere a main hope for Sunday had been wave in Wales and the Welsh Borders which Phil King first pointed out to me on Thursday. Disappointingly I under-estimated the amount of cloud around on Sunday and that was the undoing at Shobdon, from where Diana King reports that “We were unable to use the primary wave in the lee of Radnor Forest because although there were gaps from time to time they were not large enough or open for long enough. A pity because there was quite strong lift - 4 kts at 3500 - 4000′.” As I mentioned on Saturday all the models showed an area of cloud extending down into England through the Cheshire Gap; it extended just a little too far westard in the event. Eastern and central areas were all much cloudier than any model forecast (though the GFS was closer than most, surprisingly for a global model*). All was not lost at Shobdon though: “Cloud amounts a bit further downwind were not so much. There was very weak lift on what was probably the tertiary wave and Peter Reading contacted at 2800, and eventually reached 10,000′.”

The wave patterns were very apparent on satellite on Sunday, and here’s a lovely picture from Terra:

To orientate yourself the Bristol Channel is visible in the lower left and Cardigan Bay in the upper left. The wave bars coming off the Cambrian mountains are very apparent under the veil of upper level cirrus.

A look at the analysis sounding again shows classic wave indicators of little direction shear but high speed shear, and nice big stable layer:

I had said I thought lift at lower levels would be weaker, when in fact it was quite strong. My error probably was thinking that the less stable air lower down would be less conducive to wave, when thinking about it should actually be better - less resistance to the first rising side of the wave. Adding a virtual parcel shows the stability and the convective inhibition resisting uplift, though there is a little area of instability above the condensation level:

Elsewhere it was a good ridge day. Nympsfield report on their news page that their ridge worked fine, as forecast. Gary and Jim provided excellent feedback from Booker. Again it seems there was strong speed shear even at low levels and I wonder if the sudden smoothness at 1,800’ on tow could have been where turbulence from the ridge was trapped by the high static stability - on Saturday I’d found the atmosphere to be highly stratified. While there was little wave lift the Stokenchurch ridge was working well despite the light winds at Booker - Jim reports “310 / 23Kts which is perfect for the bit from the M40 to Bledlow”. Going on to Wendover is best with a bit more west in the wind, Jim says. (I must say it’s very interesting to read all this - I know the area well, but from the saddle of my mountain bike. I know Whiteleaf particularly well as I broke my wrist there ). Do pilots ever head south down past Watlington Hill from Stokenchurch?

*The GFS also consistently forecasted Monday’s wind to be much lighter than the UK meso did, and was indeed correct.

Early Outlook
Really need to go back through the last few of these and see how reliable they’ve been proving. Anyway… tomorrow (Tuesday) cloudy with a little wind from the west. Wednesday again cloudy and wet in the west as a cold fronts thinks about crossing the country, before doing so on Wednesday night leaving Thursday a brighter breezier day. A southwesterly wind on Wednesday moves a little north on Thursday, probably enough for Nympsfield’s local ridge to work at least.

There’s good agreement that things are going to get rougher going into the weekend. Friday currently looks like having a bright but windy start before heavy rain moves in. Can’t say with worth-while certainty what Saturday and Sunday’s weather will be like but it looks likely that it’s going to be wet and windy at times, but probably fine at others!

In Other News
*One of Sandhill Farm’s K13s is now in very small pieces . I wonder if the wind gradient and/or rotor turbulence had a hand in the accident.

*Some folks in Belgium have winch launched a K21 to 5,640’, a new world record. Amazing what you can do with plasma rope and a 10,000’ runway . The winch used was a Mel.

As I forecast on Wednesday, the weekend was a “real mixed bag” with sun, cloud, wind and rain all at various points on both days. Generally the forecasts were good and the weather was just ok to fly, with the wind gusting up to about 25 knots at Bicester on Saturday and a tad lower on Sunday.

What was notable was the widespread wave and good wind for ridge running. Phil King first alerted me to the possibility of wave this week on Monday via a comment to that day’s post. Certainly key indicators were there - a general anti-cyclonic curve to the isobars, and stable layers at suitable heights in forecast soundings, although cloud amounts looked likely to be a problem, and Phil reported on Saturday that Friday had indeed been restricted by cloud.

Helpfully a lot was posted to the BGA National Ladder. First off, here’s a comparison of the wind on Saturday and Sunday:

Saturday’s winds are the more westerly blue barbs and Sunday’s the more northerly orange barbs (well, that was the colours I selected, but exporting an image from the IDV seems to wash out the colours and I’m a bit colourblind, but you should get the idea).

As you can see Saturday had a more westerly wind which was also generally stronger. On Sunday the wind came round much further to the north (something I suggested on both Thursday and Friday, though not so much Saturday), however, down on the south coast the change in direction was less or even opposite.

