W E A T H E R > Honza Rejmanek
Glass-off Mechanics Originally published in Cross Country Magazine : Numerous flying sites are known for their glass-off the same amount of air move in from elsewhere. There conditions. Glass-off is characterized by widespread,
are no air pockets. In the case of glass-off, sustaining a
gentle lift that is just barely stronger than the sink rate of
wide area of air rising at 1-2m/s will, by its very nature,
the glider. Within the wide area of lift, there are slightly
require a significant amount of surface convergence.
more buoyant patches. These weakening thermals can
This will need to persist for half an hour or more to claim
justify turning circles for those pilots that are eking out
that the site really glasses-off.
the last miles of a cross-country flight and are hoping to
It is worth pondering the simple and often repeated ex-
top off for one final crossing. For less ambitious pilots, it
planation that a slope falling in the shade starts to pro-
is fun to just boat around and enjoy the scenery. Glass- duce a katabatic, downslope flow, resulting in glass-off off is also sometimes referred to as magic-air or won- conditions on the opposite sunny slope. The shortfall of der-wind. In French, it is called “restitution.”
this explanation is that the depth of this late afternoon
Soaring textbooks and ground school courses often of- downslope flow is fairly shallow, and even as it begins to fer very simple explanations for this phenomenon. This
fill in and stabilize the valley floor, it alone cannot make
might be because no one truly understands the complex
up for the large volume of air rising out over the sunny
dynamics of this late afternoon transition. Research me- slopes. teorologists are only beginning to decipher the dynam-
It is worth considering what else has happened over
ics of late afternoon and evening transitions that occur
the shady slope. The layer of air next to the surface is
in the boundary layer. Discussions on soaring forums
stable and negatively buoyant. This is why it starts to
regarding glass-off also present a diverse mix of specu- flow downslope. It also means that the shady slope (and lation, theory, and lore. One reason why a sound expla- eventually the valley floor) is no longer a region that is nation remains elusive is that this soaring marvel that
producing thermals. The stable surface air is decoupling
pilots call glass-off is not necessarily a single phenome- from the air above it. All the rising air in the area that non in all locations. The evolution of gentle, widespread evening lift at a
is glassing-off eventually approaches the capping inversion and has to diverge and eventually sink. A large, cool
particular flying site depends very much on the topog- region that is decoupled and no longer producing therraphy and surface characteristics of the site. However,
mals is an optimal region for receiving this return flow.
regardless of the flying site, certain basic meteorolog-
Next time you are making porridge on the stove, take
ical principles have to be considered when attempting
a moment to study the rising and sinking areas in the
to elucidate the cause of widespread, gentle lift in the
pot. Now move the pot so that it is well off-center. Please
evening.
take care not to spill hot porridge on yourself. You might
Most importantly, the concept of mass continuity has
notice that this allows more clustered convection in the
to be considered when pondering the plausibility of a
corner that is left over the burner. The part shifted away
mental model. Mass continuity is just a fancy way of say- from the flame is now more accepting of the widespread ing that we cannot create a vacuum in the atmosphere. Any amount of air that leaves one location has to have
38
US HP A P I L O T
return flow. A complementary mechanism for widespread, gentle
