  While most ram pressure studies have focused on ram pressure stripping in
galaxy clusters, we devise a novel approach based on a kinematic measurement of
ram pressure perturbations in HI velocity fields for intergalactic material
(IGM) densities and relative velocities that are one to two orders of magnitude
lower than in galaxies showing ram pressure stripping. Our model evaluates ram
pressure induced kinematic terms in gas disks with constant inclination as well
as those with a warped geometry. Ram pressure perturbations are characterized
by kinematic modes of even order, m=0 and m=2, corresponding to a ram wind
perpendicular and parallel to the gas disk, respectively. Long-term
consequences of ram pressure, such as warped disks as well as uncertainties in
the disk geometry typically generate uneven modes (m=1 and m=3), that are
clearly distinguishable from the kinematic ram pressure terms. We have applied
our models to three nearby isolated galaxies, utilizing Markov Chain Monte
Carlo fitting routines to determine ram pressure perturbations in the velocity
fields of NGC 6946 and NGC 3621 of ~30km s$^{-1}$ (effective line-of-sight
velocity change) at HI column densities below (4-10)$\times$10$^{20}$cm$^{-2}$
(at radial scales greater than ~15kpc). In contrast, NGC 628 is dominated by a
strongly warped disk. Our model fits reveal the three-dimensional vector of the
galaxies' movement with respect to the IGM rest-frame and provide constraints
on the product of speed with IGM density, opening a new window for
extragalactic velocity measurements and studies of the intergalactic medium.
