Effects of vernal equinox solar eclipse on temperature and wind direction in Switzerland

Abstract

Abstract. The vernal equinox total solar eclipse of 20 March 2015 produced a maximum occultation of 65.8-70.1 % over Switzerland during the morning hours (09:22 to 11:48 CET). 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 10 min 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 20 Hz resolution.During maximum occultation, the temperature drop was up to 5.8 K 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 ± 0.02 K (mean ± SE). A detailed comparison with literature values since 1834 showed a temperature decrease of 2.6 ± 1.7 K (average of all reports), with extreme values up to 11 K. On fair weather days under weak larger-scale pressure gradients, local thermotopographic wind systems develop that are driven by smallscale 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 ≈

Ensemble-Averaging Resolves Rapid Atmospheric Response to the 2017 Total Solar Eclipse

“…Similar near-surface thermal responses were also simulated by McInerney et al (2018). Second, the atmospheric pressure decreases due to the eclipse cyclone (Clayton, 1901;Eugster et al, 2017) in response to the rapid atmospheric cooling in the eclipse umbra. Pressure changes of 0.26-2.0 hPa have been observed (Anderson and Keefer, 1975;Aplin and Harrison, 2003;Eckermann et al, 2007;Aplin et al, 2016).…”

“…First, the near-surface air temperature decreases. The magnitude of this temperature change and timelag between the minimum solar radiation and the local minimum temperature is site-and event-dependent (e.g., on latitude, surface characteristics, synoptic forcing, cloud coverage, and time-of-day and duration of totality of the eclipse event), with an observed temperature drop of 1.5-10 • C (Stewart and Rouse, 1974;Anderson and Keefer, 1975;Aplin and Harrison, 2003;Aplin et al, 2016;Eugster et al, 2017), and with 15-20 min lags (Antonia et al, 1979;Aplin et al, 2016). Similar near-surface thermal responses were also simulated by McInerney et al (2018).…”

Observations of Thermally-Driven Winds in a Small Valley during the 21 August 2017 Solar Eclipse

“…We note that this average value is based on both partial and total solar eclipse events that occurred at various times throughout the day. Eugster et al [20] reported that the temperature reduction showed no dependence on eclipse type (partial vs. total), geographic location and several other factors. Additionally, the 5.0 • C recorded decrease is in line with other temperature measurements made across the continental United States by weather stations in the U.S.…”

“…A time lag between radiation changes and temperature response is visible in the main tower observations, with temperature decreasing following the decrease in radiation. The vertical temperature gradient at the main tower reversed at 14:06 EDT (35 min [20] compiled 130 historical observations of air temperature during solar eclipses dating back to 1834. The majority of these observations were taken within the paths of totality of total solar eclipses.…”

“…Anderson [19] suggested that rapid changes in thermally-driven orographic winds could be partially responsible for the anecdotal reports of the eclipse wind, a sudden increase in wind speed at totality. Eugster et al [20] used data from 165 surface weather stations in Switzerland to examine temperature and wind changes during the 20 March 2015 solar eclipse. Even though the eclipse was only partial in Switzerland, the cooling was sufficient to delay or prevent the morning transition to upvalley flows.…”