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Urban areas these days can be quite large, and they can have strong effects on the environment: the urban heat island effect being the most obvious, but there may be other, more subtle effects. The relative lack of vegetation and drainage might make them drier, the concentration of high-rise buildings might approximate the effect of small hills, and there's also the air pollution they produce.

Thus I suspect that large urban areas might have an effect on rain, but exactly how? Do they cause more rain or less around them? What about any effects downwind?

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    $\begingroup$ Related: earthscience.stackexchange.com/questions/2351/… $\endgroup$ – milancurcic Oct 2 '14 at 17:43
  • $\begingroup$ A simple, naturalist and pragmatic answer: a friend of mine living in Paris told me that it does not rain any more nowadays, in Paris city. When I did my studies in Paris, back in the 80's, it did rain quite a lot, I remember very clearly. I think that my friend was exaggerating a bit, though: I would rather say that nowadays, it rains much less in Paris than, say, 30 years ago. This is very probably a consequence of the heating that you mention, in my humble opinion. $\endgroup$ – Pierre Nov 4 '14 at 21:39
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In a study conducted in Taiwan Impact of the Urban Heat Island Effect on Precipitation over a Complex Geographic Environment in Northern Taiwan (Lin et al. 2011), found that by observations that

The UHI effect plays an important role in perturbing thermal and dynamic processes; it affects the location of thunderstorms and precipitation over the complex geographic environment in northern Taiwan.

Hance, the situation and potential models of the distribution of rainfall are further complicated by the surrounding topography.

However, NASA's Earth Observatory page The Impact of City Landscapes on Rain presents 3 hypotheses for urban effects on rainfall, paraphrased below:

  1. As you mentioned, the Urban Heat Island, forms a bubble of warmer air and potentially creates atmospheric instability, resulting in

as the city-warmed air rises, it cools and forms rain-producing clouds that soak the area downwind.

  1. The buildings and other structures cause a disruption to the ambient air flow, resulting in

the city’s buildings provide a source of lift to push warm, moist, surface air into the cooler air above it, where it can develop into rain clouds.

  1. Similar to no.2, the buildings split the flow of air around the warmer city, these parcels of air return together on the other side:

When the two halves of the storm come back together downwind of the city, the air is pushed up like the two colliding trains. The rising air forms rain clouds.

When I did studies of the effects of aerosols (particularly particulate matter) in urban environments, there were many observations to suggest that some particulate matter could potentially become the cloud condensation nuclei that changes cloud cover and hence rainfall patterns.

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