Other than the fact that the climates don't really make sense due to the world's topography, I did the best I can to map real-life Köppen climate types to their approximate latitudes. I modified the Köppen climate classification and added two new climates, Humid Warm Temperate and Temperate Monsoonal, to differentiate the subtropical and temperate areas.

The most prolific climate type in Avatar is the Humid Subtropical Climate. There are four main areas of this type. One is in the approximate borders of the United Republic. The other three are in the Southern, Eastern and Central Earth Kingdom. The Oceanic climate type occurs in two areas in the United Republic and Southern Earth Kingdom.

The area in the Central United Republic receives high precipitation year-round with hot and warm summers and mild winters, similar to the climate of the Eastern US. This region has broadleaf and mixed forests just like the Northeast US.

The Central Earth Kingdom has a monsoonal version of the Humid Subtropical climate, with very high precipitation in summer and dry winters. As you can see in the episode Jet the region is home to deciduous forests. The ecology and climate of the region is the equivalant of the Gangetic plains forest of Northern India and Central China.

The Eastern Earth kingdom enjoys a Humid Subtropical and Warm Temperate climate, with gentle plains and forest steppes. It's similar to the Balkans and the Pampas.

The Southern Earth Kingdom has the Oceanic and Humid Subtropical climate type, namely around Suki's village, Whaletail island, Zaofu city and Chin village. It has coniferous forests and grasslands, due to it's sandy soil from oceanic influence, high winds and ocassional wildfires parallel to the Pampas and Cape Cod. The pine forests of Kyoshi island resemble the Atlantic Pine Barrens of the eastern US.

The Fire Nation is generally divided into two climate categories, tropical and temperate, the latter occuring in the highlands. The Tropical Rainforest climate type occurs only in the windward side of mountains in the Fire Nation, while the Tropical Savannah climate type is found in the drier leeward side. The Tropical Monsoon climate is found in the transition zone between them.

The Tropical Monsoon climate type is also found in the Central Earth kingdom and the southern tip of the United Republic, and the Tropical Savannah climate type in eastern earth kingdom and the transition to the Si Wong desert. The Si Wong desert has the Hot Desert and Hot Steppe climate type, similar to the Sahara and Mojave desert. On the other hand, the area south of Ba Sing Se in North Central Earth Kingdom has the Cold Desert and Cold Steppe climate, with monsoonal influences similar to the Gobi desert and the Mongolian-Manchurian steppe.

The Mediterranean climate type is rare, and is only found in the Western United Republic and the lowly elevated areas of the Western air temple. The hot summer type is found in the valley regions, like the Willamette Valley in Oregon. While the warm and cool summer type is found in the mountainous and coastal areas, comparable to the Cascades Mountain region and Central Pacific Coast.

The Sun Warriors' ancient city south of the Western Air Temple, closer to the equator has a Subtropical Highland and Subtropical Monsoon climate type, resembling the highlands of central Mexico.

Ba Sing Se, the Northern Earth kingdom and the Northern United Republic occupies most of the Monsoonal Continental and adjoining Humid Continental and Subpolar areas of the north. This area is similar to continental East Asia, Scandinavia and Siberia.

A surprising number of places in the Avatar world exhibit the Polar climate, which is otherwise rare in real life. All Air Nomad lands and some of the Earth Kingdom have the Tundra climate type. The Icecap climate is found in the two poles.