  The diffuse interstellar medium is dynamic, and its chemistry and evolution
is determined by shock fronts as well as photodissociation. Shocks are implied
by the supersonic motions and velocity dispersion often statistically called
"turbulence". We compare models of magnetohydrodynamic (MHD) shocks, with
speeds typical of cloud motions through the ISM (3-25 km/s) and densities
typical of cold neutral gas (~ 100 cm-3), to archival observations of the
atomic hydrogen 21-cm line for gas kinematics, far-infrared emission for dust
mass, and mid-infrared emission for high-resolution morphology, to identify
shock fronts in three high-latitude clouds pairs with masses of order 50 suns.
The clouds have `heads' with extended `tails', and high-resolution images show
arcs on the leading edges of the "heads" that could be individual shocks. The
HI shows higher-velocity gas at the leading edges due to shock-accelerated
material. For two cloud pairs, one cloud has an active shock indicated by broad
and offset HI, while the other cloud has already been shocked and is
predominantly `CO-dark' molecular hydrogen. Two-dimensional MHD simulations for
shocks parallel to the magnetic field for pairs of clouds show a remarkable
similarity to observed cloud features, including merged "tails" due to aligned
flow and magnetic field, which leads to lateral confinement downstream. A
parallel alignment between magnetic field and gas flow may lead to formation of
small molecular clouds.
