Fog on Oak Island, Nova Scotia, Canada As air cools near Earth's surface, fog results when relative humidity rises to 100 percent. (more) What Is Relative Humidity? Written by John P. Rafferty John P. Rafferty writes about Earth processes and the environment. He serves currently as the editor of Earth and life sciences, covering climatology, geology, zoology, and other topics that relate to... John P. Rafferty Fact-checked by Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree.... Britannica Editors Last updated Apr. 7, 2026 •History Relativity, in our understanding, depends on the value of one thing compared to another. For example, a mouse is small relative to the size of a larger animal, and a person traveling in a vehicle moves relatively faster than someone riding a horse. A similar logic applies when we try to compare the amount of moisture present in a volume (or parcel) of air on a cool day with that on a warm day. When...

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