The residence time of water in a reservoir is the average time that water spends in it between entering and leaving.
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Example 9.5 (Water in the air)
The atmosphere holds of water, and fall out of it each year as rain and snow. Its residence time is year, about days: the water of the air is renewed every week or two. A cloud that rains today is made of water that left the ocean or the ground a few days ago.