ok…I found a interview with Tony Sanchez in Yogajournal.
“as long as a person is consistent with the practice, outside temperature doesn’t make that much difference”, according to Tony Sanchez, former Bikram Yoga teacher and founder of the San Francisco Yoga Studio.
“A person performing the Bikram poses in a lower temperature has to move much slower to avoid possible injury,” he says, “but as long as you move at a pace where you’re stretching without straining, then you can receive 100 percent of the benefits.”
Here is the link to the artikle.
http://www.yogajournal.com/practice/527
So maybe everything is the same in low temperature, just slower…