  The yrast states of nuclei that are spherical or weakly deformed in their
ground states are described as quadrupole waves running over the nuclear
surface, which we call "tidal waves". The energies and E2 transition
probabilities of the yrast states in nuclides with $Z$= 44, 46, 48 and $N=56,
~58,..., 66$ are calculated by means of the cranking model in a microscopic
way. The nonlinear response of the nucleonic orbitals results in a strong
coupling between shape and single particle degrees of freedom.
