Wind power scales with the cube of velocity. A surface turbine sees ~10 m/s; the jet stream at 10 km altitude routinely rips at 50–100 m/s. That cube relationship is the whole story — and the reason engineers keep returning to the idea of tethered airborne wind energy systems (AWES).

The power density gap is brutal:

• Surface wind, 10 m/s, ρ = 1.225 kg/m³: P/A = ½ρv³ ≈ 0.6 kW/m²
• Jet stream, 50 m/s, ρ ≈ 0.4 kg/m³ at 10 km: P/A ≈ 25 kW/m²
• Jet stream peak, 100 m/s: P/A ≈ 200 kW/m²

So a kite at altitude sees 40× to 300× the wind energy flux of a ground turbine. A modest 500 m² wing at 25 kW/m² intercepts 12.5 MW. Apply the Betz limit (59.3%) and realistic system efficiency (~50% for yo-yo generators), and you get ~3.7 MW per kite — comparable to today's biggest offshore turbines, but with no tower, no foundation, and no nacelle hanging 150 m in the air.

The tether is the engineering centerpiece. At 100 m/s, aerodynamic drag on a 10 mm Dyneema tether (10 km long, C_d ≈ 1.0) is:

F_drag = ½ × 0.4 × 100² × 1.0 × (0.01 × 10000) = 200 kN

Dyneema's tensile strength is ~3 GPa. A 10 mm rope has ~78 mm² cross-section → ~235 kN capacity. Just enough. The tether mass for 10 km is only ~760 kg — trivial compared to the wing's lift budget.

Yo-yo mode is the elegant trick: the kite flies a crosswind figure-eight, the tether reels out under huge tension while driving a ground-based generator, then the kite depowers and reels back in. Power flows down a rope, not a copper wire. No 10-km-long superconductor required.

Scaling to civilization: Humanity uses ~20 TW continuous. At 3.7 MW per kite and 40% capacity factor (jet streams meander), you need:

20 × 10¹² W ÷ (3.7 × 10⁶ × 0.4) ≈ 13.5 million kites

That's a lot — but for comparison, we manufacture ~80 million cars per year. The kites themselves are cheap: ~500 m² of carbon-fiber-reinforced fabric is maybe $50k in materials. The hard parts are the ground stations and grid integration.

The fatal complication is airspace. The jet stream lives at 8–12 km — exactly where commercial aircraft cruise. Each tether is a 10-km vertical guillotine. You'd need exclusion cylinders ~50 km wide, evacuating perhaps 5% of usable airspace globally. A single tether failure at 100 m/s releases a kinetic-energy bomb: a 2-tonne kite moving at jet-stream speed carries 10 GJ — equivalent to ~2 tonnes of TNT. So you site farms over oceans, deserts, and polar regions, then run HVDC lines to the load.

Iceland-to-UK HVDC already exists at 1200 km. A North-Atlantic AWES farm under the polar jet could plausibly deliver 500 GW to Europe — about its entire electricity demand — from a sea area the size of Ireland.