Yes, you read that correctly. The volcanic ash cloud over Europe might be expected to block sunlight and lower continental temperatures in a sort of nuclear winter scenario à la The Road, but according to this report in the Daily Mail the opposite may be true:
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Temperatures in Europe could rise as a result of planes being grounded across the continent, according to research.
A study conducted after commercial flights were grounded for three days following the September 11 terror attacks found the average daily temperature range in the U.S. rose markedly – exceeding the three-day periods before and after by 1.8c.
Scientists claimed this showed that clouds formed by the water vapour in the exhaust from jet planes have a small but significant effect on daily temperatures.