Thanks for the comment. Due to it's proximity to the Atacama desert, it was actually quite warm (this applies to a number of mountains on the Bolivia/Chile, Argentina/Chile border). While I was wearing thermals, I didn't put my fleece on until I was within about 100 m of the top. The warm conditions may also be due to the volcano still being semi-active (i.e. giving of heat), hence the venting on the trial.