Not so sure if the shot was taken for an extended timeline, but I do admit that the clouds provide some of the most beautiful details in the pastoral scenes on Brokeback.
The first shot where you notice a rack of clouds veiling the mountains is in the openning shot in the 'drop-off' scene where you see the flock being assembled with the mountain in the background.
In the other scenes, you are struck more by the immense shadows cast by the clouds on the mountain (as in the scene where Ennis looks at Jack riding high up in the mountains).
Finally, in the opening shot where Jack sings 'Water Walking Jesus' to Ennis, you can actually FEEL the clouds enveloping the mountains.