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xkcd #3187: High Altitude Cooking Instructions
Title text:
1,300,000-1,400,000 ft: Ask a crew member to show you how to use the ISS food warmer.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3187/
But it does change how much power you need to use. If you leave the power the same, you'll need more joules because you need more time and are sending more of it to the atmosphere.