There is a large published literature reporting associations between geomagnetic activity and health outcomes — myocardial infarction rates, stroke admissions, blood pressure, heart rate variability, mood disorders and headache — and it is mostly of poor quality. The typical study is an ecological time-series correlation between a geomagnetic index and an aggregate health statistic, with substantial multiple-comparisons exposure, inconsistent lag definitions, and results that fail to replicate across populations. Meanwhile geomagnetic storm variations at ground level are on the order of hundreds of nanotesla against a background field of around 50,000 nanotesla — a fraction of a percent — and no plausible biological mechanism operating at that magnitude has been demonstrated. The reasonable conclusion is not that the effect is disproven but that it is unestablished, and that the confident claims circulating online are far ahead of the evidence.

Here is how to read the field without either dismissing it or being taken in by it.

The best evidence on lightning and headache comes from a 2013 University of Cincinnati study that tracked 90 people with migraine in Ohio and Missouri and found that headache risk rose roughly 30 percent, and new-onset headache roughly 24 percent, on days with lightning within 40 kilometres. The effect survived statistical adjustment for barometric pressure and for the general presence of a storm, which is what made it notable. But the study could not identify a mechanism, and the leading candidate — electromagnetic radiation from lightning strokes, known as sferics — remains a hypothesis rather than a demonstrated cause. Thunderstorms deliver pressure swings, humidity jumps, ozone, pollen rupture, wind, noise, disrupted sleep and light flashes all in the same package, and disentangling an electromagnetic effect from that pile is genuinely difficult.

The honest position is that something about lightning days looks real and nobody has shown what it is.

Outdoor glare headache is caused by luminance contrast, not by total brightness, and that distinction decides what actually helps. A uniformly bright overcast sky and a low winter sun glinting off wet tarmac can measure similar overall light levels while producing completely different amounts of discomfort, because glare is what happens when one part of your visual field is dramatically brighter than the part your eye is adapted to. Snow, water, wet roads and glass are the main culprits, and all four are worst when the sun sits low. The practical responses are polarisation for reflected glare, a brimmed hat for sky glare, tinted lenses in the FL-41 range for the specific wavelengths that drive migraine photophobia, and — importantly — avoiding dark sunglasses indoors, which makes light sensitivity worse over time rather than better.

Snow blindness is a separate injury with a separate mechanism, and it is worth understanding on its own terms.

A long-haul flight delivers four separable headache mechanisms at once, and they need different countermeasures. Cabin pressure is regulated to no worse than the equivalent of 2,438 metres of altitude — typically 1,800 to 2,400 metres in cruise — which drops arterial oxygen saturation into the low nineties and is the driver behind true airplane headache. Cabin humidity runs between 5 and 20 percent, drier than most deserts, which dehydrates mucous membranes over hours. Crossing time zones misaligns your circadian system from local time, and the body resynchronises at only about one hour per day eastward and one and a half westward. And travel itself brings disrupted sleep, missed meals, alcohol, caffeine shifts and sustained stress. Working out which of these is yours is what makes the difference, because hydration will not fix a circadian problem and a sleep schedule will not fix a pressure one.

Eastward and westward flights create genuinely different problems, and that is the most useful place to start.

A weather-trigger profile is a record of what the atmosphere was doing on the days you had symptoms and — just as importantly — on the days you did not, built prospectively over at least three months so that you can compare rates rather than collect anecdotes. The method matters because the intuitive approach fails reliably: recalled triggers agree poorly with diary evidence, the premonitory phase of migraine begins up to twenty-four hours before the pain and can itself change your behaviour and perception, and once you believe in a trigger you will notice confirming days and forget the rest. The fix is to log a fixed set of variables every day regardless of how you feel, keep logging on well days, and only analyse after you have enough data that a pattern would have to be real to show up.

Done properly, this takes about ninety seconds a day for three months and gives you something no general article can: your own numbers.

The honest answer is that climate change is very likely making conditions harder for weather-sensitive people, but not primarily through the mechanism most people assume. There is no good evidence that average barometric pressure is changing in a way that matters, and the research on whether storms are becoming more frequent is genuinely mixed. What is well documented is that the supporting conditions have shifted measurably: pollen seasons across North America have lengthened by around twenty days since 1990, wildfire smoke now routinely reaches regions that never experienced it, heat waves are longer and more intense, tropical cyclones are intensifying more rapidly when they do form, and the seasonal windows during which disruptive weather is possible have widened at both ends. For someone whose attacks track weather, the practical effect is fewer genuinely quiet months in the year.