Judging by the ladder Nympsfield’s runs were working well on Saturday (e.g Trevor Stuart’s flight) but it was particulary good to read Bruce Cooper’s report from Denbigh. Bruce states that the wind was close to the northern limit for the ridge there and this can be seen in the above chart.

Gary Nuttall also left a comment that Booker’s ridges were working well on Saturday, keeping a K13 and a ASW27 up. I didn’t know pure gliders could reach and return from the ridge there - I’d only seen Alan Johnstone’s ASH26 flights on the ladder before.

There was also wave coming out of Wales. Diana King climbed well from Shobdon getting to 13,000’, but very interestingly Alison Mulder reached nearly 12,000’ from Nympsfield. Below is Alison’s track laid over a Modis 250 m image taken around half-way through her flight:

Excellent wave bars are seen standing south east of the southern Cambrian hills extending well into England, and the wave bars Alison used to climb are clearly visible.

Let’s look at a GFS forecast analysis sounding for the area:

Now I’m no expert at analysing soundings for wave but it strikes me that there’s an conditionally unstable layer up to 900 mb followed by a near-isothermic layer up to 700 mb (about 9,000’) combined with a steadily increasing (and quite strong) wind with height coming from the northwest. Diana King commented on Friday’s forecast saying that on Saturday the wave began at about 3,5-4,000’ before reaching 13,500’. My theory is that air from the stable layer was being displaced upwards by the the unstable flow over the hills lower down. However with unstable air again above 700 mb there’s nothing to reflect the energy downwards again, so there wouldn’t be more than one primary wave bar - and that doesn’t tally with the sat pics. Hmmm. If anyone can see what I’m missing, please do tell…

One pointer - I didn’t forecast the wave at all well towards the end of last week and at the weekend. However, the Met Office’s F215 charts (you’ll need to be registered for that link to work, but it’s free) were on the money every day - they run a special model for wave forecasting which they use to write the MTW forecasts on those charts. Although the lead-time is short - the 0200-1100 chart issued at 2130 the night before is the earliest warning you’ll get - they’re very good.

Now let’s turn our attention to Sunday. First off, up in Scotland Kevin Hook put in a cracking 400k reaching FL195. I decided to try overlaying Kevin’s flight on a satpic too and seeing how they lined up. This time the image is an excerpt from the afternoon NOAA-18 image with 1 km/pixel resolution: copyright 2007 RSGB, University of Bern and NOAA.

I asked Kevin how well this satpic matched what he saw from the cockpit. He wrote:

“The wavebar at AMU never moved throughout my flight as you can see from the proximity of each leg to the others in that area. The Satpic shows it just as I remember. As you go west and north, the trace is occasionally on the wrong side of wavebars or over the top of them. I think that this shows that in these areas the wave was much less stable and kept moving up and down wind. Re-forming wave often results in small gaps and corresponds to my impression of clag at TUL and TUW. The blue at the eastern turnpoints and local to Portmoak is also accurate.

“After finishing the task, I watched the wave at TUL set up properly and worked the new line out of interest. The hot spot that I was working at 13:45 is exactly at the point of greatest sink in the Satpic 45 minutes earlier.

“It never ceases to amaze me how different conditions can be on the run out from a turnpoint compared to the run in ten minutes earlier. A change of a few knots in wind speed or of a few degrees in angle can change the picture totally. If anything, I am surprised how closely this single snapshot matches the weather over a six hour flight.”

Very interesting. Again, here’s the GFS analysis sounding for the same time and place:

(Part of the dewpoint line is missing. I don’t know why). If anything it looks like just like a taller and windier version of the same sounding from south Wales on Saturday.

Back down south, judging from the ladder, the wind was not as good for ridge running on Sunday as it had been on Saturday, although Booker again flew a K13 on their ridge until the wind veered and dropped in the afternoon. That said, there appeared to be widespread wave again, and a good climb was made from Talgarth to 10,000’.

So again, thanks all for your comments, emails and ladder flights this weekend. Comparing your flights to the forecast soundings will mean I (and you!) will have a better idea of what to look for in the future, and I’ve refined my knowledge of the winds needed for the ridges too.

Early outlook
Tomorrow (Tuesday) a cloudy and damp day, wind 15 knots from the northwest once again. All back to Nympsfield and Talgarth then . Wednesday looks brighter (though possibly showery in the east) with the wind round to the north now, but quite weak - a bit borderline for Parham.

Thursday sees the high off to the southwest at the moment move a little closer in, giving a brighter day again but quite windless - possible fog risk. Friday not dissimilar (though the Met Office thinks it will be wet - I don’t buy that). The weekend sees too much divergence in the models - the GFS and ECMWF being completely opposed - to make a worthwhile forecast.

