John Hirst, the chief executive of the Met Office, said unpredictable atmospheric conditions in the Atlantic had made things more difficult, despite a multi-million-pound investment in the service.
Last month was the dullest August since records began and one of the wettest because of a temporary shift in the Gulf Stream – the warm Atlantic current from the Gulf of Mexico – and a cooling Pacific current known as La Nina.
The combined effect was to bring damp air to countries bordering the North Atlantic and more unpredictable weather patterns.
Midway through the month, the Met Office gave people a ray of hope by predicting a fine end to August.
But in the end sunshine gave way to heavy thunderstorms and Britain had just 105.5 hours of sunshine in the month, compared with an average of 165.1.
Mr Hirst revealed the difficulties as the Met Office published its annual report detailing how the service had improved reliability for the third year in a row. But it admitted that about 15 per cent of temperature forecasts had been off the mark.
Trying to plan work in the garden the night before becomes futile, when there is a different forecast the next morning.
An entire month's rain could fall today