In this work, the problem of the transient heat and mass transfer and stability within a salt-gradient solar pond has been numerically investigated by mean of a 3D-model with all properties variable as function of temperature and salt concentration. The pond has been subject to real external perturbations resulting from the variations of the local weather conditions. The initial conditions correspond to an 'artificially stabilized' pond with the linear temperature and salinity profiles in the gradient zone. Numerical results as obtained for a Tunisian experimental test-site have been satisfactorily compared and validated against measured temperature data. They have clearly shown that after a relatively short period of operation, say only a few days, the changes in the weather conditions have produced important effects not only on the internal temperature field of the pond, but also on its stability as well. In general, one may expect that some instability could develop and growth from within two critical zones: the first one beneath the water free surface and the second one in the region immediate to the bottom surface. The transient behaviours of the pond, in particular its stability, have also been investigated for a very short period of time, say 24 hours of operation. It has been clearly observed that the overnight cooling at the water free surface may rend unstable the immediate region beneath that surface. On the other hand, the solar heating effect during the daytime may have adverse effect on the stability near the bottom surface. Finally, results have also shown that the water transparency has an important effect on the pond stability itself. A pond with good transparency water has been found to be more susceptible to instabilities than a poor transparency one.