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At the risk of [mfd] for "bad science" and/or "bad maths", a quick back-of-the-envelope calculation leads me to believe that a large (~ 10^9 cubic meter or about 1/2 cubic mile) evacuated underground chamber would inhale enough air to lower atmospheric pressure by several percent over a large city's
metropolitan area. The intent is to help induce cloud formation by creating a local, artificial atmospheric low. A series of these immense chambers would enable an entire geographic area to induce a regional atmospheric low and would serve as a tool to "break" a high-pressure system associated with drought. That is to say, it wouldn't necessarily cause rain in and of itself, but it would help alter an acutely severe drought by locally changing weather patterns.
The sequestered air could be relased back into the atmosphere slowly over time. Or, in the case of a tropical cyclone, a coordinated release of mulitple chambers could possibly change the storm's course.
The size of the chamber could be reduced by increasing the pressure inside the chambers. The air being inhaled could be cooled (and thereby made denser) by being drawn over a heat exchanger filled with ground water. Condensation from the air could be collected.
see also [BunsenHoneydew, May 28 2009]
[batou, May 29 2009]
||How quickly would the half cubic-mile of air have to be enchambered to beat the air surrounding your city's air rushing in to equalise the pressure ?
||Where do you plan to put the half cubic mile of earth? How tall a hill could you create from it, and could that change local weather favourably?
||If you open the chamber on a day of low pressure and seal it, it should retain the ambient pressure of that day indefinitely. Indeed, if the chamber is cooled, the pressure would drop even further.
||Open the seal on a hot, humid, high-pressure day - a loud pop, an inrush of air, a local cooling effect, and cloud (or at least mist) formation should be your result. [+]
||// Where do you plan to put the half cubic mile of earth? How tall a hill could you create from it // Make a 1mile² pyramid from it, and it would be 1.5miles high. Influence on weather? Possible.
||Chicago has the Deep Tunnel system, originally designed to store runoff from heavy rains rather than dump it into the river. I cannot find figures on the internal volume, but it seems reasonable to suppose that it is at least 1/2 cubic mile.
With suitable modifications, this should be a nice testbed for your idea.
||SImply processing a lot of air to change its humidity could
affect local weather. Water vapor is a pretty potent
greenhouse gas. Not to mention that dry air can cause
clouds to evaporate --and clouds can affect local weather,
too. AND water is 1500 or so times denser than air, so the
volume you have to prepare, to hold the removed water
vapor, can be a lot smaller. (Yes, the heat of
condensation, of all that water, sill needs to be removed
during the processing.)