Since at least 1940, the National Electrical Code has contained a requirement for ampacity correction of installations subject to high temperature conditions. In the 2005 NEC, a new fine print note was added to inform code users that if conductors in conduit were exposed to direct sunlight in close proximity to rooftops, they might experience an additional 30°C temperature rise above ambient (NEC-2005, 310.10 FPN 2). New code language was added in the 2008 Code in 310.15(B)(2)(c), and a new Table 310.15(B)(2)(c) to address rooftop wiring installations. With these additions to the Code, it now is a requirement to apply additional corrections to conduits exposed to sunlight on rooftops. In effect, this has redefined the ambient temperature requirements for these locations. 
Ambient temperature has in the past been applied as the temperature surrounding a wiring method (i.e., room temperature, air temperature, etc.), and not within the wiring method. http://www.iaei.org/magazine/?p=4539#more-4539
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