The following is taken directly from the official website:
Cities: Skylines features three distinct terrain themes. These include north, sunny and tropic themes. Gameplay-wise the themes do not differ from each other, the effects are purely visual. This way the player can create their favorite maps using any of the themes available.
A theme includes its own set of trees (pine and conifer trees for north, broad-leaved trees for sunny and palm trees for the tropic theme – there are also some overlapping flora found in all of the themes), terrain colors, water texture and lighting conditions. For example, the sun is lower in the north theme and the grass is darker as well. The water is also a darker blue. In comparison the sunny theme has sun higher up in the sky and the lighting reflects more the Central and South European lighting conditions. The grass is greener and the water texture is somewhat more vibrant compared to the north theme. The tropic theme emulates tropical island atmosphere with sand texture that has a nice, slight glow to it when the sun hits it.
North theme represents the climate of northern countries such as those in Scandinavia. The sun has a bit colder feel to it accompanied with a toned down sky color, the grass is a bit darker as is the color of water. Pine and conifer trees make up the majority of flora but there are also a few broad-leaved trees and bushes to create variation.
Below are a few images of a small industrial town in the north.
Sunny theme represents central and south European climate. The sun is brighter and a bit warmer in the sunny theme with blue skies, the grass is brighter green giving a more vibrant feel and the color of water is lighter blue. Broad-leaved trees such as alder and beech make up the majority of the flora. Oak trees are included in the sunny tree roster as well as are few bush types.
Sunny metropolis divided by a river.
Tropic theme represents both the sunny California type climates as well as tropical island climates. The sun light is of warmer color and the sky color complements it nicely. The grass texture goes for a slightly different shade of green color compared to the sunny theme. The lush palm trees and bushes add to the grass color and create the iconic sunny beaches and tropical islands look.
Last but not least, a city stretching from the highway to the sandy beaches, just like in California.
Cities: Skylines releases March 10th for PC.
Source: Official Website