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Somes J
2015-Jun-03, 02:15 AM
My understanding is that the rule of thumb is that the east coasts of continents are wetter in the tropics. Why is it that equatorial East African countries like Kenya, Somalia, and Ethiopia have desert, grassland, and savanna landscapes while there is a huge rainforest at the same latitude to west? If I didn't know better I would expect it to be the other way around. I don't think it's because the east is more mountainous because if it was down to rain shadows then the west would get dried out too.

Does the African monsoon extend that far south?

Noclevername
2015-Jun-03, 03:35 AM
Why would rainshadows in the east dry out the west?

Jeff Root
2015-Jun-03, 06:46 AM
Knowing nothing about it, I'm wondering exactly where the
water that falls in the African rainforests comes from, and
what makes it fall where it does, rather than either closer
to or farther from the source.

-- Jeff, in Minneapolis

Cougar
2015-Jun-03, 12:42 PM
My understanding is that the rule of thumb is that the east coasts of continents are wetter in the tropics.

I don't know about your rule of thumb, but it's a good question. I've driven up to Lake Turkana, and it was like driving on Mars.

Kenya does have wet and dry seasons, but apparently not in the right proportions for rain forests (although there are some - around Mt. Kenya, for example). There must be something idiosyncratic about the weather pattern on that side by the Indian Ocean. Basically, it doesn't get enough rain. :)

grant hutchison
2015-Jun-04, 10:52 PM
The altitude has much to do with it: Nairobi is a mile above sea level. So rain coming in from the Indian Ocean tends to get dumped on the coast.
Yves Coppens "East Side Story" for human evolution suggests that the uplift of East Africa associated with the formation of the Rift Valley led to the drying of the original rainforest in that region, forcing human ancestors to make the transition from tree dwellers to savannah apes.

Grant Hutchison

dgavin
2015-Jun-06, 06:45 PM
Grant has it, it is an effect known as rain shadowing. As clouds come in from the coast, and are pushed up over high ranges, they lose water (rain) in proportion to how high they move up. Another thing that effects if it's the west or east sides that get the incoming clouds, is the trade/prevailing/polar easterlies winds. http://www.eschooltoday.com/winds/images/global-atmospheric-circulation.jpg

Jeff Root
2015-Jun-06, 08:22 PM
It looks to me like you are NOT saying that the reason it
doesn't rain in eastern Africa is that high ground shadows
the region, but that the ground is too low for the moist air
to be pushed up high enough for the water to condense.
Farther inland, where Nairobi is, as Grant says, is much
higher ground, so it rains there, not along the coast.

Looking at my map, I presume that no towns equals too
dry to sustain people. There are almost no towns near
the coast, east of the high ground.

-- Jeff, in Minneapolis

grant hutchison
2015-Jun-06, 09:46 PM
Large plateau areas exhaust rain clouds before they get all the way to the interior: they rain out around the edges. Hence Coppens thesis linking a large area of elevated ground with increasing aridity.

Also, from the Encyclopaedia Britannica entry on Eastern Africa:
Arid Somalia and northeastern Kenya are anomalous in these latitudes, with rainfall less than 10 inches per year. There, in the northern summer, airflow diverges towards the low pressure of the Indian Ocean monsoon system, resulting in a gently subsiding atmosphere rather than the uplift needed to generate precipitation.
Grant Hutchison

Cougar
2015-Jun-06, 10:04 PM
Grant has it, it is an effect known as rain shadowing. As clouds come in from the coast, and are pushed up over high ranges....

I agree that Grant has it - much of Kenya is at a high elevation. There are, however, no high ranges along the Kenya coast. The elevation gain from Mombasa up to Nairobi is generally rather smooth. Google maps satellite view shows the terrain changes to "desert-ish" about 50 miles inland from the coast. The elevation there must be about 1,000 ft. That would be a constant 0.4% grade. The grade is pretty much the same for another 50 mi. to Voi, where it's 2,000 ft. elevation. Apparently, that first 1,000 ft. catches any consistent moisture coming off the Indian Ocean. That "desert" landscape on the east side of Kenya seems to result from a huge encroachment from the north. Oddly, evidence of the great rift is more east - just east of Nairobi running north-south....

Cougar
2015-Jun-06, 10:09 PM
Also, from the Encyclopaedia Britannica entry on Eastern Africa....

Right. I wondered if the ocean interaction in that region might be somewhat anomalous, too. [ETA: Apparently that is the case.]

BigDon
2016-Jan-11, 03:53 PM
Anybody here besides me been to Mombasa?

grant hutchison
2016-Jan-14, 01:25 AM
Anybody here besides me been to Mombasa?Me. Twice, during the 80s. Happy memories, apart from the guy who chased me with a machete.

Grant Hutchison

Cougar
2016-Jan-14, 02:16 PM
Anybody here besides me been to Mombasa?

Several times in the early 70s, usually on the way up the coast to Kilifi, Malindi, and Lamu. Tent camping. Great coastline, incredible marine life....