Can the relative humidity ever be zero percent?

Dear Tom

Can the relative humidity ever be zero percent? I say yes, but my girl friend says no. Who is right?

Jerold Crissman, Omaha, Nebraska

Dear Jerold,
The concept of zero percent relative humidity — air completely devoid of water vapor — is intriguing, but given Earth’s climate and weather conditions, it’s an impossibility. Water vapor is always present in the air, even if only in trace amounts.
Practically all of Earth’s surface contributes water vapor to the atmosphere. Water evaporates from open water and ice (which covers more than three-quarters of the planet), soil moisture evaporates into the air, and water vapor is always escaping from plants. And once water vapor is in the air, wind carries it everywhere, even across arid deserts. In man-made environments without added moisture, very low humidity occurs in heated homes during winter cold spells.

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