What causes humid climates?

Answer 1

Water evaporates faster at higher temperatures, so warm climates are more likely to be humid than colder climates are (indeed, in very cold weather, even surrounded by ice and snow, the humidity will be very low). But hot climates are not always humid. Deserts can be both hot and dry. Typically, a desert is in a "rain shadow" where an intervening mountain range, between the desert and the nearest ocean, is causing the rain to fall before it reaches the desert. Being close to an ocean is a good source of humidity. Lakes and rivers are also a source of humidity. Rain forests are humid, because the vegetation is very efficient at drawing water out of the ground and releasing it through the pores (or technically, stomata) in leaves. The rain is constantly being recirculated, at it falls into the ground and is again absorbed.

Answer 2

The latitude and proximity to a body of water (an ocean) are the two major influences on a climate being classified as humid subtropical.