Would continental deserts appear at the equator of a planet?

[Barlie]

I just have one question that's been bugging me for some time since I can't seem to find the answer. Lets say Africa was shifted down some, so that the equator bisects the Sahara. Would you still expect to see a large continental desert there? I'm a little confused as to the source of moisture at the equator and how that moisture travels.

[Caenwyr]

As a rule, deserts form around the tropics, and rainforests around the equator. As with every rule, here too there are quite some exceptions, though the general pattern is easy to spot on the world desert map:

map-world-desert_6364_600x450.jpg

The reason for this pattern is mainly the wind. Near the equator, air from storms rises very high and moves away from the equator, losing all its moisture by the time it reaches the tropics. When the air falls back down to earth here, it gets heated up even more due to atmospheric compressing. This is the process leading to dry, scourching deserts on the tropics. Other deserts such as those in Central Asia are formed because rain clouds can't reach them. This might be because they're behind a mountain wall shielding off all moisture, or simply because they're too far away from any sufficiently large body of water. Deserts can also appear close to sea (e.g. the Kalahari Desert), where ocean currents make cold waters well up by the coast. Because of the low temperature, little or no water evaporates, giving the same result as when there hadn't been a sea at all.

(Interestingly, because cold water can hold more gas, cold upwelling ocean currents are usually pretty oxygen rich: a perfect condition for sea life. So while the Kalahari Desert, and other deserts close to sea are almost devoid of life, the ocean next to it is usually teeming with it.)


So to answer your question: if Africa were to move south, the deserts would (roughly) remain on the same geographical longitudes, and there for would appear to move north. If the continent would venture so far south the current Sahara region would lie on the equator, there'd be rainforests where the Sahara is now, and a desert in the Congo basin. Crazy stuff huh? ;-).

More questions? Do not hesitate and ask!

[Sapiento]

As a rule, deserts form around the tropics, and rainforests around the equator. As with every rule, here too there are quite some exceptions, though the general pattern is easy to spot on the world desert map:

map-world-desert_6364_600x450.jpg

The reason for this pattern is mainly the wind. Near the equator, air from storms rises very high and moves away from the equator, losing all its moisture by the time it reaches the tropics. When the air falls back down to earth here, it gets heated up even more due to atmospheric compressing. This is the process leading to dry, scourching deserts on the tropics. Other deserts such as those in Central Asia are formed because rain clouds can't reach them. This might be because they're behind a mountain wall shielding off all moisture, or simply because they're too far away from any sufficiently large body of water. Deserts can also appear close to sea (e.g. the Kalahari Desert), where ocean currents make cold waters well up by the coast. Because of the low temperature, little or no water evaporates, giving the same result as when there hadn't been a sea at all.

(Interestingly, because cold water can hold more gas, cold upwelling ocean currents are usually pretty oxygen rich: a perfect condition for sea life. So while the Kalahari Desert, and other deserts close to sea are almost devoid of life, the ocean next to it is usually teeming with it.)


So to answer your question: if Africa were to move south, the deserts would (roughly) remain on the same geographical longitudes, and there for would appear to move north. If the continent would venture so far south the current Sahara region would lie on the equator, there'd be rainforests where the Sahara is now, and a desert in the Congo basin. Crazy stuff huh? ;-).

More questions? Do not hesitate and ask!

I'm not really an expert in climatology, but doesn't the general climate also affect the size and quantity? I know, this wasn't the starting question, but as we are already into it, why not discuss it further.
IIRC the Sahara was green in former, warmer periods and it is still not clear why the area deteriorated into the current state.

[Barlie]

The reason I ask is because, if I understand correctly, the Sahara is a result of being in the high pressure belt, combined with the fact that that the part of Africa it occupies is a large landmass, and atmospheric moisture would have all fallen as rain before reaching the center of that landmass anyway. Thus, 2 desert creating forces serve to make the Sahara (unless I'm misunderstanding something).

At the equator (also unless I'm missing something), I'm told that the wind patterns are vertical , as in, traveling from the ground to the sky, but not from east to west or west to east. I'm also told that much of the equator's air moisture comes from the ocean. Because of the wind pattern I mentioned, I don't expect moisture to travel much across longitudes. So if I'm getting this right, I'd expect the rain to kind of just fall where the the moisture was collected in the first place.

If Africa were to be shifted as I mentioned, the Sahara would lose its position in the tropics, but still retain its location on a large continental mass with much of its area far from the ocean. So I'd guess there would be rain forests along the coast of where the Sahara is on the continent, but how far into the continent would they go? Would the Sahara disappear completely?

I ask because I'm working on a fictional world map and I have a large area land mass along the equator, significantly larger than northern Africa. If I'm wrong in anything I mentioned please correct me. This has been bugging me for weeks.

[Caenwyr]

I'm not really an expert in climatology, but doesn't the general climate also affect the size and quantity? I know, this wasn't the starting question, but as we are already into it, why not discuss it further.
IIRC the Sahara was green in former, warmer periods and it is still not clear why the area deteriorated into the current state.

There used to be hippos and other such rainforesty creatures in the Sahara, that's true. Those parts were once pretty green and wet. But that was during the ice ages, when all climatic bands were compressed towards the equator. As soon as the heat went on at the end of the last ice age (about 10,000 years ago), the climatic bands started shifting towards the poles. turning the green haven the Sahara was into the scorching desert of today. There are fossil records of hippopotami in the Sahara no older than 8,000 years. These conditions also explain why there's so many animals living in both the African rainforest and the jungles of India, while there's no climatic connection between the two: there used to be, until the ice age pushed the desert between 'em.

