>> Intro:
While cycling through the cornfields I recenly had an eureka moment 
when coming up with a really wild and crazy idea about what could be possible with underpressure based robust lighter than air metamaterial structures.
I regularly ponder about how AP technology could be applied to solve a number of problems.
The idea I had may solve at least three of them and opens up a whole bunch of other opportunities and interesting questions.
The three problems solved are:
A) The problem of keeping something stationary relative to the ground in a high up laminar large scale wind-current (e.g. CO2 collectors in the sky). This seemed to be impossible without expending energy to actively move against the current.
B) I thought about what is likely to replace today's mostly three bladed windmills that barely scratch/tap the lowest percent of the troposphere (100m of 10km). Obviously some silent sail like air accelerator/decelerator sheets/cloth/sails should become possible.
Future "power-windsails" may be quite a bit bigger than today's windmills but they still need to be linked to the ground for counter-force and counter-torque. To avoid excessively large bases advanced sail like wind generators probably would not be made excessively large (that is a large fraction of the 10km troposphere). Also giant towers permanently emanate the danger of them coming crushing down.
C) I thought about extracting the potential energy from rain-droplets: Clouldn't one look at clouds as almost everywhere available catchment lakes in the sky?
>> So here's the idea:
Specifically what came to me was to massively employ lighter than air structures in form of aerogel like "strings/filaments" (quite thick in diameter) that are tied/anchored/thethered to the ground and also intermeshed with themselves up in the sky. In the following I will refer to those structures as aerial meshes or airmeshes or airgrids. Keeping everything held at all times. This is kind of remotely similar to the principle of machine phase in the nanocosm and it too comes with a some advantages.
These structures seem to be easy to errect in giant scales. They could be applied for:
* aerial traffic
* large scale energy extraction
* and even reversely as means for super large scale strong weather control (ozone too)
Beside spanning "windsails" in the mesh loops of the "air grid" obviously "solar sails" are also possible.
Also there may be rains sails whick I'll explain later.
All sails could/should be equipped with temporary deployment capability and modes that let through part of the wind (lamellas?).
>> Wind-loads
Obviously one must worry about excessive windloads.
Even uncompensated advanced materials might be able to withstand windloads (estimations needed) the floating air strings / air filaments could be armed with a dense rope in the core. Assuming a density of 4kg/dm^3 a strong rope of about 1cm diamater needs to be embedded in an lighter than air string of at least about half a meter so that it starts floating.
To prevent getting critical loads and temporary collapse of the metamaterial due to windpressure making it temporarily non-buoyant there is the possibility of windload compensation.
Luckily with APM there's no additional cost making the whole surface an active "living" structures.
By integrating two other technologies windoads may be reducable to acceptible levels or even completely compoensatable.
Conveniently when there is windload there is also local power for the protection mechanisms.
Two main technologies usable for wind-load compensation are: (names freely invented)
A) "infinitesimalbearing parallel motionion cloaking"
B) "adiabatic normal motion cloaking"
A) "infinitesimalbearing parallel motionion cloaking" (this was presented by Josh Halls in his book "Nanofuture" as a means for propulsion) When air moves parallel to a surface the surface is moved with the same speed in the same direction. This replaces friction in air with much lower friction of "infinitesimal bearings" that are integrated in the air-vessels (or here air mesh strings) topmost surface layers.
B) "adiabatic normal motion cloaking"
When the aforementioned technique is used the air still needs to get out of the way sidewards of an obstacle.
While the aformentioned technology/technique can compensate for parallel air motion there still remains a motion component that is head on to the surface. Obviously this must be a motion of one period/impulse of incoming and then outgoing air in the frame of reference that is moving with the parallel motion compensation speed (I hope that formulation is sufficiently comprehensible).
What one would try here is to "grab" pockets of air compressing them down as they approach (this heats them up so they must be kept sufficiently thermally isolated to not loose their enegry) and then expanding them up again. This technique may be capable of reducing bow waves. (Though I'm rather wary about whether this could/would work or not.)
>> Robustness against lightning (and ice loads)
Obviously one must worry about lightning. There seem to be two polar opposite options.
A) Adding lightning protectors of highly conductive material. On a large scale this would probably be a bad Idea. They are likely to negatively influnece weather by quenching thunderstorms and air to ground potential in general.
B) Making the "air-strings" electrically highly isolating (not hard for an aerogel metamaterial out of high bandgap base material).
A thin layer of intermediately conducting water droplets that heats when lightning strikes (it converts to plasma and may damage the surface) may be avoidable by making the surfaces highly hydrophobic. As a nice side effect combined with small scale active surface movement this can also prevent any ice deposits and thus dangerously high ice loads.
A&B) A third option is to make the structures switchable between the two extreme states.
This may allow to extend the weather control to electric aspects of the atmosphere.
Avoiding long stretches of electrical conductors (km scale) generally seems to be a good idea.
By exclusively resorting to chemomechanical energy transmission one gets resillience against directly hitting solar storms (giant protuberances directly heading towards earth that would be devastating today due to induction of high voltages in long power lines) and maybe even even resilience against EMPs from not too near atomic blasts (that hopefully will never happen).
>> Exotic untapped energy forms:
There's a constant quite high electric field between ground and sky (aerostatic electricity).
I don't know how much energy is in there and what would happen if large fractions of this electric reservoire where to be extracted or boosted. There's some questionable science going on there with todays pretty limited technology.
Simple experiment:
A little more dangerous:
Slanted horizontal "sails" hanging below the clouds could be used like funnels guiding the rainwater to the "air-mesh-filaments" that then act like eavestroughs in the sky allowing to tap the full potential energy of rainwater. Then we wouldn't depend on a mountains with a suitable high up valley that can be blocked anymore.
Most of the rain must be redistributed at a lower level (like a shower head in the sky - rain sails ?!) to not negatively influence vegetation. Yes that sounds ridiculous but it might make sense.
>> The structure of the lifting metamaterial
For furter discussion of the limits of the technology I need to go a little more into the detail of the structure of the lifting metamaterial. These ultra light metamaterials are made out of cells with thin gas-tight walls and internal 1D trusses (possibly fractaly arranged) that prevent collapse from external pressure. Advanced surface functionalies of the airmesh strings are not located on every cell wall but on the outermost walls of a "sky string" or independent balloon. These outermost surface functionalities are not part of the base metamaterial. The "sky strings" have many basic cells throughought their diamater. The main function of the walls of each cell is just gas exclosure. This compartmentalisation that is finer grained than the whole air string gives some redundance and safety. If the metamaterial is made out of an uncombustible base material like sapphire then there is little to no chance that these structures come crushing down. Nice! The internal trusswork might be equipped with active components to adjust cell sizes a bit such that buoyancy can be adjusted. Too much buoyancy is bad too because of too much upward pulling force on the anchor points.
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