View Full Version : A question on realism

04-07-2011, 02:01 PM
Hi, I've posted a few times here before. I've recently returned to a map I made last summer, and have recently had a potential problem pointed out to me that I would like to check with the greater community of this forum.

A friend of mine (who is a geography minor) recently told me that a certain wooded valley I have on my map technically shouldn't exist, due to the 'rain shadow' effect. When making the map, I did my best to make it as realistic as I could. I admit, I am somewhat ignorant of the rain shadow principle however.

Would anyone mind giving me their thoughts on this? The valley in question in located on the larger continent named Aevir. There is a large mountain range running from north to south on the western side, with a much smaller range to the east of it. In between these two ranges is a wooded valley with a river running through the middle. Is this realistic, or should this not occur in nature? I would like my world to look as natural as possible, without having to resort to magical influence.

This is the link to my map:

The posted map is somewhat lower quality than the hand drawn original, however it's the only form I have access to for the time being.

Any help would be greatly appreciated.

04-07-2011, 03:27 PM
I'm also no expert on rain shadows, but I live in British Columbia, and a lot of the highways here go through valleys with a river at the bottom and mountains to either side, and they generally have plenty of trees in them. Except for the southern interior, which is nearly a desert, and doesn't have much in the way of trees, period.

04-07-2011, 03:30 PM
Depending on the latitude, the prevailing winds would be blowing either westward or eastward. Since you've got a north-south aligned mountain range on both sides of the valley, the wind would be obstructed regardless of latitude. This causes the air to cool as it is forced to ascend the mountains, and the cooling causes the moisture in the air to form raining clouds. These rains will fall on the windward side of the mountain range, rather than the valley which is behind the mountains.

You might be able to ignore this to some extent if the mountains are small. Another workaround could be that there's large glaciers on the mountains, slowly accumulated during a long time period of coldness (an ice age perhaps) and that they are now melting due to warmed up climate. The waters from the melting glaciers would then be what supports the forests.

04-07-2011, 03:56 PM
I was going to say if they were smallish mountains that clouds could easily go around but Gman beat me to it. If they are very tall, like the Himalayas, then it might be just grasslands but then Gman's glacier tip (t'was excellent) and probably the best thing that you could tell your friend to shut him up. :)

04-07-2011, 04:58 PM
As always, I'm impressed and pleased by the helpfulness of people on this forum! Thanks for the replies, and I have a new question. If I go with the glacier idea suggested by Ghostman, that would also account for the river running through the valley. How long would such a glacier last?

04-07-2011, 05:12 PM
A glacier could last thousands of years. Of course it depends on a lot of factors. I'm no expert but I have a "glacier" of sorts in my fantasy setting that has survived for almost a millennium. (no global warming yet...) :)

04-07-2011, 06:29 PM
Glaciers move a few inches per day, so there still has to be snowpack above the glacier to serve as the origin. Glaciers don't just sit still for a thousand years, they move all the time. So there has to be sufficient influx of new snow pack to 'feed' the glacier, while at the glaciers end, it breaks off in chunks, and streams of dirty blue water seep out to form rivers beyond.

Regarding rain shadow. I was in the Hawaiian Islands on vacation seven or so years ago. Kuau'i the western most Hawaiian island is the wettest place on Earth with an average annual rainfall of around 800 inches per year. However to the southwest of the extinct volcano where all this rain occurs it looks like Arizona or Australia - its dry as bone.

Or consider the Andes mountains through South America. On the western side in southern Peru and northern Argentina is a very dry desert where mummies are placed in caves and completely dry out, where as to the east of the Andes, its the Amazon rainforest.

The prevailing winds carry moisture with it. When the moisture is forced to rise against high mountains, the air cools and rain is formed to fall. Beyond the mountain the air has no moisture left, so the opposing side is often desert.

Besides glaciers those mountains on either side of it could contain snowpack that melts into streams and waterfalls into that canyon of yours - so there's more than one way to get water into that valley.


04-07-2011, 10:46 PM
You should also use Washington State as an example. While we have the Olympic Peniunsila with the only Evergreen Rainforest. And on the other side is either very Barren Wasteland's or Desert.

04-08-2011, 07:06 PM
I'm also no expert on rain shadows, but I live in British Columbia, and a lot of the highways here go through valleys with a river at the bottom and mountains to either side, and they generally have plenty of trees in them. Except for the southern interior, which is nearly a desert, and doesn't have much in the way of trees, period.

A fellow British Columbia/Vancouver guy, eh?

When Looking at BC/Washington/Oregon for rainshadow effects one must be careful not to look at the coastal mountains & valleys and apply that to a fantasy world, but rather to look to the Rocky Mountains and the terrain that lies beyound, namely Alberta, Montana, Wyoming, Utah etc to truly see a rain shadow effect.

As the moisture heavy air is blown in off the ocean onto land, it is forced upwards by mountain where is cools and as a result holds less water. So, the BC/Washington/Oregon coast is wet because the moist air almost immediately hits mountain and is forced upwards to discard their water vapor (It's why North Vancouver has way more rain than Richmond/Surrey), but the air retains a lot of moinsture, as it heads inland (In BC at Least) it crosses over the Interior Plateau, a relatively flat region sandwiched between the Coastal Mountains and the Rockies (http://en.wikipedia.org/wiki/Interior_Plateau) This region has a lower rainfall because the air is heated here from the land and thus able to hold more water content as so, will not shed what it already possesses as it heads east to the Rockies. Once it hits the Rockies, a much higher mountain chain than the coast, it dumps a lot of it's moisture before in finally flows down the eastern slopes of the rockies into Montana/Alberta etc., where the North American Rain Shadow Begins.