Onset of convective time dependence in cylindrical containers

Thermal measurements probe time dependence in a convecting helium layer with cylindrical geometry and aspect ratios Γ≡radius/height in the range 2.4<Γ<22. For all Γ, there is a first transition at a Rayleigh number R₁>Rc where convection begins at Rc. Time dependence occurs for R>R₁ when Γ4. The character of the transition at R₁ is identified over the full range of Γ. As Γ becomes large R₁Rc→1.09, a value which is difficult to reconcile with predictions for the expected skewed varicose instability.