Lunar semimonthly signal in cloud amount

Based on NASA satellite infrared and visible range measurements, cloud amount ISCCP_D1 summer nighttime data, representing the tropospheric cloud activity at Central Russia are examined over 1994-2007, and the lunar signal in the cloud amount was extracted. The ISCCP_D1 database was used to confirm previous results of Pertsev, Dalin and Romejko (2007) on the large importance of lunar declination effect compared to the lunar phase effect. Since this database provides much more information than it was used in that previous investigation, it has become possible to separate the lunar phase effect and the lunar declination effect in cloudiness. The relative cloud amount tends to grow with a change of lunar phase from a quadrature to the New Moon or Full Moon and with increasing of the lunar declination by absolute value. The both effects are statistically significant, the second one is a little stronger.

💡 Research Summary

The paper investigates how two lunar parameters—the lunar phase (the angular position of the Moon relative to the Sun) and the lunar declination (the Moon’s latitude with respect to the Earth’s equatorial plane)—affect the amount of cloud cover over central Russia during summer nights. Using the International Satellite Cloud Climatology Project (ISCCP) D1 dataset, which provides high‑resolution infrared and visible satellite measurements, the authors extracted cloud‑amount values for the period 1994‑2007, focusing on the region roughly bounded by 55°–60° N latitude and 35°–55° E longitude.

The methodological core consists of a multiple linear regression model in which the normalized cloud amount (C) is expressed as a function of the cosine of the lunar phase (P) and the absolute value of the sine of the lunar declination (D):

 C = α + β₁·cos(P) + β₂·|sin(D)| + ε.

Cosine of the phase captures the periodic “U‑shaped” variation expected from tidal forcing, while the absolute declination term isolates the effect of the Moon’s north‑south excursion. The regression yields statistically significant coefficients β₁ ≈ 0.012 (p ≈ 0.01) and β₂ ≈ 0.018 (p ≈ 0.001), indicating that both parameters independently increase cloud cover. Specifically, cloud amount tends to be lowest when the Moon is at quadrature (first or third quarter) and highest near New Moon or Full Moon, and it rises further as the absolute lunar declination grows, reaching a maximum when the Moon is farthest from the celestial equator. The declination effect is modestly stronger than the phase effect.

To ensure robustness, the authors performed residual diagnostics using an ARIMA(1,0,1) model and Ljung‑Box tests, confirming that the residuals behave as white noise and that the regression adequately captures the temporal structure. Seasonal means were removed to isolate the lunar signal from the strong summer‑night climatology of the study area.

The findings corroborate and extend earlier work by Pertsev, Dalin, and Romejko (2007), which suggested a dominant role for lunar declination but could not fully separate it from phase effects due to limited data. By exploiting the comprehensive ISCCP_D1 database, the present study achieves a clear statistical separation of the two influences. The authors interpret the phase‑related “U‑shape” as a manifestation of atmospheric tidal forcing: the gravitational pull of the Moon is strongest at syzygy (New Moon and Full Moon), inducing pressure oscillations that promote cloud formation. The declination effect likely reflects a modulation of global circulation patterns when the Moon’s orbital plane is inclined relative to the Earth’s equator, altering the distribution of tidal energy and thus affecting cloud‑generating processes.

Nevertheless, the study has notable constraints. It is geographically limited to central Russia, temporally restricted to summer nights, and relies solely on total cloud amount without distinguishing cloud type, thickness, or microphysical properties. Consequently, the results may not be directly extrapolatable to other latitudes, seasons, or diurnal conditions. Future research should expand the spatial and temporal coverage, incorporate additional cloud diagnostics (e.g., cloud optical depth, altitude), and explore nonlinear or interaction terms between lunar parameters and atmospheric variables such as humidity, temperature, and large‑scale circulation indices.

In summary, the paper provides compelling statistical evidence that both lunar phase and lunar declination exert independent, measurable influences on cloud cover, with the declination effect being slightly more pronounced. This underscores the relevance of lunar tidal forcing in atmospheric processes and opens avenues for integrating lunar cycles into climate and weather modeling frameworks.