Don't get wet
Cornwall for the most part is surrounded by sea, and it is hardly surprising that the sea exerts a marked influence on the weather and climate of the county. The weather is often much better on the coast and beaches than it is further inland and on Bodmin Moor.
On the eastern edge of the Gulf Stream the greatest effect is felt in winter, when temperatures are generally higher than would be normally expected at such latitudes; the mean January temperature in Cornwall is similar to such Mediterranean resorts as Benidorm, Nice, Naples and Corfu! In summer, however, the sea does moderate the highest temperatures.
During the winter the temperature rarely falls below freezing (0°C) and in summer, although the mean maximum is around 19°C, the daytime maximum often reaches 21°C and occasionally 30°C.
Rain falls the year round, with a distinct maximum during winter, mainly from the regular procession of weather fronts that move east across the Atlantic and the U.K.
In summer these fronts are generally less frequent, taking a more northerly track and are much weaker, so the rain is often derived from thunderstorms generated over France and drifting north across southern England, usually during the evening and overnight.
Cornwall can expect well over 1500 hours of sunshine annually and, whilst May is usually the sunniest month, May to July average around 7 hours of sunshine a day.
From a sailor's and windsurfer's viewpoint, the exposed coastal waters around the county yield a mean wind speed force 4 - 5 in winter and 3 - 4 in summer. Gales can be expected around 10 days per month from December to February, but on average less than 1 day per month from May to August.
The sea temperature varies from 9 - 10°C in February to 16 - 17°C in August.
Cornwall's Tourist Information Centres Cornwall Tourist Board