Practically none

Checking the headache topic's "real culprit" — the pressure itself

In the headache topic we said the prime suspect behind the pain isn't the full moon but low barometric pressure. That leaves one question worth nailing down: does the full moon actually move the pressure? If the barometer reliably fell on full moon nights, the moon could reach headaches *through the weather*. If it doesn't, the moon has no back door to your head either.

So we took the headache suspect — air pressure itself — and compared full-moon and new-moon days against ordinary ones.

The lunar atmospheric tide is real. It's just the wrong order of magnitude

To be honest: the moon's pull on the atmosphere (the atmospheric tide) is real. By the same physics as the ocean tide, air pressure carries a tiny lunar-cycle wobble. But its size is under 0.1 hPa at mid-latitudes — against the tens of hPa the weather moves in a single day, that's a ripple a thousand times smaller. Compare daily mean pressure on full-moon versus ordinary days and the difference vanishes into the weather's noise.

How the verdict is computed

• Data is daily mean sea-level pressure from ERA5 reanalysis (Copernicus C3S, CC-BY). Sea-level reduction removes elevation, so Tokyo, London, and New York all sit on the same ~1013 hPa footing
• Like temperature, pressure is an absolute quantity that never crosses zero, so a ratio (×1.00) is meaningless. We take the deviation (hPa) from the "same month" average, build a confidence interval, and judge against an hPa threshold
• We mark "full moon days" (instant ±24h) and "new moon days" from the lunar phase at each city's noon, then compare the mean deviation against normal (0 hPa)

See the methodology for the full criteria.

The real culprit is fronts and storms

What moves the barometer dramatically is weather, not the moon. Each city's record-low day is a storm day (a typhoon or a bomb cyclone), tens of hPa below normal even in the daily mean. And beyond the headline records, ordinary weather keeps nudging pressure by a few hPa every day. The moon's ripple is invisible inside that swell.

Low pressure really can trigger a headache. But what carries that low pressure in is a front and a storm — not a full moon. If you look up on an aching night, look at the weather map, not the phase.

Sources

• ERA5 reanalysis (Copernicus Climate Change Service (C3S), CC-BY 4.0). Daily mean sea-level pressure. Historical values accessed via Open-Meteo; annual updates pulled directly from the Copernicus CDS
• Lunar age and the instant of each phase are computed in-house from the algorithms in Jean Meeus, *Astronomical Algorithms* (UTC basis)