South of the eastern area of the subarctic climate, in the region between lat. 40 and 55 N. and from the continental interior to the east coast, lies the humid warm-summer continental climate. Located squarely between the source regions of polar continental air masses to the north and maritime or continental tropical air masses to the south, it is subject to strong seasonal contrasts in temperature as these air masses push back and forth across the continent.
In the Koppen-Trewartha system, this area is designated as Dcb, described as a cold, snowy winter climate with a warm summer (see Appendix 2, climate diagram for Iron Mountain, Michigan). The Dcb climate has 4 to 7 months when temperatures exceed 50F (10C), with no dry season. The average temperature during the coldest month is below 32F (0C). The warm summer signified by the symbol b has an average temperature during its hottest month that never exceeds 72F (22C). Precipitation is ample all year, but is substantially greater during the summer.
Needleleaf and mixed needleleaf-deciduous forest grows throughout the colder northern parts of the humid continental climate zone, extending into the mountain regions of the Adirondacks and northern New England. Here soils are Spodosols with a low supply of bases and a horizon in which organic matter, iron, and aluminum have accumulated. They are strongly leached, but have an upper layer of humus. Cool temperatures inhibit bacterial activity that would destroy this organic matter in tropical regions. Soils are deficient in calcium, potassium, and magnesium, and are generally acid. Thus, they are poorly suited to crop production, even though adequate rainfall is generally assured; but conifers thrive in them.