Practically none

Checking the moon against air pressure, a known headache trigger

In the headache topic we noted that low barometric pressure is a well-known headache trigger. That raises a question worth checking: does the full moon move the pressure? If the barometer reliably fell on full moon nights, the moon would be reaching headaches through the weather.

So we took 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 is still lost in 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.

What clearly moved the barometer

What moved the barometer dramatically was the weather. 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 can't be seen inside that swell yet.

Low pressure really can trigger a headache. In this data, what carried that low pressure in was fronts and storms. To prepare for an aching day, start with the weather map.

What we can't see yet

This result doesn't mean "the moon has nothing to do with it." Here is what the current method can't see:

• The lunar atmospheric tide itself: the moon's wave in the air comes round twice a day. In a daily mean, its peaks and troughs cancel out
• How fast pressure changes: headaches are said to follow how quickly pressure drops over a few hours, more than its level. A daily mean can't show that speed

What we want to check next

• Use hourly pressure to catch the lunar atmospheric tide directly
• Compare how fast pressure falls on full-moon versus ordinary days

The moon really does pull on the air. Whether that small wave connects to headaches — that's what we hope to find in the data one day.

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)