Northern Isles weather

storms

wind extremes

Braer storm satellite image and animation

aurora borealis

Foula lightning

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

  • the thermo-haline circulation of the North Atlantic, including the warming influence of the North Atlantic Drift.

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.