While people are at the greatest risk from high temperatures during the summer season, your Mac may be even more fragile. (People can be water-cooled and no current Mac offers that option—though some have tried.) An iPhone or iPad will warn you when it detects it’s too hot before shutting down, while a Mac may simply suddenly power off. If it doesn’t power down, you may be running it to close to its maximum capability and putting a lot of additional wear in the process on components that might fail later during other seasons.
Knowing the temperature is one thing. The other is how hot should your internal components run? Apple says you should only use a Mac when the ambient temperature–the temperature around you–is in the range of 50°F to 95°F (10° to 35°C) and 95 percent or lower humidity.
Read more at Macworld.com