The simplest definition (but the least useful) is that all areas north of the northern polar circle (‘the Arctic Circle’) are Arctic. Polar circles are found in both the northern and southern hemispheres and are defined as the lines around the globe, north (or south in the case of the southern hemisphere) at which the sun does not set for at least one night of the year.
Currently, this is the case north of 66.5° northern latitude (respectively south of 66.5° southern latitude), but this varies in the course of natural history, as the tilt of the Earth’s axis changes slowly. The closer one comes to the Poles (90°) from the polar circles, the more nights in summer the sun will stay above the horizon – about half of the year at the Poles themselves.
This phenomenon of sunlight all around the clock is called midnight sun or the polar day. In the same areas, we find also the phenomenon of polar night – ie: the sun never comes above the horizon even at midday. From a mathematical point of view, polar day and polar night should be equally long at the same position above the polar circle. In reality, however, the polar night is somewhat shorter than the polar day, because the atmosphere of the Earth bends the sunlight so that it reaches slightly above the horizon.
It is for this reason that the sun is still visible even when it has just sunk under the horizon. In central Svalbard, the polar day lasts about four months without any sunset, whereas the polar night lasts only about three months and 20 days. During the polar night, it is truly dark all around the clock only at very high latitudes, for instance around Christmas in Longyearbyen. This gives excellent opportunities for watching the relatively weak northern lights at any time of the day (provided the sky is clear).
This is possible only in places as far north as Svalbard. At the beginning and end of the polar-night period, or in areas only just above the polar circle (for instance northern Norway), there is a ‘dawn’ during daytime, with the sun being only slightly under the horizon at midday. As for the natural world, it makes very little difference whether the sun sets for one night or not. For instance, in Sweden the polar circle runs through huge boreal forests, with nothing much ‘Arctic’ on either side.
Therefore, the polar circle is not a useful definition for ‘Arctic’ when it comes to describing a certain type of nature. Instead, this definition is more popular as a boast, and the oft-romanticised attribute ‘Arctic’ is used a lot in northern Scandinavian tourism marketing, and for claiming subsidies from the European Union for northern Sweden and Finland for ‘Arctic’ agriculture. Canadians, on the other hand, would be rather less pleased with the polar circle as the border demarcating the Arctic, since both a typical Arctic landscape and polar bears reach much further south.
This is the most easily recognisable definition of Arctic – though only on land. Here, the Arctic is everything north of the treeline – ie: everywhere where there are no high-growing trees or bushes even under favourable conditions. This vegetation type without high-growing plants is called tundra. Isolated patches of tundra exist outside the Arctic and Antarctic, for instance in high mountain areas.
Botanically, there are still trees (polar willows, dwarf birches) and bushes (dryas, etc) in the tundra, as botanists define trees as plants with wood, but these plants reach maximum heights of only about 15cm in Svalbard. The advantage of this treeline definition is that the lack of high-growing plants is a clearly discernible landscape feature. The disadvantage: the definition applies only on land.
10°C July isothermal line
This is a more theoretical definition, but can be applied on land and on sea. According to this definition, the Arctic is where the long-term mean temperature of the warmest month – normally July – is less than 10°C. This looks like a random value but if this line is drawn on a map of the northern hemisphere, it matches the treeline on land. Usable solar energy is insufficient north of this 10°C July isothermal line for the development of high-growing plants during the productive season of the vegetation. Before July, the white winter snow still covers too much of the land and reflects back much of the solar energy. With monthly mean temperatures of 4–6°C in July, Svalbard is well within both this climatological definition area of the Arctic and the botanical treeline definition, and the same applies for both Jan Mayen and Franz Josef Land.
In natural sciences, a combination of the botanical treeline definition and the 10°C July isothermal line makes most sense when describing a boundary for the Arctic. This would then include Svalbard, Franz Josef Land and Jan Mayen, most of Greenland (except the southern end), all of the northern Russian islands plus a wide zone of northernmost mainland Russia from Kola to the Bering Strait, the north of Alaska, all of the northern Canadian islands, and the north of the Yukon, the Northwest Territories, Quebec and even northernmost Manitoba.
In the area of Hudson Bay, the Arctic even reaches south of the polar circle according to this definition, whereas in Scandinavia only a minor region around Vadsø would be regarded as Arctic whereas the Magerøy Island and the mass tourism of North Cape is actually still south of the treeline. Most of Iceland is also non-Arctic according to this line of definition.
Based on this, one can describe the Arctic geographically as the Arctic Ocean around the North Pole, encircled by and including the northernmost parts of the neighbouring continents and archipelagos. At the North Pole, the Arctic Ocean is about 4,200m deep, covered by drifting ice on the water surface. This is almost the opposite of the Antarctic in the south, which is basically a continent around the South Pole, surrounded by oceans and some included nearby islands. At the South Pole, the surface of the ice cap is more than 4,000m above sea level.