A humid continental climate (Köppen prefix D and a third letter of a or b) is a climatic region typified by large seasonal temperature differences, with warm to hot (and often humid) summers and cold (sometimes severely cold) winters.
Precipitation is relatively well distributed year-round in many areas with this climate, while others may see a marked reduction in wintry precipitation and even a wintertime drought. Snowfall, regardless of average seasonal totals, occurs in all areas with a humid continental climate and in many such places is more common than rain during the height of winter. In places with sufficient wintertime precipitation, the snow cover is often deep. Most summer rainfall occurs during thunderstorms and a very occasional tropical system. Though humidity levels are often high in locations with humid continental climates, the "humid" designation does not mean that the humidity levels are necessarily high, only that the climate is not dry enough to be classified as semi-arid or arid. Very few areas with a humid continental climate fall in the Dsa or Dsb categories; generally these are adjacent to Mediterranean climates where the elevation precludes such classification due to colder winters.
Humid continental climates tend to be found above 40° N latitude, within the central and northeastern portions of North America, Europe, and Asia. They are much less commonly found in the Southern Hemisphere due to the larger ocean area at that latitude and the consequent greater maritime moderation.
The Köppen definition of this climate regarding temperature is as follows: the mean temperature of the coldest month must be below −3 °C(26.6 °F) (some climatologists prefer to use the freezing mark), and there must be at least four months whose mean temperatures are at or above 10 °C(50 °F). In addition, the location in question must not be semi-arid or arid.
Under Köppen, the following variants of this climate are possible:
The second letter
f: Does not meet either of the above specifications.
s: a dry summer — the driest summer month has at most 30 millimetres (1.18 in) of rainfall and has at most ⅓ the precipitation of the wettest winter month.
w: a dry winter — the driest winter month has at most one-tenth of the precipitation found in the wettest summer month.
The third letter
a: Warmest month averages above 22 °C(71.6 °F)
b: Does not meet the requirements for a, but there still are at least four months above 10 °C(50 °F).
Dfa/Dwa/Dsa: Hot (or very warm) summer subtype
Regions with hot-summer humid continental climates
A hot (or very warm) version of a continental climate features an average temperature of at least 22 °C(71.6 °F) in its warmest month. The warmest month is usually July, though in some cases, it can be August. In this region, July afternoon temperatures average up to 32 °C(90 °F), while the January mean temperature can be far below −3 °C(26.6 °F). In Europe, it is found in areas largely inland in Romania, Moldova, Ukraine, in the Pannonian Basin in parts of Hungary, Croatia and Serbia and in the Don River estuary of Russia. In Asia, around the Caspian Sea in Russia, Kazakhstan, in parts of Iran, Turkey and parts of Kashmir in India. It covers a great swath of Northeast China, almost all of North Korea, most of South Korea and much of northern Japan (including most of Hokkaido) as well as various central areas. In East Asia, this climate exhibits a monsoonal tendency with much higher precipitation in summer than in winter, and due the effects of the strong Siberian High much colder winter temperatures than similar latitudes around the world, however with lower snowfall, the exception being western Japan with its heavy snowfall. Within North America, it includes parts of southern New England and the Middle Atlantic states, much of the Midwestern United States, and small parts of Southern Ontario, Canada, from the Atlantic to the 100th meridian, and in the general range of between 39 °N to 44 °N latitude (with a larger north-south spread in the western portion due to the lack of maritime influences); precipitation increases further eastward in this zone and is less seasonally uniform in the west.
The 0 °C(32 °F)isotherm (freeze line) and the −3 °C(27 °F) isotherm (persistent-snow line) are often both argued as the statistical dividing line between the humid continental climate dominating areas to the north and west and the humid subtropical climate dominating areas to the south and southwest. The Köppen climate classification, the most popular climate classification, uses −3 °C(27 °F) as its lower threshold criterion; however, many climatologists in the U.S. prefer to use 0 °C(32 °F) as the standard because they feel it better reflects consistency in regional floristic character (i.e. forest composition/type).
Outside of North America the Dfa climate type is present near the Black Sea in southern Ukraine, the Southern Federal District of Russia, southern Moldova, and parts of southern and western Romania, but tends to be drier and can be even semi-arid, in these places. Tohoku in Japan between Tokyo and Hokkaidō also has a climate with Köppen classification Dfa, but is wetter even than that part of North America with this climate type. A variant which has dry winters and hence much lower snowfall with monsoonal type summer rainfall is to be found in north-eastern China including coastal regions of the Yellow Sea and over much of the Korean Peninsula; it has the Köppen classification Dwa. Much of central Asia, northwestern China, and southern Mongolia have a thermal regime similar to that of the Dfa climate type, but these regions receive so little precipitation that they are more often classified as steppes (BSk) or deserts (BWk).
It appears nowhere within the Southern Hemisphere, which has no large landmasses so situated in the middle latitudes that allow the combination of hot summers and at least one month of sub-freezing temperatures.
Regions with warm summer humid continental climates
The warm summer version of the humid continental climate covers a much larger area than the hot subtype. In North America, the climate zone covers from about 44°N to 50°N latitude mostly east of the 100th meridian. However, it can be found as far north as 54°N, and further west in the Canadian Prairie Provinces and below 40°N in the high Appalachians. In Europe this subtype reaches its most northerly latitude at nearly 61° N. Areas featuring this subtype of the continental climate have an average temperature in the warmest month below 22°C. Summer high temperatures in this zone typically average between 21–28 °C (70–82 °F) during the daytime and the average temperatures in the coldest month are generally far below the −3 °C(27 °F) isotherm.
Such high-altitude locations as South Lake Tahoe, California and Aspen, Colorado in the western United States exhibit local Dfb climates. The south-central and southwestern Prairie Provinces also fits the Dfb criteria from a thermal profile, but because of semi-arid precipitation portions of it are grouped into the BSk category.
In Europe, it is also found in central Scandinavia. In eastern Central Europe (eastern Austria, (small parts of) Germany, most of Poland, Czech Republic, Slovakia, Hungary, Ukraine, northern Romania) and in coastal areas of central Scandinavia is a warm summer subtype with less severe winters, more similar to the winters of the hot summer subtype found in eastern North America – the winters here are modified by the oceanic climate influence of western Europe.
The warm summer subtype is marked by mild summers, long cold winters and less precipitation than the hot summer subtype, however, short periods of extreme heat are not uncommon. Northern Japan has a similar climate.
Much of Mongolia and parts of southern Siberia have a thermal regime fitting this climate, but they have steppe- or desert-like precipitation, and so are not really considered to have a humid continental climate.