  Dust aerosol plays a fundamental role in the behavior and evolution of the
Martian atmosphere. The first five Mars years of Mars Exploration Rover data
provide an unprecedented record of the dust load at two sites. This record is
useful for characterization of the atmosphere at the sites and as ground truth
for orbital observations. Atmospheric extinction optical depths have been
derived from solar images after calibration and correction for time-varying
dust that has accumulated on the camera windows. The record includes local,
regional, and globally extensive dust storms. Comparison with contemporaneous
thermal infrared data suggests significant variation in the size of the dust
aerosols, with a 1 {\mu}m effective radius during northern summer and a 2
{\mu}m effective radius at the onset of a dust lifting event. The solar
longitude (LS) 20-136{\deg} period is also characterized by the presence of
cirriform clouds at the Opportunity site, especially near LS=50 and 115{\deg}.
In addition to water ice clouds, a water ice haze may also be present, and
carbon dioxide clouds may be present early in the season. Variations in dust
opacity are important to the energy balance of each site, and work with
seasonal variations in insolation to control dust devil frequency at the Spirit
site.
