Monitoring and analysing the effect of solar eclipses worldwide on weather conditions in the boundary layer

Monitoring and analysing the effect of solar eclipses worldwide on weather conditions in the boundary layer: co-ordinated, led and published the first-ever analysis (of the 11 August 1999 total solar eclipse) from a comprehensive station network (Hanna, Weather, 2000).

A rare and spectacular total solar eclipse crossed south-west Britain on 11 August 1999; the rest of the country was subjected to a deep partial eclipse. For most of Britain the eclipse lasted from about 10:00 to 12:35 BST, with maximum obscuration of the Sun by the Moon between about 11:12 and 11:22 BST. Despite generally cloudy conditions in south-west England, although considerably clearer further east, a distinct dip in the daily temperature profile is notable synchronous at various stations across the UK (see the graph below).

This eclipse was exceptional in having a track crossing densely populated land masses and was a near-midday, summer eclipse. Therefore it provided a unique opportunity to assess the influence on meteorological parameters such as surface air temperature, wind, radiation and cloud, and I received a tremendous response (>140 replies) in answer to my request letter in WEATHER magazine (Royal Meteorological Society) for relevant observations.

Despite an encroaching weather front introducing synoptic changes to the relatively static weather pattern over the UK, these reports collectively show quite convincingly that the eclipse caused a widespread decline in surface air temperature of typically 1-3ºC at standard screen height (~1.5 m). As the boundary (surface) layer temporarily became decoupled from the higher atmosphere, ground temperature (where measured) fell considerably more than this. Some of the largest surface air temperature drops attributed to the eclipse (locally up to ~6ºC) were measured in south-east England and East Anglia, where skies were relatively clear and the (approximately 97%) eclipse was very near to total. However, the screened air temperature drop in these regions was more generally 2-3ºC. In south-west England (across a large part of which totality occurred) generally thick cloud cover muted the air temperature response to a modest 0.5-2.0ºC drop.

There is considerable evidence from quite a large number of stations that low-level, convective cumulus cloud dispersed during and immediately following mid-eclipse, and already light winds (any changes in which were unlikely to have caused the widespread air temperature drop) widely fell calm.

Since major solar eclipses so clearly affect local and regional weather patterns, it is hoped that my research – based here on the help of many observers, gratefully acknowledged - will stimulate similar meteorological observations and further research at future such events. Please contact me if you would be interested in collaborating or working with me in this research area.