MSL Post Landing - Commissioning Period & Early Observations, Commissioning Activity Period 1B - Sols 9 through 16 |
MSL Post Landing - Commissioning Period & Early Observations, Commissioning Activity Period 1B - Sols 9 through 16 |
Aug 26 2012, 02:45 AM
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#286
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Senior Member Group: Members Posts: 1044 Joined: 17-February 09 Member No.: 4605 |
Can someone help me out with the weather report? I've not seen absolute humidity expressed as a percentage before. Is this the humidity ratio (mass of water/mass of air) - slightly more than 1 gram water vapor per m^3?
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Aug 26 2012, 03:46 AM
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#287
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Forum Contributor Group: Members Posts: 1372 Joined: 8-February 04 From: North East Florida, USA. Member No.: 11 |
Can someone help me out with the weather report? I've not seen absolute humidity expressed as a percentage before. Is this the humidity ratio (mass of water/mass of air) - slightly more than 1 gram water vapor per m^3? It's usually the amount of water the atmosphere can hold, 8 % is like a bone dry desert value, whereas in Florida eg, you often get 90 to 100 %, which generally feels miserable. Local met offices will also include a PWAT amount of water, that is if you squashed all the water out of the air you might eg get 1 inch. Usually less than 1/2 inch of PWAT it won't rain. Right now in NE FL the humidity is a nice 48 %. Most air conditioning units will make your house around 40 % humid, which is generally a comfortable level for humans. |
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Aug 26 2012, 05:29 AM
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#288
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Merciless Robot Group: Admin Posts: 8784 Joined: 8-December 05 From: Los Angeles Member No.: 602 |
Interesting. I recall that many years ago (and, yeah, I'm getting old, no need to rub it in!) RH was always expressed as a percentage. Basically, 100% humidity meant it was gonna rain, period; the atmosphere was saturated.
Mars' atmosphere is both considerably less dense (approx. .01% of Earth sea-level) and radically different in composition than that of the Earth. The latter must have some effect on RH, but the low density is more significant: Liquid water cannot exist on the surface of Mars. Water there acts like dry ice on the Earth: two phases (solid or vapor), no waiting. So, my read on 8% RH is that there must not be much water around, or at least the local conditions are not permitting significant sublimation. I'll be interested in the diurnal variation pattern if they do acquire such data. -------------------- A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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Aug 26 2012, 06:04 AM
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#289
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Senior Member Group: Members Posts: 3419 Joined: 9-February 04 From: Minneapolis, MN, USA Member No.: 15 |
I recall reading that Mars' atmospheric humidity often reaches 100%, but that since the air is so much thinner and colder than Earth's, it can hold only a very small amount of water vapor, so that a very slight increase in the actual amount of water vapor in the air raises the percentage by a very large percentage.
So, it's relatively easy to saturate Mars' atmosphere in a given location with an overall very small amount of water vapor. A reading of less than 10% would, I would think, indicate an extremely dry air mass. -the other Doug -------------------- “The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
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Aug 26 2012, 08:51 AM
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#290
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Senior Member Group: Members Posts: 1729 Joined: 3-August 06 From: 43° 35' 53" N 1° 26' 35" E Member No.: 1004 |
I recall reading that Mars' atmospheric humidity often reaches 100%, but that since the air is so much thinner and colder than Earth's, it can hold only a very small amount of water vapor, so that a very slight increase in the actual amount of water vapor in the air raises the percentage by a very large percentage. I think you are referring to this recent paper Evidence of Water Vapor in Excess of Saturation in the Atmosphere of Mars |
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Aug 26 2012, 06:45 PM
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#291
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Martian Photographer Group: Members Posts: 352 Joined: 3-March 05 Member No.: 183 |
>100% RH up in the sky is one thing. Frost and fog tell you that there's 100% at the surface with some frequency. The absolute amount is also greater than the saturation vapor pressure for night-time temperatures in many times & places. It's easy to saturate at -90 C. During the equatorial mid-afternoon though, 8% would be a dripping oasis by Martian standards. (But of course, in Tucson on an early July afternoon, 8% looks damp too.)
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Aug 27 2012, 02:34 AM
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#292
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Junior Member Group: Members Posts: 83 Joined: 19-April 05 Member No.: 251 |
I always thought that 100% relative humidity was when the partial pressure of the water vapor in the atmosphere was equal to the vapor pressure of liquid water at a given temperature. As such, partial pressures of other gasses in the atmosphere would be irrelevant.
