Singularity (climate)
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In meteorology, a singularity is a weather phenomenon likely to occur with reasonable regularity around a specific approximate calendar date,[1] outside of more general seasonal weather patterns (e.g., that May Day is usually warmer than New Year's Day in northern locales). The existence of singularities is disputed, some considering them due to seeing patterns in noise and statistical artifacts from small samples.[2]
In North America, the most significant purported singularities are January thaw (warmer weather around January 25) and Indian summer (warmer weather in mid-autumn).
More fanciful ones include the British tradition that rain on St. Swithun's Day (15 July) will be followed by forty days and nights of rain, and similar folk beliefs around Groundhog Day.