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Could someone please benice and tell me how to find the air density when the temperature is 18 degrees celsuis and the atmospheric pressurew is 782 mm HG.
thanks a lot
thanks a lot
I believe that should be [tex]\rho = \frac{MP}{RT}[/tex]Neglecting humidity, [tex]\rho = \frac{P}{RT}[/tex]
Remember pressure is in absolute units as well as temperature (kelvine or rankine).
Relative HumidityI have always been curious about the affect of water/water vapor on the overall density. I found the above formula on the internet so I have no idea if it is correct. I don't think I fully understand the relationship between vapor pressure and the more common weather term humidity. How much does the overall density change when humidity changes from 30% to 90%. Does the vapor pressure change as well?
from - http://www.weather.gov/glossary/A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point.
Thanks. I have been reading some definitions of weather terms. I have always used this http://wahiduddin.net/calc/calc_da.htm" [Broken].Relative Humidity
from - http://www.weather.gov/glossary/
Humidity
Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity. - NWS glossary
Dew Point
(Abbrev. DWPT) - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature, and sometimes written as one word (Dewpoint). - NWS glossary
Both density and vapor pressure are related to the mole fraction of water vapor in the gas. The water molecule has a molecular mass of 18, as compared to N_{2} = 28 and O_{2} = 32, so moist air is lighter since the mean molecular mass is less.
That's the way I always do it. I'm so used to using...unless R is the normalized gas constant for air, in units of J/K-kg (not the molar gas constant), in which case, Fred's equation is correct.
If I understand what you are asking correctly, then the measured density divided by the standard (referenced) density will give you the ratio you are looking for. Both units must be the same.I think I now understand how to get the raw air density in kg/m^3; can someone help me with a formula to convert this to air density percent with 100 as the standard density?