Climate and Global Change
How humid is it anyway?
Introduction
Humidity has considerable to do with how we “feel” warmth. The more humid the air the warmer
we feel. From a climate prospective, humidity is an important parameter when considering
clouds and rain. Recall from class that we had several ways of describing atmospheric moisture.
The various measures of atmospheric water vapor content and their units included the following:
•
Absolute Humidity
Units
= Mass of water vapor / Volume of air
=> grams / meter3 (g / m3)
•
Mixing Ratio
Units
= Mass of water vapor / Mass of dry air
=> grams / kilogram (g / kg)
•
Specific Humidity
Units
= Mass of water vapor / Mass of moist air
=> grams / kilogram (g / kg)
•
Vapor Pressure
Units
= Part of the pressure caused by water vapor molecules
=> inches of mercury or millibars (in of Hg or mb),
•
Relative Humidity
Units
= (actual vapor pressure) 100 / ( saturation vapor pressure )
=> %
•
Dewpoint Temperature = Temperature at which dew (saturation) forms when air is
cooled without addition or subtraction of water vapor
Units
=> °F, °C and K
•
Wet-bulb Temperature =
Units
Temperature a thermometer can be
cooled by evaporative cooling
=> °F, °C and K
Dewpoint Temperature Measurement
In this experiment we are going to use a simple, and maybe not the most
accurate, “instrument” to measure the room’s dewpoint temperature. Take a
shiny aluminum can (a clean silver soda can will work) and fill it partially
(about half to two-thirds) full of water that is slightly warmer than the room
air temperature.
First, measure the room air temperature. Next, measure the temperature of
the water. Is the water slightly warmer than the air temperature? If not, find
some water that is warmer than the room air temperature.
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Now add ice to the water in the can. Stir the ice-water mixture slowly while looking at the
outside of the can. Record the temperature of the ice-water mixture when the can begins to
become hazy, i.e., dew or condensate starts to form on the can. This temperature is an estimate of
the room’s dewpoint temperature. If the ice melts before dew starts to form add more ice.
Why is this maybe not the most accurate measure of the dewpoint temperature? What if we had
used an iron can instead of the aluminum can? Would our “instrument” have been more or less
accurate? Hint: Think conduction (see table below). Would a copper can work better?
Material
Iron
Aluminum
Copper
Heat Conductivity
( cal / s - cm - °C )
0.161
0.50
0.918
What would it tell you if dew did not form on the can and you had cooled the water to near
freezing?
Relative Humidity Calculation
To get an idea how the dewpoint temperature relates to relative humidity, convert the room’s
dewpoint temperature you measured into relative humidity. Recall relative humidity is defined as
100 times the actual vapor pressure divided by the saturation vapor pressure, i.e.,
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RH (%) = 100 ( actual vapor pressure ) / ( saturation vapor pressure ).
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As was stated in class, the saturation value depends on the temperature (for example, see the
graph below, which relates the saturation specific humidity to temperature), warmer air has a
higher saturation value than cooler air. One can substitute the appropriate specific humidity or
mixing ratio values into the definition of relative humidity with little or no error.
To determine the saturation vapor pressure of the room air, we will use the table on the next page
to look up the saturation vapor pressure corresponding to the room temperature. This table is
comparable to the graph only for saturation vapor pressure and it is easier to select an exact
value.
From the Table below, enter the saturation vapor pressure corresponding to the room’s
temperature
______________ .
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T
(°C)
-40
-38
-36
-34
-32
-30
-28
-26
-24
-22
-20
Saturation
T
Saturation
T
Saturation
T
Saturation
Vapor
(°C)
Vapor
(°C)
Vapor
(°C)
Vapor
Pressure
Pressure
Pressure
Pressure
(mb)
(mb)
(mb)
(mb)
0.189
-18
1.49
4
8.13
26
33.61
0.232
-16
1.76
6
9.35
28
37.79
0.284
-14
2.08
8
10.72
30
42.43
0.346
-12
2.44
10
12.27
32
47.55
0.420
-10
2.86
12
14.02
34
53.20
0.509
-8
3.35
14
15.97
36
59.42
0.613
-6
3.91
16
18.17
38
66.26
0.737
-4
4.54
18
20.63
40
73.77
0.883
-2
5.27
20
23.37
1.05
0
6.11
22
26.43
1.25
2
7.05
24
29.83
Saturation vapor pressure over water in mb versus temperature in °C
The dewpoint temperature of a parcel of air represents the temperature at which the air would
become saturated. Thus, to determine the actual vapor pressure, we use the table to look up the
vapor pressure corresponding to the dewpoint temperature we measured.
Enter the actual vapor pressure corresponding to the room’s dewpoint temperature __________ .
Calculate the room’s relative humidity using the equation above. ___________
Feel the Effect of Latent Heat
Predict what will happen to your hands’ temperature as you wave your hands back and forth in
the air eight times.
Wave your hands eight times and compare the difference in how the temperature of your hands
feel. For this part of the experiment be sure your hands are dry.
Soak a paper towel in room temperature water. Place the wet towel over your right hand. Predict
what will happen to each hand’s temperature when you wave both hands in the air.
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Conduct the test and record what each hand feels. Is it cooler or warmer?
Hold your arms still. Do you feel a difference in the temperatures of your hands?
Explain the interactions between the heat energy from your body, i.e., the temperature of your
hands, the water molecules on your hand and the air during these activities.
Relative Humidity with a Sling Psychrometer
Recall from lecture that when
an object is wet, energy (the
latent heat of vaporization) is
removed from the object by
evaporation. If we place a
“sock” over the end of a
thermometer and ventilate the
thermometer, we can measure
the difference between the “drybulb” (ordinary) temperature
and the web-bulb temperature.
This difference is referred to as
the wet-bulb depression. The
difference between these two temperatures is related to
relative humidity of the room air. If the room is near
saturation, then there will be very little evaporation
from the wet-bulb thermometer. Thus, the wet-bulb
and the dry-bulb will be similar, i.e., the wet-bulb
depression will be small. If the room air is very dry,
then there will be considerable difference between the
two temperatures, i.e., the wet-bulb depression will be
large.
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Using a table or in this case the “slide rule” on the psychrometer you will be able to obtain the
relative humidity.
How does this relative humidity compare to the value you calculated above?
If they don’t agree, why do you suppose they are different?
Cloud Bottle Demonstration
This experiment demonstrates two basic principles: the Ideal Gas Law and the relationship
between saturation and temperature.
Place a small amount of warm water (use water heated in the teapot) in the bottom of the cloud
bottle. Let the water set for a few minutes so it can come to equilibrium with the air in the bottle.
Measure the temperature of the air in the bottle. Connect the pump to the bent glass tubing of the
apparatus and increase the pressure inside the bottle by squeezing the hand pump (about 15
times).
Measure the temperature of the air in the bottle again.
Did the temperature change? ––––––––––
If so, how did it change, increase or decrease? –––––––––––––––––
Using the Ideal Gas Law explain why the temperature might have changed.
Now turn off the lights and shine a flashlight through the bottle. Disconnect the pump from the
apparatus - you should hear a hiss as the pressure in the bottle is released and becomes the same
as the pressure in the room. Did anything happen inside the bottle? If so, what?
______________________________________
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Measure the temperature in the bottle once more. Did it change again? ________
If so, how did it change (increase or decrease)? _____________
Why?
If the temperature of the air in the bottle changed by increasing and decreasing the pressure inside
the bottle, what happened to the relative humidity of the bottle's air - did it change?
If so, how did it change (increase or decrease) in relation to the temperature and pressure?