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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?

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The MacBook Air does not require you to leave it open for the heat dissipation to work properly. It works just fine and is built for operating in clamshell mode. – Gerry Sep 19 '12 at 16:49

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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.

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This doesn't work on Mountain Lion. – Sandro Dzneladze Sep 19 '12 at 18:33
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It still works with my MacBook Air and Thunderbolt Display. You have to close the lid first or connect the displays when the laptop is closed. – Lauri Ranta Sep 20 '12 at 4:55
Just want to add that closing the lid seems kind of "inefficient" since this prevents hot air from escaping through the fan/keyboard/etc. – Daps0l Mar 31 at 11:06

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