In a break from forecasting, and for something to do while I was poorly today, I looked at the reports from the various European equivalents of the AAIB - the BEA (France), BFU (Germany) and confusingly the BFU in German or the AAIB in English (Switzerland - we’ll call them the Swiss). I looked at the last ten years, ‘cos that’s all that is available from the BEA and the BFU. I’ve also included British data from our AAIB.

I’m not going to draw conclusions, simply present the facts as I have found them.

So, here are the glider-to-glider collisions per country, by year. Generally the criteria for reporting was death or serious injury to at least one of the pilots involved, so many more “minor” collisions are not included.

FLARM was introduced in 2004 (and subsequently endorsed by the IGC).

I have excluded the following collisions between:

• a DG100 and a Tornado in Germany in 1998
• a Grob G103 and a Airbus A320 in France in 1999
• a LS4 and a Cessna 182 in Germany in 2003
• a Wilga and a Jantar in Germany in 2003 tug upset, glider struck tug after tug had already impacted with ground
• an ASK21 and a microlight in Switzerland in 2007

I did include a collision between a Pegase and a Rallye tug, both free-flying, in France in 2004, a collision between a Cirrus and a Pawnee tug, both on final approach, in Britain in 2001, and a collision between a Falke motorglider and a LS4 in Germany in 2000.

At least three collisions were in circuit or final approach, and three occured during thermalling. There were a multitude of collision angles, from head-on to from-behind, though at least one of the latter was due to one accident glider unsuccessfully trying to manuever clear. Several gliders were carrying high-visibility markings.

The BFUs include pictures in their accident reports. I now have a better idea of why Fossett has not been found - if there’s anything left, it may well not look like an aeroplane anymore. I’ll freely admit I found some of the images quite disturbing.

(I also discovered that in one collision a Duo Discus lost 4 m of wing, but was flown down to a controlled impact. However I did read of the incident where a brand new Duo’s wing fell clean off! It was also of note how many gliders strike terrain in the Alps, often after being caught in orographic downdrafts.)

I think the subject of “when to hold comps” deserves a post all of its own, so I’m giving it one. The first thing to say is that the weather is obviously variable and no two years are ever exactly the same. However some kinds of weather are more likely than others at certain times of the year, and if you’re planning an event beyond the forecastable-period (about three weeks), these patterns provide some guidance. (You’ll note, too, that throughout this I make no reference to 2007’s weather—-the weather of one year means nothing. This is all based on long-term averages—-at least thirty years, and often much more.)

Let’s start with some basic stats. Below is the 30-year-average (1971-2000) May-September month totals for first sunshine (hours) and then rainfall (mm) for England only, courtesy the Met Office.

As you can see, May is actually the second sunniest month of the summer (and indeed the year), and it’s also the driest, seeing few Atlantic fronts and not yet warm enough for heavy thunderstorms. However a month is a long time and England is a big place, so those numbers hide some of the patterns that we see.

While the soaring season starts in March, it tends to be a period of isolated good days due to passing ridges of high pressure, rather than any periods of sustained settled weather. May is the first month where the weather settles down for summer. This is the time of the year where there’s the least difference between sea and land temperatures, and you can often play “spot the isobar” with the synpotic charts. As a result, late April through mid-May is when north-easterlies are most likely, often bringing low cloud to the east of the country. However for the west it’s actually the sunniest time of the year. Later in May the weather tends to become quite settled. I suggest that the summer half-term is a very good time to hold a competition or at least a task week, especially for clubs in central or western areas.

Moving into early June the settled weather often persists. However late June and early July quite frequently (around 75% of the time) sees a run of poorer weather with westerlies moving through—-the “European Monsoon”. This is a result of a series of teleconnections around the world changing the flow of the jetstream over us—-the snows of North America tend to melt at this point, and the Asian jet takes a left and crosses north of the Himalayas. So maybe late June/early July isn’t a great time to hold a comp. Or a tennis championships.

Through mid-July into mid-August we are into the Dog Days of summer, so named as Sirius rises and sets with the sun (the theory was that Sirius gave extra heat!). The weather patterns of this time tend to persist for several weeks - St Swithin’s legend is based on a grain of truth. Whilst the weather in this period is not always great, if it is good, it is often as good as it gets. This is a good time to hold the most popular comps. Seven day comps may well be a good idea for this busy period, as the likelihood of good weather at this time of year is sufficient that seven days should always be enough to get several decent scoring days. Perhaps some smaller regionals should consider this.

Late August and early September often see good weather whatever preceeded it, with early September often being dry and settled as high pressure builds—-like May, a good period to hold a competition. However by late September the days and nights become equal in length and the northern hemisphere of the planet begins to steadily cool. Westerlies start to dominate again, and the first storms of the season often arrive, frequently fuelled by the remnants of Caribbean hurricanes embedded within them. Time to start planning those wave and ridge trips…