Complex sentence: The summer was one of the wettest since records began in 1766: rainfall in the three months of June, July and August was more than twice the annual average, and August, 1912, was the coldest, dullest and wettest August of the 20th century.
Split into simpler sentences: The summer was one of the wettest since records began in 1766, with rainfall in the three months of June, July and August being more than twice the annual average. August 1912 was the coldest, dullest and wettest August of the 20th century.