  We use SDSS imaging data in $ugriz$ passbands to study the shape of the point
spread function (PSF) profile and the variation of its width with wavelength
and time. We find that the PSF profile is well described by theoretical
predictions based on von K\'{a}rm\'{a}n's turbulence theory. The observed PSF
radial profile can be parametrized by only two parameters, the profile's full
width at half maximum (FWHM) and a normalization of the contribution of an
empirically determined "instrumental" PSF. The profile shape is very similar to
the "double gaussian plus power-law wing" decomposition used by SDSS image
processing pipeline, but here it is successfully modeled with two free model
parameters, rather than six as in SDSS pipeline. The FWHM variation with
wavelength follows the $\lambda^{\alpha}$ power law, where $\alpha \approx-0.3$
and is correlated with the FWHM itself. The observed behavior is much better
described by von K\'{a}rm\'{a}n's turbulence theory with the outer scale
parameter in the range 5$-$100 m, than by the Kolmogorov's turbulence theory.
We also measure the temporal and angular structure functions for FWHM and
compare them to simulations and results from literature. The angular structure
function saturates at scales beyond 0.5$-$1.0 degree. The power spectrum of the
temporal behavior is found to be broadly consistent with a damped random walk
model with characteristic timescale in the range $\sim5-30$ minutes, though
data show a shallower high-frequency behavior. The latter is well fit by a
single power law with index in the range $-1.5$ to $-1.0$. A hybrid model is
likely needed to fully capture both the low-frequency and high-frequency
behavior of the temporal variations of atmospheric seeing.
