Hoarfrost in a downward spiral

Further to my previous post about "hoarfrost", I just used google's nifty new word frequency tool, http://ngrams.googlelabs.com/graph?content=hoarfrost&year_start=1800&year_end=2010&corpus=0&smoothing=3, to track usage of the word since 1800. There is definitely a quite rapid downward trend since 1946, a trend that is common to both American and British English. I know, let's blame global warming! Or possibly people are avoiding this word because of the phonetic similarity with "whore". Intriguing!