Mars: Dissipating Behavior of the Cloud Belt

This is a brief report on the Martian climate, based on our observations in 2001 as well as those in 1997 and 1999. The focus is the dissipating behavior of the low-latitude cloud belt appearing around the aphelion, the behavior of which has never been explicitly examined. We derive the optical thickness of water ice clouds (WICs) as $ \tau_{\mathrm{WIC}} \approx 0.1$ ( $ \lambda \simeq 4400\,{Å}$ ) at the solar longitude $ L_{\mathrm{s}} = 174^\circ$ in 2001. The latitudinal coverage of the cloud belt is approximately $ L_{\mathrm{s}}$ -independent just until its end. The cloud belt divides into a “semi-encircling” cloud band and discrete WICs before its dissipation (over $ L_{\mathrm{s}} \sim 100^\circ \hbox{--} 110^\circ$ ) in many cases. We suggest that this cloud division should embody localization of the cross-equatorial Hadley circulation.