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Thread: rainfall increase

  1. #1 rainfall increase 
    Forum Radioactive Isotope sculptor's Avatar
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    I read an article last week that claimed that rainfall had increased about 2% worldwide over the past century, and 5% over north america.

    The article stopped short of causation.
    I assumed that as the temperature rose, more evaporation led to more rainfall.
    Seem likely?
    Then:
    Why more in North America than worldwide?

    .................
    Years ago, I read a study about climate change from urban heat islands. Therein it was claimed that 70 miles downwind of Atlanta Georgia there was a significant increase in rainfall and thunderstorms over the past 50 years. Atlanta had grown from a relatively large town to a large City recently, so that records were available both before Atlanta became a large city, and after.

    Are these two studies supporting each other?


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  3. #2  
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    Not sure what journal or study the article you mention is referring to.

    There is a recent paper that suggested about a 6% global increase per degree C temperature observed. And most other studies suggest rainy places will generally get more rain, while dry places get less rainfall--the exceptions being along the high latitude side of the subtropical zones because subtropical jetstreams move poleward.
    http://journals.ametsoc.org/doi/abs/...I-D-12-00502.1

    Cities are complex and have two things happening, neither of which is necessarily associated with the global averages. One is aerosols which can block sunlight and reduce rainfall over the city, the other is increased frozen condensation nuclei which can enhance down wind rainfall and convective systems. I haven't read any studies about Atlanta, but where was one about Houston a few years back.


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    Okaaay. I can't find the piece I read the other day directly about this. But if you go to the What’s driving it? heading in this item about the Colorado floods there's a pretty good rundown.

    After The Flood: How Climate Change Changed One Colorado Community Forever | ThinkProgress
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  5. #4  
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    Quote Originally Posted by sculptor View Post

    I assumed that as the temperature rose, more evaporation led to more rainfall.
    Seem likely?
    This is one aspect of climate science that I feel completely happy with - if the temperature rises, the rate of evaporation will definitely increase.

    A few years ago, I read of some research/modelling done by a group in Germany which led to the conclusion that global warming would not take place monotonically, but would be oscillatory. There would be periods of high evaporation followed by cloud formation and rain, and the temperature would then decline leading to less evaporation and less rain. This cycle was expected to recur every fifteen years. It's too early to know if the fifteen year prediction is correct but the underlying idea seems reasonable.

    In recent years, rainfall in Britain has been above average. The year 2012 was the second wettest year on record, and that was only slightly below the record set in 2000.
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  6. #5  
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    Quote Originally Posted by JonG View Post
    There would be periods of high evaporation followed by cloud formation
    Doesn't moisture require nuclei to form cloud -> droplet -> rain?

    I've assumed that irrigation of dry lands, plus controls on aerosol emissions, has decreased the availability of nuclei. Then vapour without cloud makes a nice greenhouse, no?

    EDIT: The rainfall increase is especially weird because cloud coverage has decreased (though pre-satellite data is sketchy).
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    Quote Originally Posted by Pong View Post
    Quote Originally Posted by JonG View Post
    There would be periods of high evaporation followed by cloud formation
    Doesn't moisture require nuclei to form cloud -> droplet -> rain?

    I've assumed that irrigation of dry lands, plus controls on aerosol emissions, has decreased the availability of nuclei. Then vapour without cloud makes a nice greenhouse, no?

    EDIT: The rainfall increase is especially weird because cloud coverage has decreased (though pre-satellite data is sketchy).
    They do require nuclei, two types, condensation and ice nuclei.

    Condensation nuclei are usually plentiful in the lower atmosphere and seldom lacking enough to reduce cloud formation.

    Ice nuclei are almost always in short supply which is why super cooled water droplets are such a hazard to aircraft; it's also why they are the basis for seeding attempts. Outside of the tropics most precipitation starts as ice.
    Last edited by Lynx_Fox; October 13th, 2013 at 09:24 AM.
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  8. #7  
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    Quote Originally Posted by Lynx_Fox View Post
    Ice nuclei are almost always in short supply ... most precipitation starts as ice.
    That is almost contradictory. Interesting.
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