Fluctuations in a fluid with a thermal gradient

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We report measurements of the temperature fluctuations in a horizontal layer of a pure fluid confined between parallel plates and heated from below. Consistent with earlier work we found that the structure factor Sₜ(q) (the square of the modulus of the Fourier transform of the temperature field) was consistent with Sₜ(q) q⁻⁴ for large horizontal wavenumbers q. As q decreased, S(q) increased less rapidly, passed through a maximum, and then approached zero. The results agree qualitatively, but not quantitatively, with the predictions of Ortiz de Zárate and Sengers.