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Science

NASA Confirms Rainy Cities 21

Posted by timothy from the view-from-above dept.
Devil's BSD writes: "It's true, urban areas are rainier than rural areas. Using the Tropical Rainfall Measuring Mission satelite to measure rainfall around cities, NASA found that areas downwind from cities had up to 116% more precipitation than those upwind from it. The cause? The major heat difference, up to about 10 degrees Fahrenheit (5.6 Celsius degrees), caused by the asphalt and concrete in cities. This story is posted on the Goddard Space Flight Center page."
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NASA Confirms Rainy Cities More

NASA Confirms Rainy Cities

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  • Just look at how rainy the set of Blade Runner is. Or was it it Philip K. Dick....

  • ( Pardon the pun ).

    This has been known for some time. The claim, when I heard it, was that particulate matter in the air from exhaust etc gave the water vapor something to coallese onto and hence form water droplets. I think particulate matter is more reasonable to expect than temputure variations ( even tho these do exist ).

    Why is Nasa re-inventing what is already known? This was out about 5 years ago ( or longer - memory rentention tends to blur ).
    • You are right, but it's both: When moist air is forced to rise (for instance in the north of spain, air comes from the atlantic and has to rise over the mountains. The weather there is _very_ wet.) Particles act as condensation starters, perhaps similar to impurities starting cristal growth in saturated solutions. Try this at home: Just before you take a shower, smoke a sigarette in the bathroom. Notice how much more steam forms now when you let the hot shower run? That's the cigarette smoke acting as condensation nuclei, I think. All this was in my geography class that I took 16 years ago. Hardly news. Nasa is in need of money again, I guess.

  • causation (Score:1, Insightful)

    by Anonymous Coward
    The cause? The major heat difference, up to about 10 degrees Fahrenheit (5.6 Celsius degrees),

    post hoc ergo proctor hoc

    I'm too lazy to read the science and peer review it, but lots of cities sprung up around rivers (for trade & transportation), so the area might have been rainier before the city appeared to support the river.

    Without before/after rain measurements, I can't accept their conclusions.

    • Re:causation (Score:4, Insightful)

      by Matthias Wiesmann ( 221411 ) on Monday June 24, 2002 @10:17AM (#3756691) Homepage Journal
      Indeed many cities are built on the border of rivers, but those rivers exist before they flow into the city and after they run out of it.

      So if the change of temperature was only caused by the body of water, the whole river would cause rain down-wind (this is probably true, but is an orthogonal problem).

      Claiming that the observed effects can be explained by the rivers would imply that those river do not exist outside of the city...

      • In much of the United States, the Corps of Engineers builds dams on local streams to create lakes for recreation and water supply. For instance, look at this satellite image [globexplorer.com] of Dallas. All those lakes are artificial and are made from tiny, even intermittent Texas streams, very much unlike the rivers found elsewhere. So, in a sense, there are bodies of water that exist only in the vicinity of cities in this part of the world. If they affect weather, that is more than I know.
    • The expression is " post hoc ergo propter hoc [skepdic.com]" (link to the entry in the excellent Skeptic's Dictionary [skepdic.com].
  • I am a boater and experience the "Weekend Effect" all the time where it rains in weekends with nice weather on Mondays. That seems to be related with higher temperatures by the weekend caused by the industrial weekly cycle. Maybe some real scientific data will follow soon... DumKopf

  • Nonsensical model (Score:2, Interesting)

    by kroymen ( 242910 )
    I'm no meteorologist, but it seems to me that there's a world of difference between conditions that produce clouds and conditions that cause clouds to precipitate out. Rising warm air may build a cloud, but without a coincident drop in temperature or air pressure, I don't see how that could translate to more rain in the cities. In fact, I'd suspect the heat island would reduce rain in the cities and increase rain outside of the cities.

    Their model seems primarily to be: we noticed that this condition exists and this other condition exists too; perhaps through some mysterious not-precisely-known interaction of the one is causing the other.

    The fact that rainfall is presumably (I'm not sure the observation is meaningful without comparative historical data) greater in the cities as well as downwind from the cities suggests another causative factor to me. The already posited airborn particulate output of cities seems to be a much stronger explanation in my opinion.
    • I don't know where you live, but here on the east coast, a very typical forecast in the summer is hot, muggy and partly cloudy with a chance of thunderstorms in the late afternoon. The reason? Hot air makes rain happen.

      As you correctly said, as air warms, it rises. Now, if you've ever taken chemistry, you might remember Boyle's Law of Gases that relates temperature, pressure and volume of a gas. If you lower the pressure of a gas, the gas expands, and the temperature falls. As that warm, moist air rises, atmospheric pressure falls, and the air cools. Because cooler air holds less water, moisture will condense, forming cumulus clouds. Because condensing water vapor releases a good bit of heat, that air can keep rising, building clouds higher. Rain can happen when heavy drops of water fall through those clouds absorbing other droplets until they are big enough to fall to the ground without reevaporating. But don't take my word for it. USAToday will explain it with pretty color pictures. http://www.usatoday.com/weather/basics/wworks0.htm
    • Why don't you read the rest of the summary (not to mention the actual paper itself) before posting.
      • I hit the wrong "reply to this" button.. I wasn't intending to respond to your message- which actually (unfortunately for me) contains a well thought out reply. :-)

        D'oh!

  • Let's move all our farms to cities...
  • So we needed a team of highly paid government scientists to tell us what is already obvious?

    Coming soon: The NASA water-color-detection satelitte array: are the oceans really blue?

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