  Broad Absorption Line (BAL) trough variability is predominantly due to cloud
motion transverse to our line of sight. The rate at which the variability
occurs indicates the velocity of the cloud, which can provide constraints on
the cloud's distance from the central source. This requires detailed
spectroscopy during a variability event. Such spectra have proven elusive,
suggesting either the timescale of variability is too short to be caught, or
too long to notice until a sufficient amount of time has passed. Photometric
monitoring of BAL quasar colours may potentially be used as an early warning
system to trigger time resolved spectroscopic monitoring of BAL variability.
Towards this end, we are analyzing both BAL and non-BAL colour variability
using time series photometry from Stripe 82 in the Sloan Digital Sky Survey.
