A general increase of about 80mm (3.2in) per annum occurs for every 30m (100ft) increase in altitude.

Most of the rain falls over the hills and mountains of Snowdonia. Blaenau Ffestiniog is second only to Fort William in Scotland for rain among Britain's towns, with an annual average of 2,391mm (79in) - more than twice what Manchester gets. The eastern flank of Snowdon is among the wettest places in the UK, with a yearly average of at least 180 inches (4,572mm)!

However, along the coast it is much drier. At Valley on Anglesey the annual rainfall is around 843mm (33in), while Llandudno is drier still with 750mm (29.5 inches). This is because the north coast lies in a rain shadow, sheltered from the prevailing southwesterly winds by the hills and mountains to the south west.

For this reason, the North coast has also seen some notable out-of-season heatwaves, thanks to the fohn effect whereby warm moist air blowing across Snowdonia dries out and warms further as it descends the lee slope. Thus Aber, near Llandudno, has recorded 18.3 degrees C in January and 21.3 degrees C in November.

There is relatively little seasonal variation in rainfall, except that the late spring and early summer, from March to June, tends to be the driest part of the year. Roughly half the annual rainfall falls between October and February. However, there can be marked dry or wet spells. It is possible to have 30 days in a row of dry weather; equally it is not uncommon to have periods of wet weather with rain on more than 50 consecutive days.

During a wet spell rainfall can vary greatly from place to place. Low lying and coastal areas may see very little but upland areas exposed to the wind can get a good soaking. This extra rainfall causing river levels to rise which sometimes leads to flooding.

Cool on the coast
Temperatures on the coast range from 7 Celsius in January to 16 Celsius in July (45 to 61F). The coast is the best place to be if you don't like the summer heat as sea breezes will often cool things down. However, if the wind is off the land it can get very hot on the coast with temperatures exceeding 31C (88F). 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. The coldest nights are those on which there is little wind, skies are clear, and there is a covering of snow on the ground; the lowest temperatures occur away from the influence of the sea, on the floors of inland valleys.

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. Nevertheless, with a wind off the land temperatures can plummet well below zero. Sunshine is greatest along the coast, with over 1,600 hours a year on average at Valley on Anglesey. Inland, however, especially over the hills and mountains, it is cloudier with an annual average around 1,100 hours.

As you climb higher into the hills and mountains temperatures generally fall and the weather can deteriorate rapidly.
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 on Anglesey to over 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 over 40 in the mountains. Mind you,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. Valley on Anglesey can expect a gale on 27 days or more in a typical year. Aberdaron on the tip of the Llyn Peninsula is also an exposed and windy place. On December 24th 1997 a gust of 112mph was recorded during the passage of a deep area of low pressure.

The weather in the mountains can be very different to the coast. It can be clear and sunny in Barmouth but in Betws y Coed pouring with rain or snow, with the hills and mountains shrouded in low cloud. Hill fog can be both extensive and frequent and is a potential hazard for walkers.