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Ishtar
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The diagrams confused me as as Mick said the wind is connected to earth's rotation so I had to look that up. The poles are areas of high pressure and the equator is an area of low pressure. High pressure moves to low pressure. Without rotation wind would move in a straight line. With earth's rotation bends the wind. This is what the arrows on the other chart are showing.
It seems to me that where rain falls and doesn't fall has more to do with the relationship of high pressure and low pressure air and nothing to do with mountains or other features (not that I know much about weather).
Here we see areas of high pressure and low pressure
Areas where there is only low pressure and no high pressure flowing into it seem to be where the deserts are
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Hatty
Site Admin
In: Berkshire
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| It seems to me that where rain falls and doesn't fall has more to do with the relationship of high pressure and low pressure air and nothing to do with mountains or other features (not that I know much about weather). |
That sounds plausible. Why do our weathermen tell us that low pressure means wet weather and high pressure presages a dry spell?
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Mick Harper
Site Admin
In: London
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There are two general points to make. The first is careful ignoral. Since orthodox explanations of the hydrological cycle cannot realistically have winds flowing from the ocean onto deserts, no map ever shows such a thing. This is irrespective of the actual situation whether they do or they don't. Take another look at Chad's map but this time with proper scepticism. First, the Atacama: that actually shows every possible combination including, at its southern end (but kept unobtrusive) onshore winds. Or look at Australia. That is all desert and so -- surprise, surprise -- no winds at all cross its borders either way! What are the chances of that!
This is all completely standard of course and will be familiar to you all as a general exercise in unconscious orthodox subterfuge. And it may be that, really and truly, prevailing winds don't blow onshore onto deserts. But it is all irrelevant. You cannot pick and choose with physical laws. If the atmosphere picks up water from the ocean it must do all the time; if prevailing winds prevail then non-prevailing winds must blow some of the time; if air cools when climbing mountains it must shed its water.
Every time. So the Atacama cannot be a desert. Or if it can be a desert it cannot have places with no recorded precipitation ever. So think about this, and think what it really tells you about condensation, oceans and atmospheres. In absolute terms. A correct deduction will now lead you to what all deserts have in common.
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Hatty
Site Admin
In: Berkshire
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| A correct deduction will now lead you to what all deserts have in common. |
I'll be traduced but the common feature is lack of plant life so presumably there's a connection between lack of rainfall and lack of plant life. I suppose lack of soil suitable for plant life is the underlying problem. From discussions about 'the salting of Carthage' it might be something to do with salt, the Atacama is at least fertile in its nitrate resources if nothing else.
Perhaps there was once water there which evaporated as it didn't have an outlet, like the Dead Sea.
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Mick Harper
Site Admin
In: London
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This is true, and later we shall see it is relevant too. However since lack of rainfall will produce lack of life anyway we cannot put it forward as a cause-and-effect.
Let me list the major deserts of the world in an attempt to assist you in seeing what they have in common. I will further assist you in putting one in brackets since the commonality applies here too but in a way that might otherwise mislead you:
Atacama
Kalahari
Australian
Antarctica
South-western USA
Sahara
(Gobi)
It may also assist if I point out that very recent man-made adjustments to the extent of deserts should be ignored.
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Ishtar
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| Hatty wrote: | | Why do our weathermen tell us that low pressure means wet weather and high pressure presages a dry spell? |
From my googling here is what I understand: The High pressure area is cold, and the Low pressure area is warm. The high pressure seems to form over the ocean and moves over the land. When high pressure and low pressure meet, you get a weather front. I've seen those as a band of clouds sitting on the horizon.
Warm (low pressure) air holds the moisture; this is an important distinction because it forms over the land not the sea. The cold high pressure air is what is sitting out at sea. When the colder air hits the warm land air it cools the warm air and the warm air can't hold the moisture - it rains.
Looking at the air pressure map, the fertile areas like rainforests are in areas where high pressure and low pressure can meet.
If an area is dry it should be because there is no high pressure air that can squeeze out the water.
But I don't know if that reasoning is correct.
| Mick Harper wrote: | | A correct deduction will now lead you to what all deserts have in common. |
I'm trying to think what all the deserts have in common..
- they are all very hot and dry (not completely dry though)
- a lot of them are on similar latitudes
- they all seem to be on the western shores of continents or at least near a large area of water
That last one seems important. Except for the Gobi they all seem to be on the east side of areas of water. If water comes from the ocean then they shouldn't be near the ocean, it seems.
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Hatty
Site Admin
In: Berkshire
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| Warm (low pressure) air holds the moisture this is important distinction because it forms over the land not the sea. The cold high pressure air is what is sitting out at sea. When the colder air hits the warm land air it cools the warm air and the warm air can't hold the moisture - it rains. |
That makes sense. Maybe the rainfall out at sea is affected by warm and cold water currents.
| a lot of them are on similar latitudes |
Yes, the Tropics, roughly 30 degrees north and south of the equator. I'm puzzled by this, aren't tropical zones usually associated with wet not dry conditions? It's strange, I'd expected to find deserts on the equator.
| they all seem to be on the western shores of continents or at least near a large area of water |
Even more puzzling. In England west is wet and east dry. It all seems back to front. Unless the tropical zones have a different wind pattern.
