The humid subtropical climate, marked by high humidity (especially in summer) and the absence of really cold winters, prevails in Southern Atlantic and Gulf Coast States.
In the Koppen-Trewartha system, this area lies within the Cf climate, described as temperate and rainy with hot summers (see Appendix 2, climate diagram for Atlanta, Georgia). The Cf climate has no dry season; even the driest summer month receives at least 1.2 in (30 mm) of rain. The average temperature of the warmest summer month is above 72F (22C). Rainfall is ample all year, but is markedly greater during summer. Thunderstorms, whether of thermal, squall-line, or cold-front origin, are especially frequent in summer. Tropical cyclones and hurricanes strike the coastal area occasionally, always bringing very heavy rains. Winter precipitation, some in the form of snow, is of the frontal type. Temperatures are moderately wide in range and comparable to those in tropical deserts, but without the extreme heat of a desert summer.
Soils in the moister, warmer parts of the humid subtropical regions are strongly leached Ultisols related to those of the humid tropical and equatorial climates. Rich in oxides of both iron and aluminum, these soils are poor in many of the plant nutrients essential for successful agricultural production.
Forest provides the typical vegetation throughout most of this division. Much of the sandy coastal region of the Southeastern United States is covered by second-growth forests of longleaf, loblolly, and slash pines. Inland areas have deciduous forest.