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Fair Isle weather
station
Northern Isles
weather
storms
wind extremes
Braer storm satellite image and animation
aurora borealis
Foula
lightning
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Shetland's climate
The main factors governing the
climate of Shetland are:
latitude and position
relative to large scale circulation patterns,
its position relative
to the ocean and the landmasses of Britain and Scandinavia,
its
altitude and local geographical features
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Shetland lies close to the
main depression track between Scotland and Iceland thus, in the main,
experiences an extremely mobile and often strong westerly regime. The influence
of the Atlantic and the North Atlantic Drift give Shetland a relatively small
diurnal and annual range of temperature. In more rare easterly regimes the
influence of Scandinavia and the North Sea give generally less rainfall, but a
higher incidence of haar. The islands' topography has some effect on sunshine
values between west and east and on site specific forecasts for wind and
temperature.
Temperatures
Comparison of Lerwick temperatures with individual sites in the U. K. is of
course possible from tables which are readily available, however this will not
reflect the true oceanic effects on a small landmass compared with the rest of
the U.K. There will be a greater seasonal and diurnal variation of temperature
over inland U.K. than in Shetland. This is because land has a low specific heat
relative to water, also heat exchange to air from land takes place from a
shallow surface layer (1m) as opposed to the large oceanic heat store.
Insolation, which varies with latitude, is also a consideration.
High temperatures and very low temperatures in Shetland are rare, but degree of
wind chill can be extremely high. Prolonged radiative cooling is also rare.
Sub-zero temperatures are most likely in sub-zero air masses or at night under a
clear sky with no wind and with a snow surface.
Variation of soil type has little effect on temperature extremes due to the
relative uniformity of peat cover. The effects of sandy soil around Sumburgh
airport will be offset by its low elevation (10m amsl) combined with close
proximity to the sea.
Some of the main arterial
roads and side roads pass through inland valleys where katabatic cooling can
cause frost to form. This can often happen on radiation nights in late spring
or early autumn catching the early commuter unawares.
Cloud Cover and Sunshine
Shetland's proximity to depression tracks will give a higher average cloud
cover than is experienced at lower latitudes. Extensive stratocumulus sheets
often accompany winter anticyclones and those of summer as well, especially if
they are anchored to the west of the British Isles giving the air over
Shetland a long sea track. Transient ridges of high pressure from the west or
northwest damp convection, spreading out stratocumulus, which will often
persist until the cloud from the next system arrives.
There is little seasonal variation of cloudiness at either Lerwick in the east
or at Hamnavoe on Burra Isle on the west coast. Hamnavoe however, has less
yearly average cloud cover than Lerwick due to a higher incidence of sea fog (haar)
and low stratus at Lerwick.
Building of the Scandinavian anticyclone in spring or summer can give Shetland
a long south to southeasterly sea track over a cold North Sea, advecting haar
onto eastern and southern coasts. Higher sunshine values at Hamnavoe reflect
the higher incidence of haar and stratus on the east side during the summer
months. For both Lerwick and Hamnavoe May and June are the sunniest months.
Visibility
True radiation fog in Shetland is comparatively rare and is generally short
lived. Other than in haar and precipitation, visibilities are generally good
especially with winds from northwest around to northeast. In dry southeasterly
airstreams haze can be advected north to Shetland dropping visibilities as low
as 2000m.
Persistent haar (more than
24 hours) can occur from May to September with maximum frequencies in May and
July. Advection from the south and southeast is the primary mode of occurrence
and can take some time to cover the length of the islands. Haar can continue
to keep visibilities below 1000m in winds up to 20kt. Diurnal clearance over
west Shetland is common due to entrainment of dry air from aloft by turbulence
as the barrier of hills that form the spine of Shetland lifts the
southeasterly airstream. The best conditions for this is when there is little
or no medium or upper cloud.
Precipitation
As well as being the sunniest, the months of May and June are also the driest.
This is due to the seasonal weakening of the mean pressure gradient between
high and low latitudes with the consequent weakening of the westerly regime.
As the Azores high builds during the summer this regime will become
intensified again steering frontal systems across the isles.
Local topography does have some effect on rainfall distribution. The
north-south trending high ground and Ronas Hill, produce orographic
enhancement as well as giving the eastern islands some rain shadow from
prevailing southwesterlies.
With a less marked sea/land temperature difference in summer than in mainland
U.K. showers are relatively infrequent, these need deep instability coupled
with a sharp upper trough. Thunderstorms due to daytime heating are extremely
rare, it is medium level instability moving up from the south that is most
likely to generate thunderstorms.
Deep instability in winter due to relatively warm sea temperatures can produce
vigorous convection. Cumulonimbus clouds thus produced often are limited in
vertical extent (as low as 8000 ft tops) but have all the nasty properties of
their big brothers, e.g. anvil top, severe icing and turbulence, although are
usually too limited to produce thunder. Deep convection with a sharp trough
over the area can produce thunder.
Snow falls at Lerwick around 68 days a year but only lies on about 26 days.
This due to a combination of (relatively) mild sea temperatures in early
winter and the returning sun in the spring. Frontal situations are most likely
to bring rain and snow rather than snow, however the cooling effect of
persistent precipitation can bring a change to snow. Deep instability in
winter can bring frequent heavy snow showers, however snow is most likely in
Polar Lows and troughs in north to northwest airstreams accompanied by
drifting and blowing snow in strong winds.
Winds
Due to Shetland's location vis-a-vis mean depression tracks, prevailing wind
direction is southwesterly. The frequency of Atlantic depressions passing
Shetland give average wind speeds are which noticeably higher than is the
usual experience at low level stations on mainland U. K.
Lerwick experiences 42 gale days a year, with no month gale free. January is
the windiest month with 8 gale days on average.
January 1993 was of special significance, not just for the wreck of the oil
tanker Braer, but for the staggering number of gales, which was instrumental
in clearing up the mess. This extreme month set the highest hourly mean wind
recorded at Lerwick (66kt), Also 25 gale days (10 of these were storm force or
more), 18 consecutive gale from the 1st to 18th and as if that wasn't enough
it was the wettest and second dullest on record.
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