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Environmental Change
Network
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1999 Eclipse
Researchers at Wytham used additional climate monitoring equipment to get a set of data averaged over 2 minute intervals through the eclipse, to complement the standard hourly ECN measurements. As expected, light levels fell dramatically during the eclipse, by approximately 96% in fact - the amount to which the sun was obscured by the moon in the Oxford area. The graph shows that this was greater than the various ups and downs caused by passing clouds at the beginning and end of the period. Observers did not experience this degree of darkening because our eyes compensate for changes in light levels. On the other hand many eclipse watchers reported feeling colder and the air temperature did indeed fall, but only by 2.4 °C and the minimum was reached 14 minutes later than the maximum darkness. The temperature change experienced by a person will have been somewhat greater than this. Our sensing of temperature depends on the direct warming of the sunlight hitting our skin, as well as air temperature. The temperature at the ground surface is similarly dependent partly on the sunlight actually reaching it and at Wytham the grass field surface temperature dropped by 4.5 °C during the eclipse.
Researchers at Alice Holt also made additional meteorological measurements (averaged over five minutes) during the eclipse. The temperature and solar radiation data show a similar trend to Wytham, though with a slightly bigger temperature drop.
In addition the researchers at Alice Holt made flux measurements of transpiration and photosynthesis for the whole forest canopy. These measurements were made from a tower above the canopy. Both show a decline as solar radiation drops. Photosynthesis requires light and as its intensity falls, the rate at which carbon dioxide can be taken up during photosynthesis falls. In response to this, the leaves close their stomata to minimise water loss, hence transpiration falls. There will also be less water loss from surfaces by evaporation, as the solar radiation input decreases.
Because the photosynthetic rate of the trees has fallen there is a slight increase in atmospheric CO2 concentration - this is a balance between uptake in photosynthesis by plants and release by respiration in plants, animals and microbes (which will not have changed substantially during the eclipse).
Data provided by: Wytham (Natural Environment Research Council): Dr. Mike Morecroft Alice Holt (Forest Research): Sue Benham and Dr Mark Broadmeadow
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The eclipse on 11th August 1999 could be seen from the North Atlantic to India.
In the United Kingdom the eclipse was total over the Isles of Scilly, Cornwall, Devon and
Alderney but a partial eclipse could be seen over the whole of the UK (see the
accompanying map for the percentage of the sun covered). Researchers at two of
ECN's sites, 
