Why is there less vegetation on the western coast of most continents?
This is for a research project for Earth System Science class.

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Clarification of Question by mattorpedo-ga on 25 Sep 2005 12:08 PDT

I would like to know if there is a reason why this would happen in the month of May?

THat is because the earth rotates from west to east, or
counterclockwise if you are looking down from above the North Pole.
Meaning, the eastern coasts go first against thw wind; the western
coasts later.

The low-lying wind carries humidity or water or clouds with rain. As
the wind/clouds reach a high peak or mountain range at the eartern
coast, the wind/clouds rise over and above these high slopes, thereby
cooling the water in them. The water cools on the rise, condenses,
form droplets, falls as rain.

That is why also that the eastern slopes of mountains/mountain ranges
hve more, lusher vegetation than the western slopes. The rain might
not be enough to reach the western slopes.

The wind/clouds wiil gather water again as they move inland towards
the wester coasts, but the water accumulation is not as large in
quantity as that accumulated from the evaporation over the oceans
before the wind/clouds reach the eastern shores.

Forest and large vegetations rely only on rain for their water. Lesser
water quantity from rain results into lesser vegetation on the wester
coasts.

What is the definition of "less vegetation"?  1) Less varieties of
plants?  2) Less lush vegetation?  3) Less breadth of the inland
costal area with vegetation?

Europe only has a western coast, so either it doesn't count or
contradicts the premise.
Asia has no western coast, well, Turkey's, pretty thinly vegetated,
but the western coast of India and those of SE Asia are tropically
rich in vegetation.
S. America's western coast is bordered by the Andes for a great
length, so #2 & #3 are perhaps explained by that, and maybe #1 also,
as a result of the mountains' limiting natural spread of varieties
from elsewhere.
N. America's western coast is also influence by the Sierra Nevada
mountains, but further north quite rich, perhaps not in the varieties
of plants, whereby the northern climate of Alaska has an influence.
Australia's is separated by the desert, also prevailing winds may have
an influence.
Africa?  Yes, less vegetation, just flew over it with Google Earth.
The Antarctic, to complete the count:  no eastern or western coasts
and hardly any vegetation.

So:  to me it seems that three out of six continents do not fulfil the
premise that there is less vegetation on their western coasts.  For
the other three, geography is influencial, perhaps also prevailing
winds in the Southern Hemisphere, whereby the definition of more or
less vegetation needs to be defined and documented.

Have fun with your project, Myoarin

Hmm, guess I was away from my text longer than I thought, but I have
no argument with Ticbol's explanation.  S/he hopefully not with mine. 
:)

Hmmmm...

I live in the US, and here, the air moves mostly from west to east. 
At latitudes in the 30-50 range, flow is dominated worldwide by the
westerlies.  So, the west coast gets air from over the Pacific, which
doesn't change too much in temperature.  The east coast gets air that
has just crossed the continent, over land that can be quite hot or
quite cold.  That's why the west coast has a milder climate than the
east coast.

Perhaps ticbol's argument is valid in the tropics, where flow is
dominated by the trade winds, which do blow from east to west.