There are two METAR reports below. The first is for Dulles Airport. The 2nd is for New Philadelphia OH.
Calculate the density of the moist air at both locations. Also calculate the relative humidity.
Describe and explain the steps you take. Show your work. Use online calculators only to verify your results. You may use an online calculator to determine station pressure. Google the search phrase "altimeter to station pressure" and click on the search result located at www.srh.noaa.gov.
KIAD 292352Z 20005KT 8SM BKN200 34/23 A2971 RMK AO2 SLP055
T03440228 10389 20344 55005=
KPHD 292353Z AUTO 15007KT 10SM FEW090 SCT110 22/18 A2981 RMK AO2
LTG DSNT SE AND S TSE33RAE34 SLP086 P0001 6//// T02170178
At KIAD, the temperature in °C is 34.4, the dewpoint is 22.8, the sea level pressure is 1005.5 mb, the altimeter is 29.71 inches Hg. The altitude of this station is 93 meters.
At KPHD, the temperature is 21.7°C, the dewpoint is 17.8, the sea level pressure is 1008.6, the altimeter is 29.81 inches Hg. The altitude of the station is 272 meters.
The reports are for 00Z June 30, 2012.
If you apply the ideal gas formula to water vapor, you use the partial pressure of the water vapor -- this is the "e" -- and also use the R-sub-v value for water vapor (the ideal gas constant for water vapor).
It is customary to say "vapor pressure" which means "partial pressure of water vapor".
There is a special value of "e" which is the partial pressure of water vapor at "saturation". This is the "e"-sub-"s" variable. For some parcel, the actual vapor pressure e will always be less than or equal to e-sub-s.
We can calculate "e"-sub-"s" using the formula provided in the lecture notes, or use a lookup table.
Let's say we have a parcel of air, and we know its temperature. We can determine "e"-sub-"s" for that parcel -- the saturation vapor pressure for the parcel, which is based on temperature only. BUT!! we cannot determine the actual vapor pressure for that parcel.
If we are given both the temperature and dewpoint for the parcel, we can determine the actual vapor pressure. Because we then calculate the saturation vapor pressure at the dewpoint temperature, which becomes the "e" for the parcel.
Yup. The notation is confusing. The thing to remember about the notation is: for a parcel of air, where we know the temperature and dewpoint, then:
e FOR THE PARCEL is the e-sub-s at the dewpoint of the parcel.
e-sub-s FOR THE PARCEL is the e-sub-s at the temperature of the parcel.
The above assumes that the temperature and dewpoint are measured at some pressure value, and the assumption is made that we are holding pressure constant.
If you calculate e and e-sub-s for the parcel, then you can calculate RH.
For HW6, you will be wanting to use the formula that is in the lecture notes for the "density of moist air". This formula is given about halfway through the lecture notes for "Adiabatic Processes; Moisture" on the web site.
To use that formula you will need to calculate "e" for the parcel. See above. You will also need the variable "p". This is the actual pressure of the air -- the "station pressure". You will also need T, the temperature. And you will need to know the values of R-sub-v and R-sub-d -- the gas constants for water vapor and for dry air. These values are given in the lecture notes about a third of the way down.
Remember to use SI units!