No difference

"Full-moon nights are frosty" — and also "full moons are warm"

Temperature folklore about the moon is delightfully contradictory. Farmers and mountaineers will tell you full-moon nights are clear and cold, frost-prone from radiative cooling; other traditions insist frost won't settle under a full moon. People who spend nights outdoors earned those instincts the hard way, and we won't laugh them off from an armchair. Instead, this page tests them on their own turf — decades of measured temperatures.

A real study did find full moons slightly warmer

This one is not pure occultism. In 1995, Balling & Cerveny reported from satellite data that the lower atmosphere runs a tiny fraction warmer around full moons — about two hundredths of a degree. Candidate mechanisms include the atmospheric tide raised by lunar gravity and the full moon's reflected and infrared radiation. The key number is the size: 0.02°C. No household thermometer can measure it, and it's hard for anyone to feel. The question for this page is not "is the effect exactly zero" but "is it big enough to matter to anyone."

How the verdict is computed

• Temperatures are ERA5 reanalysis daily means (Copernicus C3S) for Los Angeles, Berlin, and Tokyo — three cities on three continents, so the answer doesn't hinge on one lucky location (since 1940)
• Temperature crosses 0°C, so the ratio scale (normal = 1.00) used by our other topics doesn't work here. Instead we compare deviations (°C) from the same-month average
• For full-moon days (within ±24h of the instant, judged at each city's noon) and new-moon days, the mean deviation is evaluated with a 95% confidence interval; under ±0.3°C displays as "practically none," above ±1.0°C as "warmer/cooler"
• Monthly averages absorb the warming trend; full-moon days are spread uniformly across the decades, so the trend cancels out of the comparison

See the methodology for the verdict thresholds.

About "full moons mean clear, cold nights"

Testing the radiative-cooling story means minding the order of cause and effect. Does a full moon clear the sky, or is a clear sky what lets you see the full moon? Cloudy full moons go unseen, so the nights we remember tend to be clear and cold. A daily mean can't separate the two, so the clear nights need to be pulled out and checked on their own. On our rain topic, precipitation doesn't yet show a full-moon difference either.

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 night-time chill: the folklore is about the full-moon night. In a daily mean, the night's chill is blended with the day's warmth
• Clear nights only: radiative cooling happens on clear nights. Counting cloudy nights too makes any difference harder to see
• A very small effect: the 0.02°C reported in research is hard to pick out from three cities' day-to-day swings

What we want to check next

• Test daily minimum temperatures (the pre-dawn chill)
• Use cloud-cover data to compare clear nights only

Perhaps the instincts of people who spend nights outdoors hold something we haven't yet turned into numbers. We'd like to find it 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
• Balling RC, Cerveny RS. "Influence of lunar phase on daily global temperatures" Science 1995;267:1481-1483
• Lunar phases computed in-house from Jean Meeus, "Astronomical Algorithms" (UTC)