St Kilda has an oceanic (Atlantic) climate, modified by the physical effect of the island itself rising sharply from the sea. This often makes it cloudier than the surrounding sea area and increases rainfall and the local gustiness of the winds.

The strong oceanic influences and presence of the North Atlantic Drift result in higher winter temperatures than would be expected for the latitude. Conversely the summers are cool. The mean daily temperature in January is approximately 5.6ºC, while the July mean is approximately 11.8ºC.

The islands lie in the path of depressions approaching from the Atlantic and as a result have a high annual rainfall of approximately 1,400mm. This rainfall is well distributed throughout the year with a maximum in December-January, and the driest period, associated with anti-cyclones approaching from the Atlantic, falling between mid-April and mid-June.

Rounding Dun in a heavy seaThe prevailing winds on St Kilda are from the south-west, although approximately 20% of the winds come from the south-east, to which the village is fully exposed. Data are intermittent, but the wind speed recorded on St Kilda is normally Force 3 or above (13km per hour, 7 knots) for approximately 85% of the time and Force 5 or above (over 24km per hour, 17 knots) for more than 30% of the time. Gale force winds (63km per hour, at least 34 knots) are present for under 2% of the total time in any one year. However, within these gales, very strong gusts of over 185km per hour (100 knots) are common on the high tops.

The configuration of St Kilda, the nature of its physical landscape and its exposure to the full force of the North Atlantic amplify the effects of the weather. Sea spray has a dramatic effect on the vegetation and the weather often prevents access to the islands.