Group D: cold snow forest climates (Df, Dw)

Detail

1. What makes a climate "D"

• Koeppen's rule for Group D: two temperature tests must both be met.
• The coldest month is −3°C or below. This means there is real winter with lasting snow.

• The warmest month is above 10°C. This is warm enough for trees to grow, so the climate is still "forest" and not tundra.

• Name: "cold snow forest climates". The name joins the long snowy winter with the forest that covers these lands.

• Neighbours on the temperature scale:
• C climates are milder. Their coldest month is above −3°C.
• E climates are polar. Their warmest month is below 10°C, so there are no trees.

2. Where D climates are found

• Belt: the large continental interiors between 40° and 70°N:
• North America: from Alaska to Newfoundland (subarctic part), and the US Midwest, Great Lakes and south-eastern Canada (humid continental part) [6][7].
• Europe: eastern Europe, Russia and northern Scandinavia [7].

• Asia: Siberia, Manchuria, northern Korea and northern China.

• Two sub-belts (Britannica):

• The humid continental belt lies at about 30°–60°N in central and eastern North America and Asia. Here cold polar air masses and warm tropical air masses meet and fight [6].
• The continental subarctic belt lies further north, at about 50°–70°N. It stretches from Alaska to Newfoundland and from northern Scandinavia to Siberia [7].

3. Why D climates exist only in the Northern Hemisphere

• Cause: the southern continents become narrow between 40° and 70°S.
• South America tapers to a point. Africa and Australia end before 40°S.

• So there is almost no broad land at these latitudes in the south.

• Process: the Southern Ocean surrounds the small land areas that do exist. Water heats and cools slowly, so it keeps winters mild.

• Result: no southern place has a coldest month of −3°C or below together with a forest summer. D climates are therefore missing south of the equator.

4. Continentality: why winters get harsher poleward and inland

• Continentality means being far from the sea. Places deep inside a continent get no moderating effect from the ocean.
• How it works:
• Land heats fast in summer and cools fast in winter.
• Far from the sea, there is no water to soften these swings.

• So summers get hotter and winters get much colder → the annual range of temperature (the gap between the warmest and coldest month) becomes very large.

• Two trends inside the D belt:

• Winters become harsher towards the poles (less sunlight, longer nights).
• Winters become harsher further inland (more continentality). This is why the coldest places are in eastern Siberia, not on the coast.

5. Vegetation: taiga and mixed forest

• Taiga (the boreal coniferous forest: a forest of cone-bearing, needle-leaved trees in the cold northern lands) grows in the colder D types.
• Main trees: pine, spruce, fir and larch.

• Larch sheds its needles in winter. This helps it survive the very cold, dry winters of eastern Siberia.

• Mixed forest (a mix of broad-leaved trees such as oak and maple with conifers) grows in the warmer D types, which have longer summers.

• The taiga as a region and economy (lumbering, fur trapping, pulp and paper) is covered in world-physical-features.

6. Koeppen letters inside Group D

• Second letter (rainfall pattern) [5]:
• f = wet all year (no dry season).
• w = dry winter.

• s = dry summer (rare in Group D).

• Third letter (summer heat or winter cold) [5]:

Letter	Meaning	Example type	Common name
a	hot summer	Dfa, Dwa	Humid continental, hot summer [5][6]
b	warm summer	Dfb, Dwb	Humid continental, warm summer [5][6]
c	cool summer	Dfc, Dwc	Subarctic [5][7]
d	very cold winter	Dfd, Dwd	Subarctic, extreme winter [5][7]

• The humid continental types are Dfa, Dfb, Dwa and Dwb. The continental subarctic types are Dfc, Dfd, Dwc and Dwd (Köppen-Geiger-Pohl system) [6][7].
• Simple rule: a and b are humid continental, with mixed forest and farming. c and d are subarctic, with taiga.

7. Df: cold climate with humid winters

• Meaning: a D climate with no dry season. Snow falls in winter and rain falls in summer.
• Location: poleward of (to the north of) the marine west coast climate (Cfb) and the mid-latitude steppe (BSk).
• Main features:
• Cold, snowy winters.
• A short frost-free season, which means only a few months without frost. This limits the growing season for crops.
• Large annual temperature ranges.

• Sudden, short-lived weather changes. Passing fronts bring quick swings between warm and cold spells.

• Where the precipitation comes from [6]:

• Frontal cyclones in winter. These are low-pressure systems that form where cold polar air meets warm tropical air.
• Convectional showers in summer. Moist tropical air pushes north behind the retreating polar front (the boundary between polar and tropical air), the ground heats it, it rises, and short heavy showers fall.

• Result: many Df places get the most rain in summer, even though no season is dry [6].

• Subtypes and land use:

• Dfa and Dfb (humid continental): the US Midwest (Corn Belt), south-eastern Canada and eastern Europe. They grow maize and wheat and support dairy farming.
• Dfc (subarctic): covers most of Canada and Russia. This is the main taiga belt, with long winters and little farming.

8. Dw: cold climate with dry winters

• Meaning: a D climate where winter is dry and most precipitation falls in summer.
• Location: mainly north-eastern Asia: eastern Siberia, Manchuria (north-east China) and northern Korea.
• Why winter is dry: the Siberian High
• Siberian High (Siberian anticyclone): a large, strong anticyclone (a high-pressure system with sinking air and outward-blowing winds) that sits over central and eastern Siberia in winter [10][9].
• How it forms [10]:
  • In winter the land loses heat quickly and the air near the ground becomes very cold.
  • Cold air is dense, so it sinks.
  • More air flows in at higher levels to take its place. The weight of air over the ground grows → high pressure.

