Answer
Because there is less dense air, and lower pressure above, faster-moving air molecules migrate upward, starting convection currents.
Work Step by Step
This is discussed on pages 305-306.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.