A continental climate can be observed in the middle-latitude interiors of the large continents of the Northern Hemisphere in the zone of westerly winds. The most obvious feature of this climate is the distinct division for the four year seasons. All such climates have at least three months of temperatures in excess of 10°C and winters with at least one month below 0°C. The winter temperatures are cold enough to support a fixed snow coating each year, and relatively low atmospheric precipitates occurring mostly in summer - although east coast areas (chiefly in North America) may show an even distribution of precipitation.

Continental climates exist where cold air masses infiltrate during the winter and warm air masses form in summer under conditions of high sun and long days. Places with continental climates are as a rule either far from any moderating effects of oceans or are so situated that prevailing winds tend to make their way offshore.
According the international climate classification system that is the most widely used system for classifying the world's climates, continental climate (referred to as "D-Group") can be subdivided into three categories: Dw - dry winters; Ds - dry summers; and Df - wet all seasons.

Today we talk about humid continental climate that can be observed over large areas of land masses namely in the regions of mid latitudes. It's an area where polar air masses confronted with tropical ones. This type of climate is called humid because of its variable weather patterns, mostly humid ones, related to cyclonic storms and its large temperature range due to its interior location in mid-latitude continents. The seasonal temperature variance can be as great as 25 to 35 degrees Celsius (45 to 63 degrees Fahrenheit).

Controlling Factors:

1). Variable weather conditions are called forth by its location and year-round influence of the polar front. This climate lies in the boundary zone between many different air masses, principally polar and tropical ones.

2). Polar-type air masses collide with tropical type air masses causing uplift of the less dense and humid tropical air causing precipitation. Colliding along the polar front, these air masses turn and swirl into large extra-tropical cyclones. Being removed from the moderating influence of the oceans, the climate experiences great swings in seasonal temperature.

3). Where the periphery of the climate borders the ocean, summer temperatures are slightly cooler and winter temperatures slightly warmer than the interior. Precipitation in the humid continental climate is primarily due to invasions of maritime tropical air.

Subtypes:

1). The warm summer subtype of the humid continental climate in North America lies on the eastern and Midwestern portion of the United States. The same climate subtype is also found in east central Europe, northern China, and northern Korea. The warm summer subtype is noted for its hot, humid summers and occasional winter cold waves.

2). The cool summer subtype of the humid continental climate in North America is found in New England, throughout much of the Great Lakes region and upper Midwest extending into south and south central Canada. It is also found in much of Scandinavia, Eastern Europe, and Russia. The cool summer subtype is marked by mild summers, long cold winters and less precipitation than the warm summer subtype.

Ecosystems (in conclusion):
The humid continental climate supports a diversity of ecosystems. The type of ecosystems is determined by their geographic location within the boundaries of the climate.  Mixed broadleaf deciduous forest is common in the southern and eastern portions of the climate in the United States. In the west where the precipitation is less, forests are replaced by grasslands.