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Seeking Nirvana #4

Copyright Dixon White September 29th, 2003

Putting the pieces together.

I was on launch watching the conditions and a gaggle of pilots already in the air with a 25 year veteran hang glider pilot and he asked me why I wasn’t in the air. He listed a couple of reasons he had chosen not to fly, the upper levels were blowing the exact opposite direction from what we had on launch, according to the screaming cummies and the NWS report, and there were lenticulars. I added that the gust differentials are at least 15 mph within 5 seconds and it was really switchy. We stood for about 10 minutes not saying anything else. Finally, after watching the gaggle of hangies and para pilots fly with joyous abandon, he said, “Don’t you wish we didn’t know as much about the weather as we do, that we could be so naive?” Even though he said this, and I must admit I know where he’s coming from, neither of us would give up our ability to find reasons NOT to fly. He’s a brilliant hang glider pilot with many note worthy flights. I’ve just crossed over 7,300 flights, at least 99.9% of them were wonderful, so despite the fact we may have not chosen to fly that afternoon, and nothing bad happened, being cautious and respectful of what might happen is a healthy habit.

There are some paragliding sites that are consistent and simple. Very few bits of data need to be interpreted in order to “know” that you’re in a relatively “safe” environment. Don’t take this the wrong way, there’s always at least a few important things to consider at every site, so don’t simply fly without having some level of specific weather knowledge. In general, low elevation, humid sites, like coastal areas, are much more simple than high altitude mountain or desert sites particularly demanding when subjected to a dry atmosphere. Many pilots don’t have access to simple straight forward sites, particularly those that live in the mountains. So, these pilots, if they want to stay on the “smart” side of their flying, need to do their homework. If we are going to fly high desert/mountain sites we have to recognize the diversity of conditions that can exist. Although a coastal pilot may feel overwhelmed by the nuances of conditions that are found in the mountains, most mountain/high desert pilots become addicted to analyzing all the variables that can “cook up” a brilliant day of flying. Mountain/desert pilots sometimes comment that they are a little bored by coastal/low elevation sites. – although this kind of flying can be a guaranteed sweet relief to anyone who’s been tossed violently around the sky when they’ve misinterpreted their home site conditions. For those that want huge altitude gains and cross country routes, nothing beats choosing the right recipe for brilliant conditions and seeing their homework pay-off.

It’s important for you to understand the dynamics of your mountain/desert site on many levels and the power of thermal blocking is a significant issue. Maybe you’ve felt the gentle ebb and flow of cycles as thermals meander up though a launch, and in a “soft” atmosphere, this is just what the thermaling pilot wants. On the other hand, thermal blocking is so “real” and it can fool you into some pretty wild air if you haven’t thought through or researched some other variables. There’s a chance that prevailing synoptic winds and terrain forced flows are blocked from the surface, particularly in a low-pressure environment when there’s strong solar heating. When thermal blocking occurs, picture growing bubbles of heat and when they release we get entrainment of the ambient wind and thus a mixing that can be too turbulent for frail aircraft.

Picture a dome of heat acting like a terrain feature in its ability to block the ambient terrain based or synoptic wind. Until that dome “breaks” the relative atmosphere you are experiencing may seem quite tame. Having a sense of diurnal mountain winds, anabatic flows, and the effect of gaps, passes, and gorges mixed with regional pressure differentials may be an important analysis point. If it doesn’t make sense that conditions are calm, then you’re probably just getting a “sucker lull”. There certainly are some flying zones, those in basins or within treed low level hills or deep valleys that are resistant to ambient winds, and these can prove to be much more user friendly flying areas. Keep in mind that flying sites located downwind of mountains, especially those that are perpendicular to the prevailing wind flows, may be subject to spontaneous down drafts or gusts as a result of wave formation. These are called lee waves, mountain waves or orographic waves. Thermal blocking may very well help temporarily hide the effects from these waves. There have been spontaneous downdrafts so violent that whole forests have been smashed, particularly in areas where there are narrow gaps. Keep an eye on the inversion strength and upper level wind flows. When an inversion breaks it can power up, in almost a harmonic sense of escalation, a blast much stronger than the simple release of a thermal or the speed of the upper level wind flows. In the opposite sense of this situation, when we get pooling of cool air, i.e. in the evenings, the cool puddle of air may also cause blocking, but be aware, as with thermal blocking, the stability of air can shift and once again there can be entrainment of the upper level wind flows.

In many areas of the country a simple way to get a no-brainer data handle on possible strong blow through is to take a look at forecasted T.A.F. predictions for a nearby airport. Ask the briefer for the T.A.F. for a local airport, or even a couple nearby airports. T.A.F. stands for Terminal Aerodrome Forecast and basically lists expected wind flows and cloud cover throughout the day. As you get to know a site be aware of the T.A.F. forecasts and see if you recognize a correlation to your site. At some sites there isn’t any correlation, but at many you might see a pattern. We actually double the T.A.F. at one of our sites and it pretty much is on the money – if the forecast is for 240 degrees at 11 knots at noon we’ll see S.W. winds at 20 mph an hour before the forecasted prediction – almost like clockwork.

You can find T.A.F listing through the NWS websites and simply need to ask your local briefer for the web address if you have trouble locating the information.

The T.A.F. is just one resource. If you keep an eye on approaching fronts, isobar maps, the jet stream, upper level winds, the thermal index etc. you’ll anticipate the probability of blow through conditions – excessive turbulence is generally NO fun!!