This is a subject where overclaiming is common in both directions, so it is worth being precise about which parts are settled.

The spring clock change is a one-hour circadian phase advance delivered overnight, with no adaptation period. For people with migraine that matters because circadian disruption is one of the few triggers with consistent evidence behind it, and because the migraine brain appears unusually sensitive to changes in routine timing rather than to any particular time itself. The measured effects on health outcomes in the days after the spring change are real but modest — small increases in heart attack admissions, traffic collisions and workplace injuries — and the migraine-specific evidence is thinner than the confident headlines suggest. What is well established is the mechanism: losing an hour of sleep and shifting your light exposure by sixty minutes degrades the stability that migraine prevention depends on. The practical answer is to phase-shift gradually over the preceding week rather than absorbing the whole hour at once.

There is also a confound worth naming early, because it explains a lot of misattributed attacks: in the northern hemisphere the spring change falls in March, which is one of the most barometrically unstable months of the year.

Equinox weather is unstable for a physical reason that has nothing to do with the equinox itself being a special date. In spring and autumn the temperature difference between the pole and the tropics is changing faster than at any other time of year, because land and ocean heat and cool at very different rates. That changing contrast is the fuel supply for mid-latitude storms, and a fuel supply in flux produces more frontal passages, sharper boundaries between air masses, and more rapid barometric pressure change than the settled conditions of mid-summer or mid-winter. For weather-sensitive people this shows up as clusters of attacks in March through May and again in September through November — not because the seasons themselves are triggers, but because those months contain more transitions.

The two transitional seasons are also not mirror images of each other, and understanding the difference explains why many people find one much harder than the other.

An urban heat island is the temperature difference between a built-up area and the rural land around it, typically one to three degrees Celsius as a daily average but frequently seven degrees or more on calm clear nights. The mechanism is straightforward: cities replace vegetation with materials that absorb and store solar energy, remove the evaporative cooling that plants provide, trap heat in street canyons that radiate poorly to the sky, and add waste heat from vehicles, buildings and air conditioning. For headache and migraine the significant part is not the hot afternoon, which is only modestly warmer than the surrounding countryside, but the hot night, when the difference is at maximum and sleep is what suffers. Poor sleep is among the best-evidenced migraine triggers there is, and the urban heat island systematically attacks it.

The effect is also strikingly uneven within a single city, which turns out to matter more than the city-wide average.

The UV index is a forecast of ultraviolet radiation intensity, and ultraviolet light is almost certainly not what triggers your migraine. Photophobia in migraine is driven by visible light, with the strongest response in the blue range around 480 nanometres, through a retinal pathway that connects intrinsically photosensitive ganglion cells to pain-processing regions in the thalamus. Ultraviolet radiation sits below 400 nanometres, outside the visible range, and is largely absorbed by the cornea and lens before it reaches the retina at all. The UV index is still a useful proxy, because the conditions that produce a high UV index — clear skies, high sun angle, altitude, reflective surfaces — are the same conditions that produce intense visible brightness and glare. But treating it as a direct trigger leads to the wrong protective choices, most obviously wearing sunglasses that block UV while doing little about the brightness that is actually the problem.

Getting this distinction right changes what you buy and when you plan around it.

Going out into the cold sets off a fast, measurable circulatory response: peripheral vessels clamp down, blood pressure rises within a minute or two, and blood is pulled inward to protect the core. Your fingers get a partial reprieve through a rhythmic rewarming reflex. Your scalp, face and neck do not. That asymmetry — plus wind stripping heat away faster than still air, and damp clothing faster still — is why a cold exposure headache is more about what you are wearing than about what the thermometer says.

Usefully, this is one of the few headache triggers you can intervene on directly, with fabric.

Cold-stimulus headache — brain freeze, ice cream headache, formally ICHD-3 code 4.5 — is a brief, intense, usually frontal or temporal pain provoked either by something cold touching the palate and back of the throat or by cold applied to the head from outside. It typically peaks around 30 to 60 seconds after the stimulus and resolves in well under five minutes. The mechanism is still argued over: the three candidates are a sphenopalatine ganglion reflex, referred trigeminal pain from the palate, and rapid dilation of the anterior cerebral artery, and the evidence does not yet cleanly separate them.

It is also, unusually, a headache you can produce deliberately in a laboratory — which is why something most people treat as a joke has a serious research literature behind it.

