  The Atmospheric Dispersion Corrector (ADC) of the Gemini Planet Imager (GPI)
corrects the chromatic dispersion caused by differential atmospheric refraction
(DAR), making it an important optic for exoplanet observation. Despite
requiring less than 5 mas of residual DAR to avoid potentially affecting the
coronagraph, the GPI ADC averages $\sim7$ and $\sim11$ mas of residual DAR in
$H$ and $J$ band respectively. We analyzed GPI data in those bands to find
explanations for the underperformance. We found the model GPI uses to predict
DAR underestimates humidity's impact on incident DAR, causing on average a 0.54
mas increase in $H$ band residual DAR. Additionally, the GPI ADC consistently
undercorrects in $H$ band by about 7 mas, causing almost all the $H$ band
residual DAR. $J$ band does not have such an offset. Perpendicular dispersion
induced by the GPI ADC, potentially from a misalignment in the prisms' relative
orientation, causes 86% of the residual DAR in $J$ band. Correcting these
issues could reduce residual DAR, thereby improving exoplanet detection. We
also made a new approximation for the index of refraction of air from 0.7
microns to 1.36 microns that more accurately accounts for the effects of
humidity.
