United States, it is more common to use the isotherm instead. The average temperature was found to be roughly the minimum temperature necessary for tree reproduction and growth. Wide temperature ranges are common within this climate zone. Second letter in the classification symbol defines seasonal rainfall as follows: s: A dry summer—the driest month in the high-sun half of the year (April to September in the Northern Hemisphere, October to March in the Southern Hemisphere) has less than / of rainfall and has exactly or less than the precipitation of the wettest month in the low-sun half of the year (October to March in the Northern Hemisphere, April to September in the Southern Hemisphere). w: A dry winter—the driest month in the low-sun half of the year has exactly or less than one‑tenth of the precipitation found in the wettest month in the summer half of the year. f: No dry season—does not meet either of the alternative specifications above; precipitation and humidity are often high year-round. while the third letter denotes the extent of summer heat: a: Hot summer, warmest month averages at least , b: Warm summer, warmest month averages below but at least four months averages above . Associated precipitation Within North America, moisture within this climate regime is supplied by the Great Lakes, Gulf of Mexico and adjacent western subtropical Atlantic. Precipitation is relatively well distributed year-round in many areas with this climate (f), while others may see a marked reduction in wintry precipitation, which increases the chances of a wintertime drought (w). Snowfall 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 in North America and Asia an occasional tropical system. Though humidity levels are often high in locations with humid continental climates, the "humid" designation means that the climate is not dry enough to be classified as semi-arid or arid. Vegetation By definition, forests thrive within this climate. Biomes within this climate regime include temperate woodlands, temperate grasslands, temperate deciduous or evergreen forests, coniferous forests, and coniferous swamps. Within wetter areas, maple, spruce, pine, fir, and oak can be found. Fall foliage is noted during the autumn of deciduous forests. Hot summer subtype A hot summer version of a continental