No difference

The "full moon nights are clear" folklore

The feeling that full moon nights tend to be clear is widely shared — moon-viewing memories are, almost by definition, memories of clear nights. Farming folklore sometimes claims the opposite, that a full moon "brings rain."

Checking "full moon = clear sky" takes one bit of care: you cannot see a full moon through clouds. Memories of full moons are made mostly on clear nights, so memory alone can't settle it. That's why this page checks against actual precipitation records.

How this verdict is computed

• Data: daily precipitation from ERA5 (the European reanalysis dataset, since 1940) for three cities — Tokyo, London, and New York — to check whether the conclusion survives a change of location
• The metric is the share of rainy days (≥1mm daily precipitation). Raw rainfall amounts are heavily skewed (mostly zero, occasionally huge) and unsuitable for comparing averages
• Rain is strongly seasonal (Tokyo's rainy season, London's wet winters), so each day is compared with the average rainy-day rate for the same month
• Days are classified as full moon days (±24h around the instant) or new moon days using the moon age at local noon; each group's mean index is compared with 1.00

See the methodology for the exact criteria.

The moon does touch the atmosphere — barely

In 2016, a University of Washington study of satellite data reported that the lunar gravitational tide modulates precipitation — by roughly 1%. The moon's gravity pulls on the atmosphere as well as the oceans; that much is real physics.

A ~1% modulation is at the edge of what long-term statistics can detect. Even so, it's a real clue that the moon touches the rain.

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:

• A ~1% wave: the modulation found in research is hard to pick out from three cities' daily rainy-day rates
• Night-time rain: the folklore is about the full-moon night. Daily data can't show a night that clears while the day stays wet
• Place: the research used satellites over wide areas. Three cities can't show regional differences

What we want to check next

• Use hourly precipitation to compare night hours only
• Check more cities, and rainfall over the oceans

The moon's pull really does reach the air. How that connects to rain is something we hope to find in the data one day.

Sources

• ERA5 reanalysis — generated using Copernicus Climate Change Service (C3S) information (CC BY 4.0); historical data accessed via Open-Meteo
• Moon phase instants computed in-house from Jean Meeus, "Astronomical Algorithms" (UTC)