NEW YORK -- As heat waves have become more frequent and lengthy because of climate change, an estimated 39 million Americans currently live with migraine, according to the American Migraine Foundation. 

Half of those with migraine report the weather as one of their headache triggers, according to Elizabeth Loder, chief of the Brigham and Women's Hospital Headache Division. 

Heat can be a major factor, experts agreed, though the onset of a headache is often a mix of compounding environmental factors," reported The Washington Post about this finding on Wednesday. 

High temperatures are frequently accompanied by changes in barometric pressure, direct sun exposure and humidity, and these environment changes can trigger a headache for those who have migraine. 

"Migraine brain does not like variability," Jessica Ailani, neurologist and director of the Headache Center at MedStar Georgetown, was quoted as saying. "It wants you to sleep at the same time, eat the same thing. So big shifts in temperature and weather don't do well with migraine." 

Experts are unsure of the precise mechanism by which heat can trigger a headache, though heat can lead to processes that are known to cause headache. 

Extreme dehydration can cause your brain to shrink and pull on the blood vessels lining the brain, which can lead to physical pain, Loder said. 

In extreme cases, heat can affect the function of brain neurons, according to Mayo Clinic neurologist Narayan Kissoon. Altered cell function leads to an increase in activity in the pain centers of the brain, he noted.