The idea of feeling "under the weather" is as old as Hippocrates, one of the fathers of Western medicine, who recorded that certain illnesses seemed to worsen under particular meteorological conditions. Today, people still claim they can feel the approach of a storm or a cold snap in their arthritis, sinuses, headaches or teeth. But does grandma's "rheumatiz" acting up make for a good barometer, or is this just another case of anecdotal evidence run amok?
Modern scientists have found no conclusive evidence for a broad correlation between pain and weather, but studies have shown statistically significant links in a few specific cases [source: Shah]. For example, a 2007 American Journal of Medicine study of 200 subjects with knee osteoarthritis found a link between barometric pressure and pain level [source: McAlindon et al.].
Barometric pressure turning rain into pain makes intuitive sense. Your bodily fluids exist in a constant balance with ambient air pressure, so as the barometer falls -- as happens with an approaching storm -- your tissues can swell in response, irritating nerve endings and causing you additional ouches [source: Shah].