The Rainshadow

This LandSat image is of the northern end of the big island of Hawaii. LandSat images are an awesome way for meteorologists to analyze the impacts of weather. So what is so cool about this picture? Take a look at the vegetation differences from the northeast side of the island to the west end. A stark difference between lush greens and drought-like conditions. What causes this phenomena to occur so close together? It's known as the "Rainshadow" effect.
The Rainshadow is caused by two things: topography and prevailing winds. In this photo, the dominant winds are coming from the northeast and moving directly across to the west of the island. The prevailing winds encounter the land and it's topography. On Hawaii, winds meet the mountains of Mauna Kea to the south and Kohala to the north. The winds are forced to ride up and over the mountains. That process is called orographic lift.




As the prevailing winds are forced UP the mountain, that air will begin to cool. This side of the mountain is called the "windward" side. As the air rises and cools, it will condense and begin to form a cloud. We even see some clouds forming in the image. If and when the clouds cool enough, they will begin to dump rain as it continues to rise over the mountains. With all the rain that occurs on the windward side of the mountain, that provides ample water to keep the vegetation nice and green!
The prevailing wind is now on its way down the back side of the mountains, or the "leeward" side. Here, the opposite of the "windward" process takes place. As air sinks, it warms up. The air running down the leeward side of the mountain has thus lost all of its precipitation and is beginning to warm up, resulting in drier conditions. This process explains why it is so dry on the back side of the mountains! Sometimes, this whole process can lead to severe droughts near and behind mountainous terrains.

The Rainshadow phenomena occurs all over the world, including right here in Oregon! Our dominant wind pattern is from the west off the Pacific and to the east. Add the coastal mountain range and the larger Cascade range and presto, we have a rainshadow!
The dominant rainshadow is in Central Oregon. This is what gives places like Bend such a desert-like climate. The rainshadow isn't as dominant between the coastal range and the Cascades, but it is evident when you look at rain totals around the metro area. In large rain events, higher rain totals normally are expected EAST of the Portland-metro area into the foothills of the Cascades because that air is beginning to ride up the Cascades.