In it, 34 patients with chronic migraine kept diaries of their health under the supervision of doctors. All of them felt good at an atmospheric pressure of 759.8 mm of mercury. But the study participants suffered from migraines even with relatively small decreases in atmospheric pressure - only 5-8 mm mercury.

I often get headaches a few hours before rain storms - not migraines (I think), but sinus headaches. Sometimes, that's my first inkling that rain is on the way. I have always assumed that it has to do with rapidly changing barometric pressure.