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."

The "full moon = clear sky" impression comes with a built-in trick: you cannot see a full moon through clouds. Memories of full moons are only ever created on clear nights, so the impression accumulates one-sidedly. It is a textbook case of observation bias (a cousin of survivorship bias). This page checks that impression 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.

But a ~1% modulation is at the edge of what long-term statistics can even detect, let alone something you would plan an umbrella around. Compare this page's verdict with the "Higher" on the tides page, and you can see the distance between "an effect exists" and "an effect matters."

Sources

• Open-Meteo Historical Weather API (ERA5 reanalysis, CC BY 4.0) / ERA5: Copernicus Climate Change Service
• Moon phase instants computed in-house from Jean Meeus, "Astronomical Algorithms" (UTC)