sunspot cycle

Variation in the output of the sun, especially in the ultraviolet, on an eleven year cycle. Low sunspot activity leads to a weakening of westerly winds over the Atlantic, bringing colder winter weather to north-west Europe. The last minimum was in 2009, the next maximum (the smallest for 80 years) is predicted for 2013.

This definition is abridged from A – Z of tree terms: A companion to British arboriculture.
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