Turn off internal display without closing lid I learned that a MacBook Air's built-in screen turns off when I close the lid so I can continue working on an external display set to be primary. But the MacBook Air has a heat system that requires it to be open to work properly. Heat accumulates faster when it is in clamshell mode.

Is there any way to turn off just the built-in screen without closing the lid but keep the external display active?
Run sudo nvram boot-args=iog=0x0 and restart. It restores the behavior on 10.6 and earlier where the internal display is not turned back on if the lid is opened in clamshell mode.

You can delete the variable with sudo nvram -d boot-args.

Other related questions here and at Super User.