The sub-arctic climate is the most extreme type of micro-thermal climates. In a micro-thermal climate the average temperature of the coldest month is below 0°C, and at least one month has an average temperature above 0°C.
Thus, the pole-ward boundary of the sub-arctic climate is set by the 0°C isotherm for the warmest month of the year.
This isotherm coincides with the pole-ward limit of the tree growth. Thus, the northern boundary of the sub-arctic climate is the pole-ward limit of forest growth as well.
The sub-arctic climate is found only in North America and Eurasia between 50″ or 55°N and 65° or 70°N latitudes. Only these two continents have large land masses in the higher middle latitudes to have this severe climate.
This climate in Eurasia extends from Sweden and Finland in Europe across much of the former Soviet Union to the Bering Sea and the Sea of Okhotsk. Taiga is the name given to the sub-arctic lands by the Russians.
Now, the term Taiga applies to the sub-arctic climates both in North America as well as in Eurasia. In North America the sub arctic climate extends from Alaska, across Canada, to Labrador and Newfoundland.
On its pole-ward margin this climate merges into tundra climate north of the 0° isotherm for the warmest month. On its southern margin the sub-arctic climate grades into the mild- summer type of humid continental climate.
In some parts of the continental interiors, the sub-arctic climate gradually passes into the semi-arid steppe climate where, located on the west coast of a continent, the boreal climate borders the marine west coast climate.
In North America the western, segment of the boreal climate is found a little beyond the Arctic Circle (66V2 °N). On the contrary, the boreal climate is pushed farther south towards the eastern margins of both North America and Eurasia.
It may be noted that the development of the sub-arctic climate is more extensive in Eurasia than it is in North America. Besides, the Eurasian sub-arctic climate is more severe as well.
The reasons are twofold. Firstly, the Eurasian land mass being larger, the effect of continentally is increased. Secondly, the effect of continentally is modified by the vast water surface of the Hudson Bay in Canada.
The sub arctic regions in the higher middle latitudes of North America and Eurasia are the major source regions for the cold polar air masses which invade other climatic regions further south. The higher middle latitude regions are characterized by high pressures, low temperatures, and relatively dry air.
The continental position occupied by this climate results in intense heating and cooling of the land. However, because of their high-latitude position, the winter cooling is more intense than summer heating and lasts longer.
Therefore these climates have long and bitterly cold winters, whereas summers are very short and cool lasting for only one to three months.
Winter in this climatic region is of 8 months duration. Average January temperatures for a number of inland stations may give an idea of the winter conditions in the boreal climate : Eagle, Alaska -26°C; Dawson, Canada, -30.6°C; Okhotsk, U.S.S.R., -24°C; Yakutsk, U.S.S.R. -43.3°C; and Verkhoyansk, U.S.S.R., -50.6°C.
The Siberian sub-arctic climate has the lowest minimum temperature ever recorded on plains. Verkhoyansk and Oimekon have recorded the lowest temperature of -66.8°C, whereas the lowest temperature ever recorded in North American sub-arctic regions is only -62.8°C at Snag, Yukon. In Alaska and Canada winters are relatively less severe because of the smaller land masses and the moderating influence of the Hudson Bay.
The long winter nights in the sub-arctic regions further reduce the already low temperatures. Besides, long lasting snow cover reflects the small amount of radiation that reaches it with the result that little is absorbed. Snow cover cools the air so that areas of high atmospheric pressure are formed.
In addition, the extremely low temperatures of this climatic region during the long winter months cause vast areas to be permanently frozen to great depths. This is known as ‘permafrost’.
During summer, the temperature rises rapidly. July, the warmest month, has an average temperature around 16°C. Daily maximum of 28°C or above is common. In June and August the average temperatures vary from 10° to 15°C. In the months of May and September the average temperatures are between 4° and 5°C.
There are many locations which record freezing temperatures in July and August. It is noteworthy that the shortness of summer is more important than its coolness. But the longer duration of sunshine in this region more than compensates the shortness of summer.
At 60°N latitude the length of days in June is 18.8 hr., while at 65°N it is 22.1 hr. In fact, at the time of the summer solstice the amount of insolation received at the surface at latitude 60° N is almost equal to that at the equator. Thus, long summer days permit large quantities of heat and light to reach the earth’s surface.
Summer days in this climate are generally warm or even hot. But the growing season in this climate is very short ranging from 50 to 75 days. Midsummer frosts are not uncommon in the sub-arctic or taiga climate.
The annual ranges of temperature in this climate are very large. In Verkhoyansk where the January temperature drops to -50″C, the annual range is 64″C. There are some of the largest temperature ranges found anywhere on the earth. Diurnal ranges of temperature in this climate are also large.
The sub-arctic climate is characterized by meager precipitation distributed throughout the year. In Siberia the precipitation averages about 38 cm. while in Canada it varies from 38 cm. to 50 cm.
However, most of the precipitation occurs during summer when the polar anticyclone is weaker, and higher temperatures increase the moisture-holding capacity of the air.
Extremely low temperatures cause the winter precipitation to fall as fine, dry snow. Snow cover stays on the ground for a long period of 7 or 8 months. It is noteworthy that because of low temperature even the modest amount of precipitation is more than what can be evaporated. Therefore the taiga regions are moist enough for forest growth.
Coniferous forest is the natural vegetation of the sub-arctic climate. These needle-leaf coniferous forests are composed of evergreen spruce, fir and pine. In the Soviet Union this forest is called taiga which means ‘snow forest’. In the inner Siberia larches are the only coniferous trees that survive the rigours of the winter icy storms.
Larches shed their leaves during winter. Pole-ward the coniferous forests are less dense, and trees are not as tall and thick as in southerly parts. Taiga is not found beyond the 10°C isotherm for the warmest month.
Timber obtained from the taiga forests is not valuable for good lumber. However, these forests are valuable for the supply of fire wood and wood-pulp.
In the more humid and warmer regions deciduous trees, such as alder, aspen, mountain ash, birch, willow, and poplar are mixed with the coniferous trees.
The deciduous trees generally occupy the low swampy areas. Most of the trees are xerophytes. Ground in densely forested areas is covered with mosses and lichens. Taiga is the most valuable source for high-quality furs.
Because of the severity of climate, animal life is not very rich in this climatic region. Some of the animals which are the most important source for furs are the mink, ermine, otter, muskrat, fox, and wolf.
Other animals found in this climate are reindeer, deer, elk and moose. Besides, wild cat and other little carnivorous bird-hunters such as marten and pole-cat are also found. Various types of insect-eater birds like wood peckers, and fruit-eaters are also found in taiga.