II.

Climate

The tundra climate is characterized by low average temperatures, harsh winters, and a short period of thaw during the summer when most plant growth occurs. Winter temperatures can be extremely low, typically averaging -20^o C to -30^o C (-4° F to –22° F) between November and February, and sometimes falling below -50^o C (-58° F). The summer lasts for three to four months but at the highest latitudes, average summer monthly temperatures may not climb above freezing. Typical average monthly temperatures recorded on the tundra around Fort Yukon, Alaska, are -30^o C (-22° F) in January, -5^o C (23° F) in April, 10^o C (50° F) in July, -10^o C (14° F) in October, and -30^o C (-22° F) again in December. The tree line marking the southern boundary of the Arctic tundra corresponds fairly closely to the 10^o C (50° F) July isotherm, so tundra and higher polar regions are often considered to be those where the average temperature in the warmest month does not exceed 10^o C (50° F).

High atmospheric pressure over the tundra means that the cold air is relatively dry. Annual precipitation is generally less than 380 mm (15 in) over most of the Arctic tundra. In some regions, drought conditions may prevail for periods of up to two months during the summer when temperatures are at their maximum. However, evaporation of moisture from the ground surface and upper soil horizons is slow in most areas because of the low ambient temperatures, so there is usually sufficient moisture on low-lying ground to support a continuous ground cover of plants. Indeed, the combination of frozen soil at depth and flat terrain can impede drainage so that the ground, when not frozen solid, becomes marshy and dotted with pools. These boggy areas support rich communities of plants and an abundance of insects which are, in turn, consumed by many species of bird which migrate north to the tundra each year to breed.