The climate is essentially oceanic with relatively small seasonal variations in temperature and rainfall - in other words, cool summers and mild winters. Mean monthly temperatures vary between 5 and 15 degrees C.

Annual rainfall varies from 800-1100mm and periods of snowfall and hard frosts occur only occasionally. This is in marked contrast to adjacent areas of the Welsh mainland and the mild climate helped the island secure the role of "Mon Mam Cymru" (the breadbasket of Wales), as a major grain producing area until recent times. Also in contrast to the mainland is the frequency of sunshine - wet weather fronts from the west pass rapidly over the low terrain, lingering over the mountains so that Holyhead on the west coast has 1550 hours of sunshine per annum in contrast to <1400 hours in the southeast of the island near the mountains of Snowdonia.

Details of local conditions are available on the official Meteorological Office site at RAF Valley Airfield on the west coast. Real-time data is available from Pentraeth on the east coast and a remarkable record of weather conditions and phenomena from Llansadwrn, also in the east of the island.