Thanks you guys, your replies encouraged me to find out what happened to Detritus' cooling helmet. It gave out in the deserts of Klatch in Jingo (page 273). Then I came across this extract on page 295:

"What do you want me to do with him, Mr Vimes?"
"Detritus?"
The troll saluted with the hand that still held the D'reg.
"All present and correct, sir!"
"But--" And then Vimes realized. "It's freezing cold! Your brain's working again?"
"With rather more efficiency, sir."

This occurred at about three in the morning. And it does say it's freezing cold. Is it possible it's colder at night in the desert than it is in Uberwald? Or does it have something to do with Detritus being from Uberwald originally? I would really appreciate your thoughts on this. Thanks