If you have a thundering headache, it might be to do with the weather, research suggests.
Scientists have confirmed a link between lightning and headaches and migraines.
A study found that lightning striking up to 25 miles away can increase the risk of headache by 31%. Thunderbolts also led to a 28% increased risk of migraine attacks.
US researchers looked at 90 chronic headache sufferers with an average age of 44 living in Ohio and Missouri.
All had conditions that fulfilled the criteria for migraines defined by the International Headache Society.
Research suggests that there are a number of ways in which lightning might trigger headaches
Participants recorded their headache experiences in a daily journal for three to six months.
During this time, scientists recorded lightning strikes within 25 miles of people's homes. The magnitude and polarity of the lightning current was also measured.
Geoffrey Martin, from the University of Cincinnati, who co-led the research, said: "Many studies show conflicting findings on how weather, including elements like barometric pressure and humidity, affect the onset of headaches. However, this study very clearly shows a correlation between lightning, associated meteorological factors and headaches."
Mr Martin, a medical student, conducted the study with his father Vincent, a professor of internal medicine at the University of Cincinnati and headache expert.
Prof Martin said there were a number of ways in which lightning might trigger headaches.
He explained: "Electromagnetic waves emitted from lightning could trigger headaches. In addition, lightning produces increases in air pollutants like ozone and can cause release of fungal spores that might lead to migraine."
The research, published in the online edition of the International Headache Society journal Cephalalgia, showed that negatively charged lightning currents were particularly associated with headaches.
When the scientists adjusted their results to take account of other weather factors linked to thunderstorms, they still found a 19% increased risk of headaches on lightning days.
"This suggests that lightning has its own unique effect on headache," said Prof Martin.
His son added: "This study gives some insight into the tie between headaches or migraines, lightning and other meteorologic factors.
"However, the exact mechanisms through which lightning and/or its associated meteorologic factors trigger headache are unknown, although we do have speculations.
"Ultimately, the effect of weather on headache is complex, and future studies will be needed to define more precisely the role of lightning and thunderstorms on headache."