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Aug 27 2012, 03:55 AM
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#293
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Senior Member Group: Members Posts: 1044 Joined: 17-February 09 Member No.: 4605 |
Yeah, therein lies my query. Relative Humidity is a ratio (percentage), being the water vapor partial pressure to saturation vapor pressure. But the weather report provides the absolute humidity which I thought was the mass of water in a cubic metre of air? I am at a loss as to how to interperet absolute humidity as a percentage although, as with nprev, new methods may have overtaken my ageing knowledge base. If they mean the ratio of mass of water vapor to mass of air then the amount of water available for phase transition would seem really miniscule.
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Aug 27 2012, 06:19 AM
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#294
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Newbie Group: Members Posts: 15 Joined: 25-June 06 From: Las Vegas, NV Member No.: 934 |
I already have visions of Coyote Buttes, Utah, with its crazy wind-sculpted hoodoos: http://www.google.com/search?q=coyote+buttes+hoodoo&.... Just so long as Curiousity does not have to apply to the BLM permit lottery to visit that part of the mountain, I'm good with that We'll see some sights when Curiosity starts driving through this lot, won't we..? [attachment=27654:Promised...osaic_1c.jpg] That is a remarkable view. Should be spectacular as climb through all that in stereo! |
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Aug 27 2012, 02:43 PM
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#295
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Newbie Group: Members Posts: 11 Joined: 12-August 12 Member No.: 6550 |
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Aug 27 2012, 05:57 PM
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#296
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Member Group: Members Posts: 507 Joined: 10-September 08 Member No.: 4338 |
I've not seen absolute humidity expressed as a percentage before. http://books.google.com/books?id=nWbmRj7Yb...CGoEA&hl=en Percentage absolute humidity. It seems to be the ratio, expressed as a percentage, of the absolute humidity over what the absolute humidity would be if the atmosphere was saturated and at a particular temperature referred to as the "dry bulb" temperature. A complicated concept, but it appears to be independent of temperature and pressure. Not sure how it would apply to Mars. [EDIT: The dry bulb temperature is not a specific temperature, it is the aiir temperature when the thermometer bulb is shielded from moisture and radiation. So now I am confused, because the definition would seem to be essentially the same as relative humidity.] [EDIT2: Ah, some clarity. The only difference between relative humiidity and percentage absolute humidity is that the former is the ratio of the partial pressure to the saturated partial pressure whereas the latter is the ratio of the absolute humidity to the saturated absolute humidity. http://books.google.com/books?id=rWwzHsJ5G...TG0YE&hl=en] |
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Aug 27 2012, 06:26 PM
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#297
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Senior Member Group: Members Posts: 1643 Joined: 5-March 05 From: Boulder, CO Member No.: 184 |
Is this the link to the weather report with humidity?
http://cab.inta-csic.es/rems/marsweather.html Here's a Wikipedia entry on absolute humidity, normally defined as a vapor density in grams per cubic meter. However there can be some confusion in terminology. http://en.wikipedia.org/wiki/Humidity#Absolute_humidity What are the units in the report? If it is percent, then maybe they really mean specific humidity (numerically almost the same as mixing ratio). http://en.wikipedia.org/wiki/Humidity#Specific_humidity It would be interesting to convert the reports to relative humidity, given that we know the temperature and pressure. By the way, relative humidity can be specified with respect to either water or ice, another fine point. -------------------- Steve [ my home page and planetary maps page ]
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Aug 28 2012, 11:31 PM
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#298
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Senior Member Group: Members Posts: 1044 Joined: 17-February 09 Member No.: 4605 |
All is explained. The Vaisala data sheet indicates that the probe measures relative humidity so this is probably no more than an descriptive error on the weather page.
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Aug 29 2012, 09:32 AM
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#299
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Merciless Robot Group: Admin Posts: 8784 Joined: 8-December 05 From: Los Angeles Member No.: 602 |
I'm sure you meant to say 1% I did; was thinking 1/100th & wrote the wrong thing. Good catch, and thanks! -------------------- A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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Aug 29 2012, 12:11 PM
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#300
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Member Group: Members Posts: 154 Joined: 21-April 05 From: Rochester, New York, USA Member No.: 336 |
I always thought that 100% relative humidity was when the partial pressure of the water vapor in the atmosphere was equal to the vapor pressure of liquid water at a given temperature. As such, partial pressures of other gasses in the atmosphere would be irrelevant. This is true. Each compound (water, carbon dioxide, etc.) exerts it's own vapor pressure based on the temperature. At equilibrium, you add them up to get the total pressure. |
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