Mid Wales has a variety of climates and there can be considerable variation in the weather from place to place and from day to day. Micro climates exist, and exposure and topography explain a large proportion of the detail in variation.
Rainfall is plentiful. Most of the rain falls over the hills and the Cambrian Mountains, with some spots receiving over 2500 mm (98 inches) on average a year. For the Elan and Claerwen Valleys west of Rhayader, the annual rainfall is around 1760 mm (69 inches). The driest months are May and June with a monthly average of 99 mm (3.9 inches). The wettest month is December with a monthly average rainfall of 205 mm (8.1inches).
Along the Ceredigion coast, it is drier. At Aberporth, the annual rainfall is 872 mm (34 inches) while eastern Powys receives the least rainfall. Places such as Knighton and Presteigne along the border have less than 750 mm of rain a year (29.5 inches). This is because they lie in a rain shadow, sheltered from the prevailing south-westerly winds by the hills and mountains to the west.
Sunshine is greatest along the coast while it's cloudier inland, especially over the hills and mountains. At Cwm Ystwyth, the annual average sunshine total is just 1127 hours compared to 1547 hours at Aberporth on the coast. In January 1962, Llwynon in Powys recorded only 2.7 hours of sunshine.
On a summer's day, it can be clear and sunny in Aberystwyth with a refreshing breeze off Cardigan Bay - but warm and humid in Llanidloes. The higher temperatures inland cause clouds to develop, to rise over the mountains and, given the right conditions, thunderstorms and downpours may result.
In winter, temperatures are influenced to a large extent by the sea which is coolest in late February or early March and warmest in the autumn. On the coast, February is normally the coldest month but inland there is little difference between January and February as the coldest month. The coldest nights are those on which there is little wind, clear skies and a covering of snow on the ground. The lowest temperatures occur away from the influence of the sea, on the floors of inland valleys into which the cold air drains. It was under such conditions that the temperature fell to an amazing -23.3°C, the lowest ever recorded in Wales, at Rhayader on 21 January 1940.
Sea Breezes
Coastal areas do not experience such cold nights due to their closeness to the sea which acts like a blanket and here frost is less common. In summer, it can get hot inland but on the coast, sea breezes will often develop keeping the beaches pleasantly cooler.
Snow is fairly rare near sea level, but much more frequent over the hills and mountains. The average number of days each year when sleet or snow falls varies from about 10 or less in Aberaeron to more than 40 in the mountains. Snow rarely lies on the ground at sea level before December or after March, and the average number of days with snow lying varies from six or less on the coast to more than 30 in the mountains. The number of days of snowfall and snow cover varies enormously from one year to the next.
The windiest places are over the summits of hills and mountains, and along the coast. Aberporth on the west coast can expect a gale on 22 days or more in a typical year, with the number of days decreasing inland to 5 or less.
Inland and over high ground, fog statistics are scarce but given the mountainous nature and proximity to the sea, hill fog can be both extensive and frequent and is a potential hazard to be borne in mind by walkers.
