36-variable model of atmospheric dynamics on a latitude circle — chaos onset at forcing F ≈ 5.7
Lorenz (1996): dXₙ/dt = (Xₙ₊₁−Xₙ₋₂)Xₙ₋₁ − Xₙ + F. The quadratic term models advection, −Xₙ is dissipation, F is external forcing. For F > 5.7, Lyapunov exponent becomes positive — the "butterfly effect" makes weather prediction fundamentally limited beyond ~2 weeks.