The western ridges and valleys of the Allegheny Mountains are significantly wetter, colder, and snowier than the eastern ones. The prevailing weather systems carry precipitation from the midwest eastward. As the air masses collide with the WV highlands, they are forced upwards by the mountains. The air becomes less dense, cools, and is no longer able to hold as much moisture as it rises so it falls as rain or snow (scientists call this orographic lifting). Cooler temperatures and greater amounts of moisture in the Alleghenies also means more fog in the western Alleghenies compared to further east. Precipitation on the weatern slope runs between 50 and 70 inches a year which is similar to Seattle. Cool wet weather has a a profound effect on the plants and wildlife that can thrive in a forest. Black cherry, yellow birch, poplar, oak and hemloch are most common with a wide variety of generally lush understory. A comparison of the images in this gallery with the images in the dry eastern forest gallery visually demonstrate differences in habitats.