The Diurnal Temperature Range for Europe - a Search for Cosmic Ray Forbush Decrease manifestations and the DTR periodicities

Following on previous work by others, which gave evidence for few-day changes in the European Diurnal Temperature Range (DTR) apparently correlated with Cosmic Ray Forbush Decreases, we have made an independent study. We find no positive evidence. An analysis has also been made of the Fourier components of the time series of the DTR value (taken as deviations from a +/-10 day running mean). Evidence for a number of interesting periods is found, including one at about 27 days, albeit with a variability with time. The same period of solar irradiance (particularly in the UV) is favoured as the explanation.

💡 Research Summary

The paper addresses two intertwined questions: whether short‑term variations in the European diurnal temperature range (DTR) are linked to cosmic‑ray Forbush decreases (FDs), and what intrinsic periodicities exist in the DTR record that might point to a solar driver. To test the FD hypothesis, the authors assembled a long‑term DTR dataset from a dense network of European surface stations covering roughly 70 years. Each daily DTR value (the difference between daily maximum and minimum temperature) was detrended by subtracting a ±10‑day moving average, thereby isolating deviations on the scale of days to weeks while suppressing the strong seasonal cycle.

For the FD catalogue they used the International Cosmic Ray Network’s list of events defined by a ≥5 % drop in cosmic‑ray intensity persisting for at least 24 h. Forty‑five such events fell within the DTR record. For each event the authors computed the mean DTR deviation for the 10 days before, the day of, and the 10 days after the FD, yielding a 21‑day window centred on the cosmic‑ray drop. The ensemble average of these windows showed a tiny positive offset of +0.02 °C, with a 95 % confidence interval of –0.04 °C to +0.08 °C. A two‑tailed t‑test failed to reject the null hypothesis of zero effect, and bootstrap resampling produced identical non‑significant results. In short, the statistical analysis finds no robust, reproducible DTR response to Forbush decreases, suggesting that any cosmic‑ray‑induced changes in cloud microphysics or radiative balance are too small to be detected in the surface temperature range at continental scales.

The second part of the study turns to spectral analysis. After linearly interpolating occasional missing values, the detrended DTR series was subjected to a discrete Fourier transform. The power spectrum revealed distinct peaks near 27 days, 13 days and 9 days. The 27‑day peak corresponds to the solar rotation period and is the most prominent, although its amplitude varies over the multi‑decadal record, indicating a non‑stationary modulation. The 13‑day and 9‑day peaks are harmonics of the solar rotation, often associated with the appearance of multiple active regions on the solar surface.

The authors argue that these periodicities are more plausibly linked to variations in solar irradiance, especially in the ultraviolet (UV) band, rather than to changes in cosmic‑ray flux. UV radiation strongly influences stratospheric ozone production; ozone, in turn, modulates the absorption of solar energy and can affect tropospheric temperature gradients. A fluctuating UV flux on the 27‑day solar‑rotation timescale could therefore imprint a weak but detectable signal on the DTR through radiative‑convective adjustments.

The paper also discusses methodological caveats. Surface DTR measurements are subject to station‑specific biases, urban heat‑island effects, and changes in instrumentation over decades, all of which can introduce spurious variability. The FD selection criterion (≥5 % drop) may miss smaller but more frequent events, reducing statistical power. Fourier analysis assumes stationarity; non‑stationary trends or abrupt climate shifts could distort the spectrum, leading to over‑interpretation of peaks.

In conclusion, the authors’ independent re‑examination does not support the previously reported correlation between Forbush decreases and short‑term DTR changes. Instead, the detection of a quasi‑27‑day periodicity, together with its harmonics, points toward a solar‑irradiance driver—most likely UV variability—as the more credible source of the observed DTR modulation. The study recommends future work that couples high‑resolution satellite UV measurements with atmospheric chemistry‑radiation models to quantify the mechanistic pathway from solar UV fluctuations to surface diurnal temperature range changes.