Transition to large aspect ratio convection

We describe experiments with aspect ratio Γ in the range 44≤Γ≤90 that characterize the transition from moderate- to large-Γ convection. Up to Γ=60, the flows are steady until well above the convective onset. For Γ≥60, we find a slow periodic state near the convective onset. A clear rise in the noise floor is seen just above onset for Γ=70. For Γ≃90, the flows are irregular and time dependent arbitrarily close to the convective onset, and the amplitude of the noise signal is proportional to the Nusselt number.