If Africa were to be shifted as I mentioned, the Sahara would lose its position in the tropics, but still retain its location on a large continental mass with much of its area far from the ocean. So I'd guess there would be rain forests along the coast of where the Sahara is on the continent, but how far into the continent would they go? Would the Sahara disappear completely?

I ask because I'm working on a fictional world map and I have a large area land mass along the equator, significantly larger than northern Africa. If I'm wrong in anything I mentioned please correct me. This has been bugging me for weeks.

As a matter of fact, most moisture in a rainforest is generated by the forest itself. It evaporates, rains down, evaporates again etc. Of course it initially got its moisture because of its proximity to an ocean, but if you give it long enough, even the hypothetical Sahara-on-the-equator would eventually be 'colonized' by a rainforest, since it conveniently carries its own water supply with it. Best example of this is the Amazon Forest. While only being climatically connected to an ocean on the east, even the westernmost stretches of the South American plains are covered in lush forests. Take for example the Peruvian rainforest, which gets its moisture from the Atlantic Ocean (about 1700 miles away) and not from the far closer Pacific (about 200 miles tops), which does not induce any rainfall in the Amazon due to the fact that there's this insurpassable wall, the Andes Mountain Range, blocking all westerly winds.

Another example? Take the African Rain forest, 3300 miles on its longest (reaching from the coast of Senegal to the Congo-Uganda border). The easternmost stretch hugs the mountain ranges of the Great Rift Valley, and is more than 1500 miles away from the ocean that feeds it. If these mountain ranges hadn't been there, it'd have marched all the way to the Indian Ocean, where the conditions for rainforest formation are just as good. The GRV works in both directions by the way: there's another rainforest on the other side of the ridge as well, stretching through northern Kenya all the way to the coast.


Basically, as long as there's no mountain ranges blocking its path, a rainforest will just keep creeping on. (But only within that 'rainforest friendly' band around the equator of course)





EDIT:
This might be important for your map: deserts might form on the equator if they're completely shut off from any ocean. Take for example the Nyiri Desert in southern Kenya. So if a desert on the equator is what you're after, just put some walls around the spot .

[Barlie]

Oh man, thank you SO much! I had this gnawing feeling that I'd screw up the map if I just went ahead with what I understood. It's just for a homebrew pnp rpg game so I'm not quite sure why I care so much, but I do. Now I can move on with this map (and my life) xD.

I'm not really an expert in climatology, but doesn't the general climate also affect the size and quantity? I know, this wasn't the starting question, but as we are already into it, why not discuss it further.
IIRC the Sahara was green in former, warmer periods and it is still not clear why the area deteriorated into the current state.

I'm no expert myself, this is just what I found while unsuccessfully trying to figure out what Caenwyr so graciously told me. There's a theory out that says that the earth wobbles on its axis like a spinning top, periodically changing the orientation of the earth's axis. Along with that change, you'd get changes in locations of temperature zones, wind patterns and ocean currents. The tilt change doesn't even need to be drastic. It seems the most recent green period for the Sahara (between 6000 and 9000 years ago) was destroyed by an axis tilt change of only 0.69 degrees from the vertical. This is based on computer models and evidence from collected sediments, which support each other.

http://www.redorbit.com/news/science...as_once_green/

http://www.independent.co.uk/news/ti...t-1117177.html

Edit: I have just one more question. Assuming no terrain blocks the wind, how far do you have to travel inland before the area is no longer climatically connected to the ocean?

[Caenwyr]

Edit: I have just one more question. Assuming no terrain blocks the wind, how far do you have to travel inland before the area is no longer climatically connected to the ocean?

The group of climates not (or very weakly) influenced by ocean currents are called continental climates. In order to understand what these are, I suggest we first take a loot at the picture below.

Attachment 48323

As you can see continental climates only occur in cold and temperate regions. Sometimes they begin directly at the coast, sometimes they're hundreds of miles away from the sea. You also notice that, the more north you go, the closer the continental climate comes to the sea. The tempering, 'moisturizing' influence of the sea gets weaker. And the less influence you get, the more extreme the temperatures will be. In Siberia, temperatures can go as high as 90°F in summer and as low as -10°F in winter. The range in temperatures in sea climates is far smaller.

But actually I don't think you can rule out the influence of the terrain: it's one of the strongest factors! Just like with deserts, mountain ranges decide how much of the ocean's influence you get. You can have a continental climate right next to the sea, as long as it's separated from it by a mountain range.

But if you're asking about the maximum distance between the sea and the 'start' of a zone with a continental climate, I guess it's in the hundreds of miles, at least here on earth.

[Barlie]

The attachment you posted seems to not be working. Just to make sure I understand, you're saying that the weather tempering effect of large bodies of water on the climates of their coasts increases as you approach the equator and decreases as you approach the poles? And yeah, I wouldn't rule out the effect of mountains and other terrain types. Just needed as simple a case as I could think of to get the general idea. Thanks again by the way. In the 3 posts you've made in this thread, I've gotten more useable information for my purposes than in a few of weeks of research.

[waldronate]

An excellent search term here is "Hadley circulation", which offers suggestions about the latitudinal desert patterns. Also, there are a number of physical geography sites out there with discussions on these topics (including the ever popular "Is the Gobi desert a rain shadow desert").