Geo Engineering Weather Modification

Here are some considerations:

The common mechanism for rain transport comes by way of our Sun, evaporation of water, and water having re condensed in our upper atmosphere to form rain droplets...
see also http://en.wikipedia.org/wiki/Water_cycle

While this is the general mechanism for our given rain water cycle, this becomes much more complex in consideration to the huge number of variables that should effect where rain may likely fall versus those area which should unlikely not see as much rainfall.  For instance, it may be that living in proximity to a large oceanic body of water and being coastal does not always guarantee that rain will fall owing to any number of other factors effecting water vapor transport, subsequent condensation and rain fall in such location.  This likely can be given predictable likely seasonal variations at times, especially given the predominance of atmospheric conditions predominating.

A brief consideration of climate change and the impacts of glaciation and all ramification of an Earth headed the pathway of the snowball.  When glaciation is on the rise, of course, more water, not less tends to be in the Earth's atmosphere relative to warmer times.  Not that rain doesn't fall during inter glacial minimums, but one would expect a much drier and colder Earth in general.  On the other hand, when our oceans are warmer and have subsequently more heat energy added into the mix, one might expect overall that atmospheric concentrations of water vapor to be higher here relative to inter glacial minimums.  Thus while some area might be impacted also by climate change in the way of less rain fall others might be impacted by more rainfall not less.

Obviously if you wanted to geo engineer rainfall.  You might find ways of ensuring that energy transport mechanisms are in place to do what needs to be done to ensure that atmospheric concentrations of water vapor are higher, not less.  But here is the interesting rub...water evaporation (owing to thermodynamics) tends to cool a given surface while the heat energy used in forming such water vapor gives way to water vapor rising from the Earth's surface into its atmosphere, and then putting more water vapor into the atmosphere tends to also lead not only to more rain, but gives way to a heat exchange cycle.  Once rain condenses upon returning back to the earth's surface leads yet to more cooling because of water's high specific heat capacity (that is, the amount of energy necessary to make water boil).   This is why the differences between deserts and the tropics amount to vastly different micro climates because desert environments to hold a lot less water, and thus have less thermal insulation ensuring less variability between say daytime highs and night time lows.  On the other hand, the most sub tropical wet regions not only receive the most amount of average sunlight energy throughout the year, but are also likely to have much higher latent moisture found distributed throughout such region.  This moisture both acts as an energy insulator, and tends to buffer a region from much higher temperatures that might result, for example, in the same latitude but being generally much drier.   Also forming clouds themselves, tend also to lead to less input energy, for instance, where cloud formations on average are more often and where water evaporation transport is greater.  For instance, where there are more clouds found over our oceans may certainly impact the degree of water evaporation since there is, in theory, less sunlight energy being absorbed by a major body of water such as an ocean in a given region.  Thus if there are year that one might characteristically describe as having much higher densities of cloud formation, one might expect that the effect could be a cooling one in so far as potential global climate and temperature change.  Then to add complications to this, ocean surface temperatures should likely effect evaporation transport, or as water molecules that are in generally more kinetically excited across the board are likely to be subject to evaporation transport.  Warmer oceans aid this, but at times there may oscillations in the energy exchange cycles of our oceans, that is, where body masses of water accumulate in more northern or southerly regions relative to say an originating source from which it came that likely lead to the potential of a greater chance of precipitation over such region and elsewhere.  It is also interesting to note that seasonally speaking desert environments tend to have less latent relationships between seasonal solar maximums versus non desert environments.  This is no quinky dink!  Obviously desert environments take less input energy to warm up relative to non desert environments that tend to be more water latent and hence requiring more input sunlight energy to hit a maximum.  Thus likely for thermal seasonal maximums, one might expect in much wetter regions to come later relative to a given solar seasonal maximum.

On the other hand, where climate were teetering between being warmer versus colder, there might be things aiding in the residual way to increase the likelihood of a wetter atmosphere.  Obviously water while shielding our planet from temperature extremes, also is a heat trapping compound, but not only water, other compounds such as CO2 may do the same trick when given to high enough concentrations.  The warmer the atmosphere means less interdependence upon the sun in provisioning necessary energy needed for water evaporation transport.  Or simply put, while one could put a mirror array up in the sky to focus say sunlight energy in further stimulating energy input to a given region (this might be a big undertaking by the way), for instance, directly.  On the other hand, if you wanted to stimulate a wetter earth in general, you'd need simply to raise global temperatures worldwide.  Heat trapping gases and compounds seem an excellent way to go to geo engineering things this way!  Now if you were wondering why places like SoCal or California in general might drier than one might expect (more recent extreme drought not withstanding)...the Pacific Ocean after all is near by, on the other hand relative to the Atlantic Ocean which for our North American continent side has different off shore flow currents.  For instance, the Atlantic on our side of the pond tends to run from south to north, where more Northerly areas of the Atlantic receive more energy not less from a given water body.  This has the effect, on the Atlantic side, of not only making more Northerly extremes, in terms of climate, moderate in so far as temperatures, but also aids in the increased likelihood of precipitation, not only on the Amercias side but also in Europe.  On the other hand, the Pacific waters on the North American side run from North to South, or much cooler waters relatively speaking, for those having been to both coasts can testify.  These cooler waters not only make on shore living a lot cooler during what might be potential summer extremes (especially right on the coast), but this tends to inhibit rainfall.  It is why the Western Coastal regions of the United States are generally drier and cooler relative the Atlantic side during a given summer season (and generally year round...excepting some unique locations), not to mention that on the Atlantic coastal side that there is a large continental body mass also pushing atmospheric heat energy into these regions with predominate westerly to easterly air flow.

Fortunately for us, the NSA need not handle the task of handling our planet's climate change alone!  We are likely well up to handling this task for ourselves each and everyday we step into our cars, fill our tanks, and subsequently filling our upper atmosphere with the necessary ingredients for more rain!  At least in some regions!   Fortunately water and water vapor alike shield our planet, for instance from getting to hot and extreme, but when it does get hot or cold, it seems it can take some to either cool things off, or warm things back up.  Climate records, for instance, show these sorts of trends.