Heat Migraines: Why Hot Weather Triggers Attacks in Summer

The forecast says 95. By mid afternoon your temples are tightening, and by evening you are in a dark room trying to work out whether the heat did this or whether the attack was coming anyway. Heat is a real migraine trigger, reported by 30.3% of patients in one of the largest trigger surveys ever published, and emergency department records show headache risk rising steadily as the thermometer does.

So the useful question is not whether hot weather can set off an attack. It is which part of a hot day is doing it to you, because heat never shows up alone. It brings humidity, glare, fluid loss, short nights, and sometimes a pressure swing, and those factors have very different fixes. Here is what the research separates out, and how to find the temperature where your own risk starts to climb.

How many people actually get heat migraines

The most detailed count comes from a 2007 Cephalalgia study by Lawrence Kelman, who surveyed 1,207 patients meeting International Classification of Headache Disorders criteria and asked what set their attacks off. Heat was named by 30.3%. Weather as a broad category scored 53.2%.

Put that next to the rest of the list and heat lands mid-table. Stress topped the survey at 79.7%, not eating at 57.3%, sleep disturbance at 49.8%. Below heat sat food at 26.9% and exercise at 22.1%. Roughly a quarter of participants reported no reliable triggers at all.

Two things follow. Heat is common enough that suspecting it after a bad July is reasonable, and far from universal enough that assuming it without checking your own record points about seven people in ten at the wrong thing.

What the temperature data shows

Survey answers describe what people believe about their triggers. Two studies measured the weather instead, and both landed in similar territory.

The first, published in Neurology in 2009 by Mukamal, Wellenius, Suh, and Mittleman, matched 7,054 patients who came to a Boston emergency department with headache between May 2000 and December 2007 against local weather records. For every 5 degrees Celsius, about 9 degrees Fahrenheit, of higher mean temperature in the 24 hours before someone arrived, the odds of that headache visit rose by 7.5%. The relationship was linear, with no comfortable plateau, and it held for migraine and non-migraine headaches alike. Air pollutant levels, notably, showed no effect at all.

The second came from a University of Cincinnati team led by Vincent Martin and was presented at the American Headache Society's 2024 meeting. Researchers cross-referenced 71,030 daily diary records from 660 people with episodic migraine against regional weather data. Each 10 degree Fahrenheit rise in daily average temperature came with a 6% increase in headache days.

Neither figure is dramatic by itself, and neither decides any single afternoon. What they describe is a slope. As a heat wave stacks up, the odds tilt a little further each morning, and across a summer that tilt is large enough to show up in a year of logs.

Humidity does its own damage

Temperature gets the headlines because it is the number on the forecast, but a 2019 Environment International study suggests moisture matters just as much. Wenyuan Li and colleagues followed 98 adults with episodic migraine around Greater Boston for an average of 45 days each, across 4,406 observation days. Higher relative humidity was associated with higher odds of a migraine starting, and here is the detail worth keeping: the association appeared only in the warm season, April through September. The same humidity in January did nothing.

That seasonal split points away from moisture itself being the problem. Sweat evaporates poorly in humid air, so a body already working to shed heat has to work harder for the same result. A 90 degree day in Houston costs you more fluid and more effort than a 90 degree day in Phoenix, which is one reason the heat index predicts how you feel better than the temperature does.

Dehydration is the most fixable piece

Fluid loss deserves its own heading: it is the one variable in a hot day you can change within the hour. A 2004 Headache study by Blau, Kell, and Sperling documented what they called water-deprivation headache in 34 people: an aching pain made worse by head movement, bending down, or walking, arriving with poor concentration and irritability.

The recovery numbers are the striking part. Twenty-two of those 34 got complete relief within 30 minutes of drinking 200 to 1,500 mL of water. The remaining 11 needed one to three hours and 500 to 1,000 mL. The authors also raised the possibility that water deprivation prolongs migraine attacks, though that remains an open question.

Precision matters here. This is a headache caused by fluid loss, which is not the same claim as dehydration causing migraine. In practice the two run together all summer, and a 30 minute response to water is a fast, cheap piece of information about which one you are having.

Glare, and the light load of a long day

Light was reported as a trigger by 38.1% of Kelman's participants, a higher share than heat itself. Summer raises that load in ways that never appear on a forecast. Daylight runs longer at both ends, the sun sits higher and bounces off pavement, water, and windshields, and every trip between a dim air-conditioned room and a bright parking lot repeats the adjustment.

