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.

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.

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

I think you are referring to this recent paper Evidence of Water Vapor in Excess of Saturation in the Atmosphere of Mars

>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.)

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.

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.

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



That is a remarkable view. Should be spectacular as climb through all that in stereo!

I'm sure you meant to say 1%

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]

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.

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.

I did; was thinking 1/100th & wrote the wrong thing. Good catch, and thanks!

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.