Visual FOG Explained

Gunning FOG Index scoring for the Sherlock Holmes tale Scandal in Bohemia.

Lines indicating the Gunning Fog Index score for the each of the first fifty sentences of Arthur Conan-Doyle's Scandal in Bohemia. Horizontal length of each line indicates sentence length. (Sentences are sorted by score.)
The Fog index bases its score on sentence length and the number of long words in a passage- more particularly, the percent of words of three or more syllables. The hills in Visual Fog are created by turning long words into their short-word equivalents.
These three sentences, for example, have the same Fog score, equivalent to that of a twenty-five word sentence without any long words:
His manner was not effusive.[5 words/1 word with more than two syllables]

He was pacing the room swiftly, eagerly, with his head sunk upon his chest and his hands clasped behind him. [20/1]

"How do I know that you have been getting yourself very wet lately, and that you have a most clumsy and careless servant girl?” [25/0]

Since these three sentences are Fog-equivalent*, the 3+ syllable word is equivalent to an additional twenty words for the five-word sentence, and to an additional five words for the twenty-word sentence.

*Fog equivalence would actually require each of these sentences to be the average, in terms of length and long words, for a larger passage.