If your hot-weather attacks cluster on sunny days and skip overcast ones at the same temperature, glare is a reasonable suspect, and a cheaper problem to solve than heat.

The pressure question

Many people who suspect heat are half right and half tracking something else. Summer thunderstorms deliver a temperature spike, then a pressure drop, then rain, all inside a few hours, and separating those in memory afterward is close to impossible.

The Neurology emergency department study looked at both. Lower barometric pressure did raise risk, but only for non-migraine headaches, and only at a lag of 48 to 72 hours. Temperature was the variable that held for migraine cases. Other research has found pressure effects in migraine, so the picture is genuinely mixed, and we covered that evidence in our guide to barometric pressure and migraines.

In a hot climate the point is practical: record the two separately, or you cannot tell which one is yours. And if you have wondered whether somewhere else would be kinder, we worked through those tradeoffs in our article on moving for weather migraines.

Finding your own heat threshold

Weather triggers tend to behave like thresholds rather than smooth gradients. Nothing much happens at 82, and then something happens at 88, and the person it happens to often has no idea where their line sits because nobody has ever written it down beside their attacks.

Finding it takes two columns: the conditions on every day, and the attacks on some of them. Comparing attack days against your own non-attack days is what separates a trigger from a coincidence, and that comparison is the whole method behind tracking migraine triggers properly. One hot day that ended badly proves nothing. Fourteen hot days where eleven ended badly is a pattern you can plan around.

The obstacle is that nobody logs weather by hand for long. Some tracking apps ask you to type the temperature in yourself, the sort of chore people quietly drop by week three, right before the data would have become useful. MigrAid, for example, records temperature, humidity, and barometric pressure automatically with every entry using Open-Meteo data, so the conditions are attached whether or not weather was on your mind that day.

Heat also changes what people drink. Iced coffee in July can easily run double a February mug, and caffeine has its own relationship with attacks, which we covered in our caffeine and migraines guide.

When a summer headache is not a migraine

Heat illness and migraine overlap uncomfortably at the start, and telling them apart matters more than any tracking question here.

The National Weather Service lists headache among the symptoms of heat exhaustion, alongside heavy sweating, weakness, cool and clammy skin, a fast weak pulse, muscle cramps, dizziness, and nausea. Heat stroke is the emergency, and the NWS lists a throbbing headache as its first symptom, together with confusion, slurred speech, nausea, dizziness, a body temperature above 103 degrees Fahrenheit, hot red dry or damp skin, a rapid strong pulse, and fainting.

Read those two lists back to back and the trap is obvious: throbbing headache with nausea and dizziness describes a migraine attack perfectly well. The signs that do not belong to migraine are the ones to act on. Skin that has stopped sweating and turned hot and dry, confusion or slurred speech, or a temperature above 103 degrees mean heat stroke, which the NWS calls a severe medical emergency, and the instruction is to call 911 rather than wait it out.

Short of that, if your summer headaches have changed character this year, arrive with symptoms you have not had before, or stop responding to what usually works, bring the pattern to your doctor instead of solving it alone.

Getting through a hot stretch

No treatment exists for weather itself, but people who track through hot months converge on the same handful of adjustments.

Move the day earlier. Errands, exercise, and anything outdoors cost less at 8am than at 4pm, when heat, glare, and accumulated fluid loss all peak together.

Drink ahead of thirst. Thirst lags behind fluid loss, and the Blau study's 30 minute response window suggests catching it early is worth more than catching up later.

Treat glare as its own problem. Sunglasses in the car and a hat outdoors address a trigger reported by 38.1% of patients, independent of anything the temperature is doing.

Protect the night. Hot nights fragment sleep, and sleep disturbance was reported as a trigger by 49.8% of Kelman's participants. Cooling the bedroom is a migraine measure as much as a comfort one.

The bottom line

Heat is a trigger for roughly three in ten people with migraine, and the effect is a slope rather than a switch: 7.5% more headache risk per 5 degrees Celsius in emergency department data, 6% more headache days per 10 degrees Fahrenheit in diary data. Humidity contributes on its own, but only in the warm season. Fluid loss produces its own headache that water resolves within half an hour for most people. Glare and short nights ride along with it.

Untangling those in the moment is not possible. Untangling them over a summer is, and it takes nothing more than logging the attacks you have, the days you do not, and the conditions attached to both. By September you will know your own number, and next July you will get to plan around it instead of guessing.