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Chad
In: Ramsbottom
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| Ishtar wrote: | The high pressure seems to form over the ocean and moves over the land. When high pressure and low pressure meet, you get a weather front....
Warm (low pressure) air holds the moisture this is important distinction because it forms over the land not the sea. The cold high pressure air is what is sitting out at sea. When the colder air hits the warm land air it cools the warm air and the warm air can't hold the moisture - it rains. |
I think Ishtar has gotten to the very heart of the matter.
It can't rain in the desert because it is simply impossible for a low pressure (moisture laden) zone of air to form above it.
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Ishtar
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| Chad wrote: | | It can't rain in the desert because it is simply impossible for a low pressure (moisture laden) zone of air to form above it. |
Or is it that the high pressure air just doesn't move into the deserts?
Musing here... Oceans should logically work the same way as air. If there were no land and the earth did not rotate the oceans should flow from cold to warm, high pressure or density to low.
The cold water will push the warm water. The swirling water matches to the high pressure zones in direction. The cold arctic water is cooling the warm equator water.
Where that water meets is where it rains in the ocean. Here you can see where the cold water swirls is the same places the high pressure cool air forms.
In the southern hemisphere the water should flow counterclockwise because the coldwater is in the south moving north and in the northern hemisphere the water should flow clockwise because the coldwater is to the north. Well, that's how I look at it logically from the pictures it makes sense.
What the picture tells me is that the cold side is always on the east. That is where the deserts are to the east of the cold currents and the cold air. The cold currents and cold air hold no water so no rain.
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Hatty
Site Admin
In: Berkshire
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| What the picture tells me is that the cold side is always on the east. That is where the deserts are to the east of the cold currents and the cold air. The cold currents and cold air hold no water so no rain. |
That's really clever. Astonishing. Wouldn't the cold eastern shore correlate to high pressure i.e. on the desert side, or have I got it back to front as usual?
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Chad
In: Ramsbottom
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| Ishtar wrote: | | Chad wrote: | | It can't rain in the desert because it is simply impossible for a low pressure (moisture laden) zone of air to form above it. |
Or is it that the high pressure air just doesn't move into the deserts?
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I don't think that can be quite the case. Your earlier diagram...
...shows a high pressure area sitting smack bang over the Australian desert.
If high pressure can only form over the ocean and its arrival over the Australian desert failed to trigger rain, it can only mean that it didn't encounter a moisture bearing low pressure zone over the desert... hence no weather front, no cloud formation and no rain.
If there is no moisture at ground level in the desert, it is impossible for a moisture bearing low pressure zone to form above it.
| What the picture tells me is that the cold side is always on the east. That is where the deserts are to the east of the cold currents and the cold air. The cold currents and cold air hold no water so no rain. |
If that was the whole story, the entire east coast of North America would be desert.
But as you pointed out earlier, it isn't this cold dry air that brings the rain... it simply causes the warm, low pressure air (above the land) to dump its moisture on contact... and as I said above, a moisture bearing low pressure zone can't form above the desert... (But it can where vegetation grows.)
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Ishtar
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| Chad wrote: | | But as you pointed out earlier, it isn't this cold dry air that brings the rain... it simply causes the warm, low pressure air (above the land) to dump its moisture on contact... and as I said above, a moisture bearing low pressure zone can't form above the desert... (But it can where vegetation grows.) |
Right, that seems closer to the truth. I'm thinking too much about the water cycle pictures again.
Low pressure over the land holds water, it dumps it when it meets the high pressure over the ocean. The high pressure is caused by cold currents. The high pressure has to move over the land to cause rain.
But how did the deserts get there in the first place? The air must not meet (as often) over those areas.
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Mick Harper
Site Admin
In: London
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This is a very familiar Treasure Hunt phenomenon: the hunters all chat amongst themselves while ignoring what the Master of the Hunt keeps on telling them they ought to be looking for. Now each one of you (you can repeat what others say if you wish) address the problem set before you: how can parts of the Atacama be rain-free for centuries (or even a desert at all) when it is the easternmost terminus of the longest stretch of water on earth?
And I'll repeat the mantra just once more since it doesn't seem to have sunk in. It is no use chatting about this or that tendency -- this pressure is higher than that pressure, this wind blows more than that wind, that's what orthodoxy does and it's called special pleading. Every desert ends up with its own special set of causes. When dealing with deserts, and especially utterly rain-free deserts, we are looking for absolute causes.
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Hatty
Site Admin
In: Berkshire
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| how can parts of the Atacama be rain-free for centuries (or even a desert at all) when it is the easternmost terminus of the longest stretch of water on earth? |
The inference is that winds blowing westwards over large stretches of water have lost any moisture by the time they get to land, hence deserts are on the west coast. The only reason for the lack of moisture must be because no plants grow in the sea.
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Chad
In: Ramsbottom
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| Mick Harper wrote: | | how can parts of the Atacama be rain-free for centuries...? |
Are we then free to ignore your earlier dictat?
| Let us ignore the silly bollocks from areas of the world where measurement is just not possible. | |
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