Less rain falls from April to August although no month receives less than 50 mm (2 in).
Fog is common during summer due to the cooling effect of the sea on mild southerly airflows.
Another possible early written reference to the islands is Tacitus' report in Agricola in AD 98, after describing the discovery and conquest of Orkney, that the Roman fleet had seen "Thule, too".—"the Isles of Cats", which may have been the pre-Norse inhabitants' name for the islands.
The Cat tribe also occupied parts of the northern Scottish mainland and their name can be found in Caithness, and in the Gaelic name for Sutherland ( This is also the source of the ZE postcode used for Shetland. The next largest are Yell, Unst, and Fetlar, which lie to the north, and Bressay and Whalsay, which lie to the east.
Shetland has an oceanic temperate maritime climate (Köppen: Cfb), bordering on, but very slightly above in terms of summer average temperatures, the subpolar variety, with long but cool winters and short mild summers.
In contrast, winters are considerably milder than those expected in nearby continental areas, even comparable to winter temperatures of many parts of England and Wales much further south.
The general character of the climate is windy and cloudy with at least 2 mm (0.08 in) of rain falling on more than 250 days a year.
The most distinctive features are the ultrabasic Geological evidence shows that in around 6100 BC a tsunami caused by the Storegga Slides hit Shetland, as well as the rest of the east coast of Scotland, and may have created a wave of up to 25 metres (82 ft) high in the voes where modern populations are highest.
The highest point of Shetland is Ronas Hill at 450 metres (1,480 ft).