I looked up several articles about humidity, relative, absolute and dew points by altitude and was not able to resolve this issue to my satisfaction. How about we try this one: When clouds form over the ocean that temporarily creates an area of lower humidity below them. This in turn causes the water to evaporate and raise the humidity level back to some equilibrium. The temperature of the water and air all contribute, but the reason you don't have humidity of 100% over the ocean surface most of the time is that the air does rise and takes the water vapor with it and the wind can blow it around as well.
I added some water to some terrariums I have this morning. The tanks are mostly sealed to limit evaporation for the benefit of the residents and so I don't have to replace the water every day. Within a few minutes, water vapor had condensed on the insides of the glass. The ocean and the air above it is not sealed like the tanks. Thus, the humidity shouldn't approach 100%. I would be interested to see humidity readings by altitude over the ocean with and without cloud cover at the same temperatures. Regarding the rainfall issue and the marine stratocumulus clouds, these are low clouds, generally topping out at below 5000-7500 ft. To produce rain, you need clouds closer to 15000-20,000 ft and to produce monsoonal rains you need clouds that go all the way up to the tropopause, 50,000 ft. These are generalizations and I know that some rainfall is possible from low clouds, but these low clouds just aren't a major rain producer and the marine stratocumulus have no inpact on rainfall in India or practically anywhere else regardless of what you might do to them. I doubt that enhancing of marine stratocumulus cloud whiteness will increase the Indian monsoon unless done in the waters immediately surrounding the subcontinent. It certainly wouldn't be expected to decrease it. BTW, did anyone notice that rainfall in India is 25% below normal this year? So wide swings are typical. What is to be avoided is a long term significant diminution in rainfall. ----- Original Message ----- From: "Stephen Salter" <[email protected]> To: "Alvia Gaskill" <[email protected]> Cc: <[email protected]>; <[email protected]>; <[email protected]> Sent: Monday, August 31, 2009 6:03 AM Subject: Re: [clim] Re: Fwd: geoengineering and hydrological cycle > Alvia > > I very much hope that treating low level clouds is not going to affect > rainfall but people have said it will and wanted to have the possibility > out in the open rather than be accused of covering it up. Perhaps I should > have written ' . . . might happen . . '. > > However I am puzzled about how moving air can dry it especially as it > seems to be harder to move it past the top of the boundary layer. I can > see that this might happen for a while but not forever. Can you explain > where it goes? > > Stephen > > Emeritus Professor of Engineering Design > School of Engineering and Electronics > University of Edinburgh > Mayfield Road > Edinburgh EH9 3JL > Scotland > tel +44 131 650 5704 > fax +44 131 650 5702 > Mobile 07795 203 195 > [email protected] > http://www.see.ed.ac.uk/~shs > > > Alvia Gaskill wrote: >> "It is known that rain is less likely if there are too many small drops >> due to smoke and fine dust. This will happen as a result of >> cloud albedo control." >> >> It was my understanding that marine stratocumulus clouds, the target for >> the spray vessels, seldom produce much rain and that the concerns about >> the cloud whitening strategy were instead related to indirect impacts on >> other types of clouds in that they are the ones that are the source of >> most of the rainfall. The ocean does evaporate quite efficiently. >> That's where the clouds come from over the ocean and also the source of >> hurricane energy. The reason the air above the water isn't 100% saturated >> is because the wind moves it around, allowing it to dry out. >> >> ----- Original Message ----- From: "Stephen Salter" <[email protected]> >> To: <[email protected]> >> Cc: <[email protected]> >> Sent: Sunday, August 30, 2009 11:41 AM >> Subject: [clim] Re: Fwd: geoengineering and hydrological cycle >> >> >>> >>> Dear Professor Bala >>> >>> I will need lots of time to study your most important paper but could >>> you tell me what you think would happen if we slowly increased the spray >>> quantity used for cloud albedo modification to keep pace with a >>> hopefully reducing but probably increasing emission rate and were able >>> to exercise choice over where the spraying was done. We know that la >>> Nina and el Nino work in opposite directions and we might be able to >>> copy them. >>> >>> With all the ignorance of an engineer new to this field I have >>> identified the following possible mechanisms. >>> >>> 1. The production of rain is very complicated but we