A quasi-static theory is derived for the relaxation of the velocity held resulting from a temperature change in an infinite vertical fluid layer when the temperature gradient is perpendicular to the gravity vector. Taking the viscosity temperature dependence into account, experimental results fit remarkably well the reference data for the thermal diffusivity of the studied systems. The laser-Doppler velocimetry (LDV) technique, applied to transient velocity measurements, is thus recommended to determine the thermal diffusivity in fluids.