Visibility
Given the distance of many parts of Wales from the industrial and populous
areas of Britain and mainland Europe, much of Wales enjoys excellent visibility.
Given the mountainous nature of the country and its proximity to the sea,
hill fog can be both extensive and frequent and is a potential hazard
to be borne in mind by walkers in Snowdonia.
Rainfall
Rainfall in Wales varies widely, with the highest average annual totals
being recorded in the mountainous areas of Snowdonia, where the yearly
rain fall is comparable with that in the English Lake District or the
western Highlands of Scotland.
Throughout Wales, the months from October to January are significantly
wetter than those between February and September, unlike places in south-east
Scotland or in the English Midlands where July and August are often the
wettest months of the year.
Winds
There is a close relationship between surface isobars (lines joining points
of equal air pressure) and wind speed and direction over open, level terrain.
However, local topography also has a very significant effect, with winds
tending to be aligned along well-defined valleys.
It is this gustiness which causes much of the damage to buildings and
trees during storms and is the greatest danger in the mountains.
The strongest winds in Britain are associated with the passage of deep
depressions across or close to the country; these are most frequent during
the winter, so that is when gales are most frequent. These depressions
are usually at their most intense over the open Atlantic Ocean, the Strongest
winds being observed over the summits of hills and mountains.
Temperature
Over Wales the mean annual temperature at low altitudes varies from about
9.5 °C to 10.5 °C, with the higher values occurring around or
near to the coasts. The mean annual temperature decreases by approximately
0.5 °C for each 100 m increase in height so that, on this basis, Snowdon
(at 1,085 m) would have an annual mean temperature of about 5 °C.
In winter, temperatures in the British Isles are influenced to a very
large extent by those of the surface of the surrounding sea, which reach
their lowest values in late February or early March.
Around the coasts February is thus normally the coldest month, but inland
there is little to choose between January and February as the coldest
month.
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 moderating 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 -23.3 °C, the lowest ever recorded
in Wales, at Rhyader on 21 January 1940.
Coastal areas do not experience such cold nights.
At the opposite extreme, some of the highest winter temperatures in the
British Isles have been recorded in North Wales. These high winter temperatures
(up to 18 °C on occasion) occur when a moist south to south-easterly
airflow warms up downwind of Snowdonia after crossing the mountains, an
effect known as the föhn after its more dramatic manifestations in
the Alps. July is normally the warmest month in Wales, and the highest
temperatures of all have occurred furthest away from the cooling influence
of the Atlantic.
Snow
Snow is comparatively rare near sea level in Wales, but much more frequent
over the hills. The average number of days each year when sleet or snow
falls in Wales varies from about 10 or less in some south-western coastal
areas to over 40 here in Snowdonia.
The average number of days with snow lying in Wales varies from six or
less around the coasts to over 30 in Snowdonia.
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