Abstract. The vernal equinox total solar eclipse of 20 March 2015 produced a maximum occultation of 65.8â70.1% over Switzerland during the morning hours (0922 to 1148CET). Skies were generally clear over the Swiss Alps due to a persistent high-pressure band between the UK and Russia associated with a rather weak pressure gradient over the continent. To assess the effects of penumbral shading on near-surface meteorology across Switzerland, air temperature data measured at 10min intervals at 184 MeteoSwiss weather stations were used. Wind speed and direction data were available from 165 of these stations. Additionally, six Swiss FluxNet eddy covariance flux (ECF) sites provided turbulent measurements at 20Hz resolution.
During maximum occultation, the temperature drop was up to 5.8K at a mountain site where cold air can pool in a topographic depression. The bootstrapped average of the maximum temperature drops of all 184 MeteoSwiss sites during the solar eclipse was 1.51±0.02K (mean±SE). A detailed comparison with literature values since 1834 showed a temperature decrease of 2.6±1.7K (average of all reports), with extreme values up to 11K. On fair weather days under weak larger-scale pressure gradients, local thermo-topographic wind systems develop that are driven by small-scale pressure and temperature gradients. At one ECF site, the penumbral shading delayed the morning transition from down-valley to up-valley wind conditions. At another site, it prevented this transition from occurring at all. Data from the 165 MeteoSwiss sites measuring wind direction did not show a consistent pattern of wind direction response to the passing of the penumbral shadow. These results suggest that the local topographic setting had an important influence on the temperature drop and the wind flow patterns during the eclipse. A significant cyclonic effect of the passing penumbral shadow was found in the elevation range ââ1700â2700mâa.âs.âl., but not at lower elevations of the Swiss Plateau. This contrasts with an earlier theory that the anticyclonic outflow should reach as far as ââ2400km from the center of the eclipse, which would have included all of Switzerland during the 2015 eclipse. Thus, measurable effects of penumbral shading on the local wind system could be even found at ââ2000km from the path of the eclipse (that is, Switzerland during the 2015 eclipse), and our results tend to lend support to a newer theory that the anticyclonic cold-air outflow from the center of the eclipse only extends ââ1600km outwards, with cyclonic flow beyond that distance.
