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During the partial (~50%) solar eclipse occurred on March 20, 2015, morning, various meteorological parameters were monitored to study their evolution above the urban area of Naples, central Italy. The experimental conditions were optimal because of the clear sky situation all over Italy, and in Naples in particular. The eclipse last about 2 hours between 9:25:06 (UT+1, local Italian time) and 11:43:09 (local Italian time, UT+1). From the observation site, the incoming solar radiation diminished by about 50% at the eclipse peak at 10:32:18 (local Italian time, UT+1), as expected. On the contrary, the UV radiation diminished significantly less, about 25%. This frequency response was likely due to Rayleigh scattering. It suggests that about 50% of the UV radiation reaching the surface was direct light and 50% scattered light. During the eclipse, the urban surface temperature and humidity values stayed almost constant instead of increasing and decreasing, respectively, as predicted by their daily cycle. This result was used to estimate that the average emissivity of the city of Naples is about f = 0.86. The wind speed decreased significantly during the event while the atmospheric pressure stayed constant to decrease only after the eclipse. Finally, we propose a simple empirical method to approximately estimate the cooling effect of an eclipse, which meteorologists could use to correct the temperature model forecast that normally ignores the occurrence of an eclipse. Consistency of these results with the literature and its importance is briefly discussed.
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