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Weather
WEATHER > Honza Rejmanek
Glass-off Mechanics Originally published in Cross Country Magazine
:Numerous flying sites are known for their glass-off conditions. Glass-off is characterized by widespread, gentle lift that is just barely stronger than the sink rate of the glider. Within the wide area of lift, there are slightly more buoyant patches. These weakening thermals can justify turning circles for those pilots that are eking out the last miles of a cross-country flight and are hoping to top off for one final crossing. For less ambitious pilots, it is fun to just boat around and enjoy the scenery. Glassoff is also sometimes referred to as magic-air or wonder-wind. In French, it is called “restitution.”
Soaring textbooks and ground school courses often offer very simple explanations for this phenomenon. This might be because no one truly understands the complex dynamics of this late afternoon transition. Research meteorologists are only beginning to decipher the dynamics of late afternoon and evening transitions that occur in the boundary layer. Discussions on soaring forums regarding glass-off also present a diverse mix of speculation, theory, and lore. One reason why a sound explanation remains elusive is that this soaring marvel that pilots call glass-off is not necessarily a single phenomenon in all locations.
The evolution of gentle, widespread evening lift at a particular flying site depends very much on the topography and surface characteristics of the site. However, regardless of the flying site, certain basic meteorological principles have to be considered when attempting to elucidate the cause of widespread, gentle lift in the evening.
Most importantly, the concept of mass continuity has to be considered when pondering the plausibility of a mental model. Mass continuity is just a fancy way of saying that we cannot create a vacuum in the atmosphere. Any amount of air that leaves one location has to have the same amount of air move in from elsewhere. There are no air pockets. In the case of glass-off, sustaining a wide area of air rising at 1-2m/s will, by its very nature, require a significant amount of surface convergence. This will need to persist for half an hour or more to claim that the site really glasses-off.
It is worth pondering the simple and often repeated explanation that a slope falling in the shade starts to produce a katabatic, downslope flow, resulting in glass-off conditions on the opposite sunny slope. The shortfall of this explanation is that the depth of this late afternoon downslope flow is fairly shallow, and even as it begins to fill in and stabilize the valley floor, it alone cannot make up for the large volume of air rising out over the sunny slopes.
It is worth considering what else has happened over the shady slope. The layer of air next to the surface is stable and negatively buoyant. This is why it starts to flow downslope. It also means that the shady slope (and eventually the valley floor) is no longer a region that is producing thermals. The stable surface air is decoupling from the air above it. All the rising air in the area that is glassing-off eventually approaches the capping inversion and has to diverge and eventually sink. A large, cool region that is decoupled and no longer producing thermals is an optimal region for receiving this return flow.
Next time you are making porridge on the stove, take a moment to study the rising and sinking areas in the pot. Now move the pot so that it is well off-center. Please take care not to spill hot porridge on yourself. You might notice that this allows more clustered convection in the corner that is left over the burner. The part shifted away from the flame is now more accepting of the widespread return flow.
A complementary mechanism for widespread, gentle
lift might be a convergence of valley winds that sets up late in the day. Topography that channels branches of valley winds toward each other can make certain flying sites prime for such convergence. If the day has been thermally active, then the temperature profile of the air within the valleys will be very close to dry neutral because convection does a good job mixing the air in the boundary layer. Dry neutral refers to a lapse rate of 1 degree Celcius cooling per 100m elevation gain. This is the same cooling rate as that of air that is being lifted. This means that converging valley winds or those impeding the back of a box canyon will be happy to rise and provide areas of widespread lift. Of course, just as the thermals throughout the day eventually reached the capping inversion, the same vertical restriction will apply to dynamically lifted or converging valley winds. The capping inversion is a soft celling. The underlying terrain is a hard floor. All that goes up must come down in between these two surfaces. All that converges below a lifting region has to diverge at the capping inversion. This is where this ties into the discussed idea of providing for return flow. If large shaded and stabilizing surface areas develop in the late afternoon, then the return flow is free to sink over these regions. This might not have been the case earlier in the afternoon while they were still sunlit.
Lastly, it should be noted that glass-off conditions are not always limited to a valley system. A region of higher ground such as a table mountain or a mesa can have a rim that faces west. If dry, rocky terrain intercepts the rays of the afternoon sun, then it can act as a source of buoyancy later than the surrounding flats. Out over the flats, a west wind can start to decouple from the surface as stable, cool air pools start to form. The wind can hit the west-facing rim and be deflected upward for quite a ways. With a neutral lapse rate, wind that is given an upward tilt is happy to continue upwards. This works best when the wind is fairly light. The exact dynamics of late afternoon transition in the convective boundary layer are still an area of active research. Late afternoon is a time of day when certain regions fall in the shade, and the surface air decouples from the flow above. Thermals get weaker and, in some cases, broader, especially if there is a supply of converging air to keep them going. Some fortunate flying sites glass-off! Even if glass-off is a phenomenon that is hard to pin down, it is certainly worth enjoying!
Parting is such sweet sorrow.
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