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Convergent, Divergent, Mixed-Up Air

Copyright 4/13/03 Dixon White

Seems like almost every new pilot says that they wish they had goggles for seeing the movement of the air. I think we can develop something of a third sense of how the air can move and then intuitively have “goggles”. We can keep in mind many of the different influences that affect the air - observe the layout of the land, the different terrain features and sense the glider’s immediate behavior within the air. The flow of the air reminds me of the ocean. It can be smooth, rippled, choppy, swelling and crushing. There is the saying that, “It’s better to fly either in thermals or in wind, but combining the two can get you more than you bargained for”. Let’s spend some time looking at how complex the air can be as different pieces of air converge or diverge.

At any one time you may be surrounded by air that is moving for one reason or a combination of reasons. Air may move on a large scale because two air masses (low and high pressure systems) are pushing against each other (which would make it “regionally windy” check an isobar map), localized anabatic or catabatic flows, rising pockets or zones of heat, outflows from storms (gust fronts) and the huge truck that drove past your landing area as you made your approach. You may also find yourself in rotor, air that is turbulent from passing around hills, trees, or houses. (Possible graphic here of someone landing with a thermal taking off, rotor from a house, a truck going past and a large wave of upper level air crashing down)

As valleys heat up they generally start a wind flow moving up the valley to higher elevations. In general you can count on landing facing up valley early in the morning while there’s still cool air flowing down valley and then land facing down valley when there’s been enough heating for warm air to start flowing up the valley. Heat flows upward, since the whole valley is heating it’s almost like a chimney of heated air flowing up the valley, and it can get very strong, particularly when the Thermal Index (T.I.) is strong and there’s high pressure and the valley is more narrow. Of course some of the heating breaks lose and moves straight up, isolated thermals, and some of the sunward facing slopes will have localizes anabatic flows upward, but, in general, there will be a strong up valley flow. Suppose there’s an upper level wind, the morning sun heats the ground which in turn heats the air and temperatures equalize the atmosphere to the point that the upper level winds aren’t held at bay by subtle temperature stratification of the atmosphere, so the wind can try fill in the atmosphere all the way to the ground. In other words, the wind that you noticed on the “upper level winds” National Weather Service data are mixing down to the surface because the inversion has broken, or “coupled” as the NWS staff call it. When the upper level winds are directionally opposed to the valley wind they can set-up a convergence point, a point at which they meet and create an area of buoyancy, lift. There can be many different convergences:

  1. The valley winds are running into the prevailing winds.
  2. The valley winds run into a gust front from a storm cell.
  3. The valley winds also run into a thermal.
  4. The valley winds run into anabatic flow on slope.

Anytime one piece of air runs into another piece of air you have convergence, and this can be useful to the soaring pilot. Let’s use this same thinking for coastal flying.

  1. The sea breeze runs into a prevailing opposed breeze.
  2. The sea breeze runs into a gust front from a storm cell.
  3. The sea breeze runs into a thermal.

In addition, you can have thermals combining with the anabatic flows up a slope for convergence. This can be an explanation for evening soaring sessions as well. There can be tremendous heating throughout the day, so much heating that the upper level winds are lifted away from the surface. Late in the afternoon the heating subsides and the upper level winds mix back down and converge with the exhausting of accumulated heat to create super buoyancy.

We can also learn to predict zones that might be full of divergent air, air that sinks. If you have zones of lifting air you are going to have zones of sinking air. In general, if in the mountains and you want to go up, you need to follow along spines and ridges, you will avoid the sinking air in the center of the valleys. If over flat lands you will want to identify potential “wicks” – fence lines, roads, tractors working a field, power lines, tree lines, etc. that will be potential release points for accumulated heat. You will fly “fast” through the non-lifting zones and slow down in the buoyant lifing zones, making circles when the lift is clearly happening.

In some cases the air all gets long just fine, it works in concert, but in others, it can get downright nasty. One wonderful, but potentially nasty converging event of air masses is the famous Lake Elsinore, California convergence. Here we have an East facing ridge taking morning sun and thus producing an upslope anabatic flow with strong thermals. The Westerly winds eventually push through from the coast and converge with these flows. Mitch McAleer, one of the most experienced local pilots to the area describes this regular occurrence in greater detail, “The lift is gathered somewhere near the smog line, sometimes right in the haze sometimes out away from it. On early spring days when there's still a lot of water in the area, clouds will mark the convergence line making it easy to spot and fly in the abundant lift. In the summer when it's dry, the line will be less obvious but still usually marked by haze domes and water vapor up to 5000 feet to 8000 feet asl to the west with clear air to the south. During the day the haze line will be in constant motion, usually moving north as the southerly winds increase as the heat increases and the pressure drops, sometimes it makes abrupt turns, it's influenced by the local weather, large clouds and down-bursting cells from over-development. Once away from the Elsinore range, the flying is mostly flatland style thermaling, with some 1000ft. rocky bumps making good triggers, and setting up good lee side thermal sources.

Do your homework on the potential prevailing winds and thermal index before you fly so that you can make sense of the air that’s working around you. Put an eye on the terrain and think through how, depending on sun angle and slope angles, air is going to flow. There’s a great deal to consider, but eventually, with practice, you’ll be able to “see” the air.