South of the warm continental climate lies another division in the Humid Temperate Domain, one with hot summers and cool winters (see Appendix 2, climate diagram for Fort Wayne, Indiana). The boundary between the two is the isotherm of 72F (22C) for the warmest month. In the warmer sections of the Hot Continental Division, the frost-free or growing season continues for 5 to 6 months, in the colder sections only 3 to 5 months. Snow cover is deeper and lasts longer in the northerly areas.
In the Koppen-Trewartha system, areas in this division are classified as Dca (a signifies hot summer). We include in the Hot Continental Division the northern part of Koppen's Cf climate region in the eastern United States. Koppen uses as boundary between C and D climates the isotherm of 26.6F (-3C) for the coldest month. For example, Koppen places New Haven, Connecticut, and Cleveland, Ohio, in the same climatic region as New Orleans, Louisiana, and Tampa, Florida, despite obvious sharp differences in January mean temperatures, soil groups, and natural vegetation between these northern and southern zones. Trewartha (1968) redefined the boundary between C and D climates as the isotherm of 32F (0C) for the coldest month, thereby pushing the climate boundary south to a line extending roughly from St. Louis to New York City. Trewartha's boundary is adopted here in distinguishing between humid continental and humid subtropical climates.
Vegetation in this climate division is winter deciduous forest, dominated by tall broadleaf trees that provide a continuous dense canopy in summer, but shed their leaves completely in winter. Lower layers of small trees and shrubs are weakly developed. In spring, a luxuriant ground cover of herbs quickly develops, but is greatly reduced after trees reach full foliage and shade the ground.
Soils are chiefly Inceptisols, Ultisols, and Alfisols, rich in humus and moderately leached, with a distinct light-colored leached zone under the dark upper layer. The Ultisols have a low supply of bases and a horizon in which clay has accumulated. Where topography is favorable, diversified farming and dairying are the most successful agricultural practices.
Rainfall decreases with distance from the ocean. Therefore, this division is subdivided into moist oceanic and dry continental provinces.