  Consider the indicator function $f$ of a two-dimensional percolation crossing
event. In this paper, the Fourier transform of $f$ is studied and sharp bounds
are obtained for its lower tail in several situations. Various applications of
these bounds are derived. In particular, we show that the set of exceptional
times of dynamical critical site percolation on the triangular grid in which
the origin percolates has dimension 31/36 a.s., and the corresponding dimension
in the half-plane is 5/9. It is also proved that critical bond percolation on
the square grid has exceptional times a.s. Also, the asymptotics of the number
of sites that need to be resampled in order to significantly perturb the global
percolation configuration in a large square is determined.
