Dear Tom,
Is it possible for the relative humidity ever to be zero percent?
Tom Herald, Palatine

Dear Tom,
Given the Earth’s present weather and climate, it is an impossibility. Some water vapor is always present in the air, even if only in trace amounts.

Nearly all of the Earth’s surface passes water vapor into the atmosphere. Water evaporates from open water that cover approximately three-quarters of the planet. Water also evaporates from snow and ice, and water vapor is always escaping from plants. In addition, soil moisture evaporates into the air.

Once water vapor is in the air, wind carries it everywhere — even across arid deserts.

The very lowest relative humidity values are found in desert locations. The simultaneous occurrence of high daytime temperatures and very little water vapor in the air results, at times, in relative humidity values of only a few percent.

Very low relative humidity values can also occur in heated homes during winter cold spells. Frigid outside air, when heated to a comfortable room temperature, can have relative humidity values of only several percent if no moisture is added.