No thermostat setting gives you warm, humid, fresh and clean air at once. Heating collapses relative humidity. Sealing the house to keep that heat in also keeps carbon dioxide, VOCs and combustion by-products in. Ventilating fixes that but costs heat and, during a winter inversion, imports outdoor particulates. Combustion heating warms the room and emits nitrogen dioxide into it. The goal is not to win every trade but to keep each variable in a workable band: 40 to 50 per cent relative humidity, carbon dioxide under about 1,000 parts per million, and no unvented flame in a room you sleep in.

Most winter air advice hands you one lever and tells you to pull it hard. That is how a dryness problem fixed in January becomes a mould problem in March.

A winter light headache is usually driven by two separate changes, not one. The first is light quantity — indoor winter light sits somewhere around 100 to 500 lux, while even a heavily overcast outdoor day delivers several thousand, so most people spend the dark months receiving roughly an order of magnitude less light than their circadian system evolved to expect. The second is light timing — a later sunrise moves the signal that sets your body clock, which drifts sleep and melatonin release later and leaves you running a mild, permanent jet lag. Both can lower the migraine threshold. They respond to different fixes, and neither is the same thing as seasonal depression.

The distinction matters, because a lamp only fixes one of them, and only if you use it at the right hour.

The winter migraine season concentrates in January and February for one dominant reason and several supporting ones. The dominant reason is physical: the polar jet reaches its strongest and most southerly position in mid-winter, which makes extratropical storms more frequent, deeper and faster-moving, and makes surface barometric pressure more variable then than at any other point in the year. Everything else piles on top — the collapse of routine after the holidays, accumulated dry air and darkness, peak illness season, lost outdoor movement, the swing from December excess to abrupt January restriction. But the seasonal epidemiology of migraine is genuinely mixed, with several good studies finding no seasonal effect at all, so the honest argument explains why mid-winter should be harder and then shows you how to check whether it is.

That second half matters more, because individual variation in seasonal sensitivity dwarfs any average seasonal effect.

A cold snap headache is rarely caused by the cold itself. What arrives with a sudden cold outbreak is a bundle of four separate changes — a sharp barometric rise, a collapse in outdoor humidity, a hard wind shift, and a set of behavioural changes you make without noticing. Each has its own mechanism and its own timing, and the useful work is figuring out which one is yours, because they call for different responses.

That framing matters because most advice about cold weather migraine stops at "stay warm", which helps with exactly one of the four.

Freezing rain needs a very specific vertical arrangement of the atmosphere: a warm layer aloft to melt falling snow, and a shallow cold layer at the surface to supercool the resulting droplets so they freeze on contact. That arrangement is narrow, unstable and hard to forecast — and it produces a long, shallow barometric decline over a day or more rather than the sharp plunge of a passing low. For most weather-sensitive people the serious risks in an ice storm are not the pressure at all, but the multi-day power outage that follows.

Ice storms are the winter hazard where the health planning and the symptom planning diverge most sharply, and the health planning is the one that matters.

The polar vortex is a band of fast westerly winds circling the Arctic stratosphere about 30 kilometres up. It is always there in winter. What makes the news is when it weakens or splits, because that allows cold air that was previously locked over the pole to spill south — and crucially, the stratospheric warning appears two to six weeks before the cold reaches the surface. No other winter weather signal gives weather-sensitive people that much lead time.

Most coverage gets the framing backwards, describing the vortex as something that "arrives". It does not arrive. It breaks down, and the consequences follow weeks later.

In a typical snowstorm the barometer does most of its work 12 to 36 hours before any snow falls. By the time the flakes are coming down hard, pressure is usually near its minimum and flattening out — and in some storm types, snow falls under a rising barometer entirely. If you react to pressure change rather than to weather, the difficult window is almost always the day before the storm everyone else is talking about.

That mismatch — between when the barometer moves and when the weather looks dramatic — is why so many people describe a snowstorm headache that was already fading by the time the snow arrived.

Heated indoor air in winter routinely sits at 10 to 20 per cent relative humidity — drier than the Sahara Desert, which averages around 25. Your nose evolved to humidify incoming air to nearly 100 per cent before it reaches your lungs, and in those conditions it cannot keep up. The result is a predictable cluster: nosebleeds, sinus pain, a dull morning headache, a sore throat on waking, and worse sleep. All of it is fixable, and the fix costs less than most people assume.

This is the most under-diagnosed winter symptom driver there is, largely because it does not look like weather. It arrives quietly over days, not with a front.