Rainfall is caused by the condensation of the water in air that is being lifted and cooled to its dew point. Rainfall tends to be associated with Atlantic depressions or with convection. The Atlantic Lows are more vigorous in autumn and winter and bring most of the rain that falls in these seasons. In summer, convection caused by solar surface heating sometimes forms shower clouds and a large proportion of rainfall is from showers and thunderstorms then. A further factor that greatly affects the rainfall distribution is altitude. Moist air that is forced to ascend hills may be cooled to the dew point, to produce cloud and rain. A map of average annual rainfall therefore looks similar to a topographic map.
Rainfall in Wales varies widely, with the highest average annual totals being recorded in the central upland spine from Snowdonia to the Brecon Beacons. Snowdonia is the wettest area with average annual totals exceeding 3000 mm, comparable to those in the English Lake District or the western Highlands of Scotland. In contrast, places along the coast and, particularly, close to the border with England, are drier, receiving less than 1000 mm a year.
Throughout Wales, the months from October to January are significantly wetter than those between February and September, unlike places in eastern England where July and August are often the wettest months of the year. This seasonal pattern is a reflection of the high frequency of winter Atlantic depressions and the relatively low frequency of summer thunderstorms. For example, at Cardiff, thunder occurs on an average of 11 days a year, compared with 15 to 20 days at many places in England. In west and north-west Wales the frequency drops to around 8 days per year.
The course of mean monthly rainfall for 1971 - 2000 for 4 sites is shown below. The pattern of rainfall is similar at each, with the months October to January the wettest and the late spring and early summer months the driest.
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Over much of Wales, the number of days with a rainfall total of 1 mm or more ('wet days') tends to follow a pattern similar to the monthly rainfall totals. In the higher parts, over 50 days is the norm in winter (December-February) and over 35 days in summer (June-August). In the driest areas of the east and south, about 40 days in winter and about 25 days in summer are typical.
The combination of close proximity to active weather systems arriving from the Atlantic and the extensive areas of upland can lead to notable daily and monthly falls. Daily totals exceeding 50 mm occur every other year, on average, in most parts of Wales. Periods of prolonged rainfall can lead to widespread flooding. A notable example was 17-18 October 1987, when up to 36 hours of rain resulted in totals of over 100 mm and severe flooding in west and north Wales. There was transport disruption, damage to property and 4 people drowned. Following an exceptionally wet autumn with over twice the normal rainfall, late October and early November 2000 also saw severe flooding, with the River Dee in north Wales particularly badly affected. Flooding and landslides also occurred in early February 2004, the area around Llanwrst in the Conwy valley of north Wales being hit hard. Preceding this, there was prolonged heavy rainfall across Snowdonia, with over 300 mm of rain falling at Capel Curig in 6 days.
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