know that it >>> needs some large drops to fall through deep clouds fast enough to >>> coalesce with drops in their path so that they grow large enough to fall >>> fast enough to reach the ground without evaporating in the dry air below >>> a cloud. It is known that rain is less likely if there are too many >>> small drops due to smoke and fine dust. This will happen as a result of >>> cloud albedo control. >>> 2. However if more small cloud drops means that there is less >>> rainfall /over the sea/ it follows that there will be more water left in >>> the air mass when it reaches land. The air mass will travel further >>> inland before the original drop numbers are restored by coalescence. >>> Rain inland is more valuable than rain at the coast. >>> 3. Any reduction in sea temperature will tend to reduce >>> evaporation and increase condensation from air back to the sea surface >>> and this will reduce the water content of the atmosphere. >>> 4. But cooling the sea means a larger difference between the >>> temperatures of sea and land and so more monsoon effect with stronger >>> winds bringing more water to land masses. >>> 5. Wind is caused by differences in pressure which are a result of >>> local temperature gradients. Wind causes turbulence in the marine >>> boundary layer. The relative humidity is very high in the stagnant layer >>> of air at distances of millimetres above the sea surface but falls to >>> around 65% a few metres above. Turbulence has a much greater influence >>> on evaporation rate than water temperature. >>> 6. Wind makes waves steeper. Spilling breakers mix the thin top >>> layer of water that has been chilled by evaporation. Plunging breakers >>> drive bubbles below the surface and throw spray above it to transfer >>> more water vapour in the air. >>> 7. Finally we know that the regions with the most severe drought >>> problems are dry because air which has been dried by being high in the >>> atmosphere is subsiding and moving out to sea. This means what we do at >>> sea cannot affect the very driest land. >>> >>> We could accept large reductions in rainfall over the sea provided that >>> we could control the rates, up or down, over land. Local effects would >>> then be much more important than the mean global change. >>> >>> I am also intrigued at why the evaporation of the sea is so slow when >>> the relative humidity a fer metres above the surface is quite low. I >>> feel that it should be possible to modify this by increasing turbulence >>> close to the surface to break through the stagnant layer in immediate >>> contact. Perhaps two controls are better than one. >>> >>> With best wishes >>> >>> Stephen Salter >>> >>> Emeritus Professor of Engineering Design >>> School of Engineering and Electronics >>> University of Edinburgh >>> Mayfield Road >>> Edinburgh EH9 3JL >>> Scotland >>> tel +44 131 650 5704 >>> fax +44 131 650 5702 >>> Mobile 07795 203 195 >>> [email protected] >>> http://www.see.ed.ac.uk/~shs >>> >>> >>> >>> Govindasamy bala wrote: >>>> >>>> >>>> ---------- Forwarded message ---------- >>>> From: *Govindasamy bala* <bala.gov <http://bala.gov>@gmail.com >>>> <http://gmail.com>> >>>> Date: Tue, Aug 25, 2009 at 3:10 PM >>>> Subject: geoengineering and hydrological cycle >>>> To: [email protected] >>>> <mailto:[email protected]> >>>> >>>> >>>> >>>> The attached paper gets into the details of why the climate system has >>>> different hydrological sensitivity to CO2 and Solar forcing. It turns >>>> out that the slow response of the system is the same for the two >>>> forcing but the fast (short-term) responses are very different. Which >>>> explains why the total sensitivities are different. >>>> >>>> The website for news release on this work is >>>> http://www.iisc.ernet.in/content_hydrologic.html >>>> >>>> -- >>>> Best wishes, >>>> >>>> ------------------------------------------------------------------- >>>> Dr. G. Bala >>>> Associate Professor >>>> Center for Atmospheric and Oceanic Sciences >>>> Indian Institute of Science >>>> Bangalore - 560 012 >>>> India >>>> >>>> Tel: +91 80 2293 2698 >>>> +91 80 2293 2505 x206 >>>> +91 9741991621 (cell) >>>> Fax: +91 80 2360 0865 >>>> Email: [email protected] <mailto:[email protected]> >>>> bala.gov <http://bala.gov>@gmail.com <http://gmail.com> >>>> Web:http://caos.iisc.ernet.in/faculty/gbala/gbala.html >>>> ------------------------------------------------------------------- >>>> >>>> >>>> >>>> > >>> >>> -- >>> The University of Edinburgh is a charitable body, registered in >>> Scotland, with registration number SC005336. >>> >>> >>> >>> >> >> > > -- > The University of Edinburgh is a charitable body, registered in > Scotland, with registration number SC005336. > --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "geoengineering" group. 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