Lagrangian motion in a quasi-two dimensional time dependent, convective
flow is studied at different Rayleigh numbers (Ra). Particle Tracking
Velocimetry technique is used to reconstruct Lagrangian trajectories of
passive tracers. In the investigated range of the control parameter
(6.87·10^7;2.17·10^9), we study an intermediate regime in which an
almost periodic Eulerian flow is observed. It is well known that in this
regime, Lagrangian motion of passive particles can be very complex due
to chaotic advection. As a matter of fact, particle trajectories can
display Hamiltonian chaos and, therefore, strong sensitivity to initial
conditions. Dispersion phenomena occurring in the tank are investigated
in a Lagrangian framework. The classical way of looking at the relative
dispersion by computing average separation at fixed time is compared
with an alternative method known as Finite Size Lyapunov Exponent (FSLE)
technique consisting in the evaluation of average rate of particle 
separation time at fixed scale. 

The obtained results can be exemplifying in approaching geophysical
problems characterised by a not sharp separation between the scale of
the velocity field and the dimension of the domain allowing us to
discuss about the better way of describing spreading of pollutants in
closed basins.