I'm drawing a blank.....I vaguely remember the rain....I can't remember why he tore his shirt off...please refresh my memory...what point in the movie was this?
His escape. He crawls though a sewer pipe which empties into a pond of some sort............there's a thunderstorm happening..........he's all smelly from the sewage............he rips off his shirt and lets the rain wash him clean.
Right after he escapes through the culvert he is all muddy and it is raining. At that point he rips off his shirt and raises his arms to the lightening riddled sky in triumph.
It's when he looks up at the pouring rain and yells, "Mennnnnddooooozzzzaaaaaaaaa!!!!"
A little NRO reference for those who know :-)