  We have used time-resolved spectroscopy to measure the colour dependence of
pulsation amplitudes in the DAV white dwarf G 29-38. Model atmospheres predict
that mode amplitudes should change with wavelength in a manner that depends on
the spherical harmonic degree l of the mode. This dependence arises from the
convolution of mode geometry with wavelength-dependent limb darkening. Our
analysis of the six largest normal modes detected in Keck observations of G
29-38 reveals one mode with a colour dependence different from the other five,
permitting us to identify the l value of all six modes and to test the model
predictions. The Keck observations also show pulsation amplitudes that are
unexpectedly asymmetric within absorption lines. We show that these asymmetries
arise from surface motions associated with the non-radial pulsations (which are
discussed in detail in a companion paper). By incorporating surface velocity
fields into line profile calculations, we are able to produce models that more
closely resemble the observations.
