Measuring Humidity
Meteo 380
Lab 4
Measuring humidity
Background
Measurement of the moisture content of the air has been a fundamental problem in meteorology for over one hundred years. Phase transformations of colorless, odorless, water vapor gas are the fundamental transactions in nearly all weather phenomena, but the amount of this gas in the air is frustratingly difficult to measure with accuracy.
Amazingly, most simple dial-hygrometers used to indicate relative humidity operate through calibration of the humidity-sensitive lengthening and contraction of human hair. Unfortunately, such devices usually respond poorly to rapid changes in moisture content and have and error of at least 10%.
Scientific measurements require more precision. Exact measurements of moisture content are essential for understanding climate and are critical for accurate prediction of weather events. Poor measurement of atmospheric moisture is still an significant cause of bad weather forecasts and incorrect output from computer models.
In this lab we are going to make scientific measurements and calculations of moisture content in the laboratory in two ways that are commonly applied by sophisticated weather instruments.
Procedure
Part I: Wet-bulb pyschrometer
A variant of this approach is used to make most of the humidity measurements at surface weather stations.
1. Use a digital thermometer to take a reading of the temperature of the air in the laboratory. This is called the dry bulb temperature. Be careful not to touch the tip of the probe, which will cause errors. Record both C, K, and F.
T= ______(C)
T= ______(K)
T= ______(F)
2. Take a piece of wick material, soak it in water and carefully thread at least 1 inch over the tip of the probe. Rest your probe against the front of the fan to promote rapid evaporation. The evaporative cooling caused by the latent heat of vaporization should cause the reading on the thermometer to drop. Wait several minutes until the reading is no longer decreasing. This is your wet-bulb temperature.
Tw= ______(C)
Tw= ______(K)
Tw= ______(F)
3. The wet-bulb temperature is related to relative humidity and dewpoint, but it will require some calculations to determine exactly how. You will need to manipulate both the Clausius-Clapeyron equation and the pyschometric equation.
You will first need to find the pv-w, the wet-bulb vapor pressure using the C-C. equation with the wet-bulb temperature. Remember that you must use temperature in Kelvin in this equation. Lv=2.27x106 J/kg and Rv=461.5 J/kgK.
pv-w=______(mb)
Next, you’ll want to find the total vapor pressure of the moisture in the room using the pyschometric equation. The total air pressure will be provided on the board. In this equation, temperature should be in degrees C.
pv=______(mb)
4. Now convert this vapor pressure to a relative humidity:
RH= pv /pv-sat
To do this you will need to find pv-satby using the C-C equation again, but this time with the dry bulb temperature, not the wet-bulb.
pv-sat=______(mb)
RH=______%
5. Working backwards with C-C equation, you can also now find the dewpoint. Use your total vapor pressure and solve the C-C equation for T. This would now be your dewpoint temperature because it tells you what temperature would achieve 100% humidity (i.e dew) with the amount of vapor that you measured. Give your dewpoint temperature in Kelvin, Celcius, and Fahrenheit.
Td= ______(K)
Td= ______(C)
Td= ______(F)
Part II: Chilled surface-hygrometer
This approach makes a direct measurement of the dewpoint, though it requires careful techniques and observation.
1. Fill a shiny aluminum can half-way with room-temperature water.
2. Obtain a cup of cold ice-water in a styrofoam cup.
3. Add about one tablespoon of cold water to your aluminum can and stir well with the digital thermometer. The temperature of your water should decrease slightly.
4. Wait until you have a stable temperature reading and repeat step 3 until you get within a few degrees of the dewpoint temperature measured in part I.
5. Continue with repetition of step 3, but observe the outside of your can carefully with each addition of cold water. You are looking for the very first formation of dew on the outside of the can. As you approach the suspected dewpoint, be sure to add only a little cold water at a time and wait a minute or two at each lower temperature to give dew a chance to form.
6. As soon as you observe dew formation, record the temperature of your water. This is your dewpoint as measured directly by a chilled-surface.
Td= ______(C)
Td= ______(K)
Td= ______(F)
7. Use the C.C. equation to find the total vapor pressure based on this measurement, and convert this to RH%, using the same equation from part I, step 4. The saturation vapor pressure based on the dry-bulb temperature will be the same as previously calculated.
pv =______(mb) (based on Td from chilled surface)
RH=______%
Part III: Moisture calculations.
1. Which of these measurements of moisture do you think is more accurate? Give a rational for your choice and numerical estimate of the experimental uncertainty in your more accurate RH%.
2. Using the total vapor-pressure, calculate the mass of water in this room using the ideal gas law, PV=nRT. You will need to measure the volume of the room, and use pressure in the SI unit, Pa, not mb. (1 mb=100 Pa) Once you solve for the number of moles, n, you can convert to mass through the molecular weight of water, 18 g/mol. R, the gas constant, = 8.314 J/mol K.
Total amount of water in this room=______g
3. Another common way to talk about moisture content of the air is called the “mixing ratio”. This is just a ratio of the number of grams of water to kg of air. To find this ration, calculate the mass of air in the room as you did with the water in step 2, then just take a ratio. The pressure of the air will be (total pressure –vapor pressure). The average molecular weight of air is 29 g/mol.
Total amount of water in this room=______kg
Mixing ratio=______g(H20)/kg(air)
4. In general the dewpoint indoors is usually not too much different than the dewpoint outdoors, because, afterall, it is the air from outside that is brought indoors through fans, windows and circulation systems. However, the the RH% is often quite different indoors because the temperature of the air is often intentionally heated or cooled. There are, however, also a number of factors that can affect the indoor dewpoint. See if you can think of some.
a) How does the current outdoor dewpoint compare to that measured in the room now? Can you explain why it is higher/lower inside?
b) What factors could cause the indoor air to have a lower dewpoint than outside?
c) What factors could cause the indoor air to have a higher dewpoint than outside?