• Result: the air stays still over central and eastern Siberia, and some of the coldest temperatures on Earth occur there [9]. Dry, cold air flows outward from the high, so winters get very little snow.

• Summer: monsoon-like wind reversal

• In summer the land warms and the high weakens.
• The winds reverse and moist air comes in from the ocean side.

• This seasonal reversal is monsoon-like: dry outblowing winds in winter, moist inblowing winds in summer.

• Rainfall: concentrated in summer and low, 12–15 cm a year (NCERT).

• Winter: extremely cold, with up to seven months below freezing.

Feature	Df	Dw
Rainfall pattern	No dry season; snow in winter, rain in summer	Dry winter; most precipitation in summer
Main control	Frontal cyclones plus summer convection [6]	Siberian High in winter, wind reversal in summer [10]
Main regions	Canada, US Midwest, eastern Europe, Russia	Eastern Siberia, Manchuria, northern Korea
Coldest subtype	Dfd	Dwd (Verkhoyansk–Oymyakon)

9. Dwd and the Northern Hemisphere cold pole

• Cold pole means the place with the lowest temperatures in a hemisphere. In the Northern Hemisphere it lies in the Dwd zone of Yakutia (Sakha Republic), eastern Siberia, Russia.
• Verkhoyansk:
• It is about 115 km north of the Arctic Circle (about 67.5°N) [4].
• Its January mean is about −49°C [8].

• Its record low is −67.8°C (February 1892) [2].

• Oymyakon:

• It lies at 63°28′N, 142°23′E, at an altitude of 800 m [3].
• WMO lists its record low as −67.8°C on 6 February 1933. This is the lowest temperature recorded in WMO Region II (Asia) [3]. It is also often quoted as −67.7°C.
• Since the 1930s, Oymyakon has been colder than Verkhoyansk almost every winter, by an average of 2.3°C over a 43-year period in the 20th century [3].

• The 1892 Verkhoyansk value is disputed, because it was read on a spirit thermometer, which is less accurate than a mercury one [11].

• Update on the hemisphere record:

• In 2020 the WMO accepted −69.6°C, recorded on 22 December 1991 at the Klinck automatic weather station. This station stands at 3,105 m near the summit of the Greenland Ice Sheet [2].
• This value beat the Siberian figures and is now the Northern Hemisphere's lowest recorded temperature [2].

• Klinck is on an ice cap (an E climate). So the Siberian cold pole remains the coldest D-climate and inhabited region.

• World's largest annual temperature range:

• The annual range here is over 60°C.
• At Yakutsk, the January and July means differ by about 64°C. Its average winter temperature is about −42°C [9].

• Verkhoyansk recorded +38°C on 20 June 2020. The WMO accepted this as the Arctic's highest temperature on 14 December 2021 [4]. Put next to −67.8°C, this gives a gap of about 106°C between the town's recorded extremes [2][4].

• Climate change signal (2020 Siberian heatwave):

• Average temperatures over Arctic Siberia were up to 10°C above normal for much of summer 2020 [4].
• The Arctic is warming more than twice as fast as the global average [4].
• The heat also contributed to large fires and heavy loss of sea ice [4].

Prelims Hooks

• D climate rule: coldest month ≤ −3°C and warmest month > 10°C. The 10°C summer test separates D (forest) from E (tundra, ice cap).
• D climates are absent from the Southern Hemisphere: there is no broad land between 40° and 70°S, and the ocean keeps winters mild.
• Second letters: f = no dry season, w = dry winter, s = dry summer. Third letters: a = hot summer, b = warm summer, c = cool summer, d = very cold winter [5].
• Humid continental = Dfa, Dfb, Dwa, Dwb. Subarctic = Dfc, Dfd, Dwc, Dwd [6][7].
• Dw is found mainly in eastern Siberia, Manchuria and northern Korea. Its dry winter comes from the Siberian High. Its precipitation is low (12–15 cm a year, NCERT) and falls in summer.
• Taiga trees: pine, spruce, fir, larch. Larch is a conifer that sheds its needles in winter.
• Trap: the Northern Hemisphere's lowest recorded temperature is now −69.6°C at Klinck, Greenland (22 Dec 1991, accepted by WMO in 2020), not Verkhoyansk or Oymyakon (−67.8°C) [2].
• Trap: Verkhoyansk is both a cold pole and the site of the Arctic heat record (+38°C, 20 June 2020) [4].
• Oymyakon holds Asia's (WMO Region II) lowest temperature: −67.8°C, 6 Feb 1933 [3].

Mains Points

• Continentality shapes Group D (GS-I):
• D climates grow out of large land masses far from the sea, with huge annual ranges (Yakutsk about 64°C [9]).

• They are absent in the Southern Hemisphere because the land there is too narrow at these latitudes. This shows how the land–sea pattern, and not latitude alone, controls climate.

• Pressure systems and seasonal winds (GS-I):

• The Siberian High builds through winter cooling of the land. It brings dry, outblowing winter winds, then weakens in summer, and the winds reverse.

• This is a monsoon-like system. It shows the same land–sea heating contrast that drives the Indian monsoon, and it also feeds the cold, dry winter monsoon of East Asia.

• Resources and land use (GS-I/III):

• Dfa and Dfb belts are major grain and dairy areas (US Midwest, Canada, eastern Europe), with maize, wheat and dairy farming.
• Dfc and Dfd taiga supply softwood timber, pulp and fur.

• Farming is limited by the short frost-free season.

• Climate change in high latitudes (GS-III):

• The 2020 Siberian heatwave (+38°C at Verkhoyansk; summer up to 10°C above normal) brought fires and sea-ice loss [4].
• The Arctic is warming at more than twice the global rate [4]. This threatens the taiga, permafrost-based towns and the boreal forest's role as a carbon store.