Convergence in the Sistema Central

Introduction

Back in 1996, Skywings published my article on convergence in the sistema central. The ideas came from observations since my move to Piedrahita in 1991, and the laboratory of competition: Setting hundreds of the worlds best pilots upon tasks to test the theory. The latter a little like throwing handfuls of coloured confetti into the air to watch the wind patterns. Since '96 we have had 3 more pwcs, the Europeans, and countless more national events, and with the bones of the convergence idea have been able to set more effective tasks. Likewise on a daily basis I am able to investigate the phenomenon with my XC groups helping to build up a more accurate picture.

The '96 article gives the feeling of those days gone by when we all wore purple and pink. It was a time when the world open distance record would still have been possible on the Spanish mainland and my focus was very much upon that. The convergence across central Spain is perhaps singular in the world because of the long length of a relatively narrow range of mountains with limited lateral ramifications, separating ample high plains which often have different climatic conditions. Its length from from Plasencia to Moncayo is approx. 400km and one of the places in the world where more yo-yo circuit flights of more than 1000 km sailplane flights have been made. As well as the principle convergence line, there are also some other convergence effects running at right angles to it, which although much smaller, give us important lines of lift allowing us to run upwind for triangle tasks.

If the central system were completely straight and uniform in altitude and width, on any particular day the convergence would run neatly parallel to it and we could trim our glider to fly in a straight line 400km from one end to another and back again (if we have enough speed) with little need to circle at all. However, the geography of the landscape is far more complex, with the air mass moving through and over passes, irregularities and changing levels of the plains. This shifts the convergence further north or south and sometimes prevents it. So the real picture although near to the ideal, is really one of undulating lines of lift, broken by gaps and lateral shifts.

The convergence effect is generally found on the mountains and on the northern plains. Fuentemilanos is the principal site for Sailplane activity on the north side, and over 100km east of Piedrahita towards Segovia. Their main area activity is the Guadarrama range, the central section of the chain north of Madrid. On good days they regularly fly west of Piedrahita, but normally would return at the Villatoro pass if the day quality is poor. A recent book published by Glider pilot Carlos Gómez-Mira(Volando a vela por la sierra central) covers in much detail flying around that area and further east to the limits of the chain at Moncayo. His own descriptions of flying the range back up my own theories and fill in a lot of detail which I have been unable to observe regarding flying conditions after Riaza (200km along from Piedrahita) where I rarely get to fly.

If you are limited to foot launching there are a number of sites along the chain, El Pitolero(SE/NW) some 50 km east of Piedrahita, Lastra(S.S/W) Chia( S. S/E) and Peña Negra(NW), close to Piedrahita, and Arcones(NW) and el Nevero(S) some 150km to the east.. Take offs on the southern side of the Gredos (Pedro Bernardo, Piedraslaves) tend not to be effective sites for accessing the convergence.

It is best if we divide up the range into sections, both for clarity, and also since conditions often change dramatically from area to area. Map 1 shows most of the length of the central system, The blue areas show an approximate idea of the convergence on a south westerly wind day. The mauve areas mark some lateral convergence effects we can also use during north to south east wind days. Red spots mark foot launch take offs. We will start in the west and mover eastwards examining each area in detail with regard to convergence effects.

Plasencia to Tornavacas

From Plasencia, our main retrieve highway, the N-110, runs up the Jerte Valley, within high mountains, with those to the north tending work better. This valley usually has poor conditions and few landing fields (terraced mountain slopes full of cherry trees). If clouds are present, they are often only present beyond our first possible take of point (El Pitolero), becoming better formed towards the Bejar/Candelario range. The latter often the first to form cloud on the whole western system, and often the first to form Cbs. With a north or north easterly wind, when we can imagine ourselves flying towards the east from Peña Negra (Piedrahita), it is common to find a convergence at the Tornavacas pass. This from the westerly valley breeze in the Jerte valley and the geosrtophic and valley Breeze coming along the valley from Barco de Avila. This is shown on map 2. The geostrophic wind shown with large green arrows, and the lower breezes shown with red arrows. The turquoise area is the convergence area. This section of lift can be used to fly up the lee side of the Sierra de Bejar from TP 19. With this flight you in a convergence neatly marked by the frontier of the province of Avila and Caceres, and then once on the Bejar range, you can fly eastwards again along the Avila/Salamanca frontier. The Tornavacas pass (TP19) marks the beginning/end of the high plains and you will generally see a marked difference in the air quality when you look down in to the Jerte valley: haze and much reduced Cu´s. Your best option is to run north along the convergence to the Bejar range, to then either continue west or east from there.

Tornavacas to Villatoro

Map 3 centres on the Corneja (Piedrahita) valley and the Villatoro pass separating it from the Ambles (Avila) valley. The Villatoro pass has achieved a mythical status in the free flying world, though often wrongly considered a difficult spot. The Villatoro pass is often last and only contact of the mighty south west wind convergence with the high mountain range after which it runs along the shallow hills of the Sierra de Avila and from then on stays well to the north of the mountain range. For a foot launched glider this is a key position, allowing our transition from the mountains to the easy run across the plains in the convergence.

On our perfect forcast day with light soutwesterly winds, Cu´s and a high base, we will be able to climb out from Peña Negra (earlier in Lastra del Cano), follow a well formed cloud street along the mountains, or a little further out, Make an easy crossing from the end of the Piedrahita chain across to the spines running up to the Villatoro pass (perhaps the most critical transition of the day with the chance of a bomb out before the pass around Villafranca). You will then climb at the pass and probably cross to the convergence formed and running north of the the N-110 highway for the next 300km (allowing for some breaks and north or south shifts along the way).

With a light northerly through to south easterly it is seldom worth continuing east of the pass, and even on a light north westerly day the wind beyond Muñogalindo (about 25km before Avila) will become east low down. With these conditions (except NW) we can plan on returning back over the pass after going into the Ambles valley, or we can use the line of convergence to run to the North west from the pass for triangle flights. The convergence are is marked in turquoise on map 3, and the blue area indicates the convergence earlier in the day, which often moves north during the afternoon. Depending upon where the clouds are marking the area during the morning, we can choose either turnpoints at TP11.12 or 13. If you get to 11 or 12 to late, you may find the convergence has already moved north, leaving you with a headwind. The classic route the it to TP 16, then back to TPs to the Piedrahita valley with a tailwind. An 80km triangle can be easily achieved with 2 legs of convergence and a tailwind return.

Villatoro to Avila

Map 4

The section from Villatoro to Avila is normally run along the south side of the Sierra de Avila on a classic SW day (map 4), but conditions right across the valley to the higher mountains (Sierra de Paramera) generally work well. However, if you are looking to go a long way stick north of the N-110. Usually there is a shift to the north at Avila

Understanding the convergence will improve your flights in this area. Wave effects across the central chain are also an important aspect of soaring here.