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GEOG 101 Physical Geography
Lab 5: Air Pressure, Humidity and Adiabatic Lapse Rates
(Credit: Based on UCSB Geography Department laboratory with modifications by D. Fairbanks and N. Sato)
Name
Answer Key
Lab Section
Date
Materials and sources
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a sling psychrometer and water
a psychrometric chart, included with the psychrometers
a Kestrel weather tracker
the elevator and stairwells in Butte Hall
colored pencils: blue, red, and green
Ruler
Introduction
Humidity, temperature, and air pressure are key environmental variables. They play critical roles in
controlling processes such as evaporation, condensation, cloud creation, and wind. Evaporation transfers
moisture from the surface to the atmosphere where it becomes available for cloud formation and
precipitation. In Section 1 of this lab, we will use a device called a sling psychrometer to measure
relative humidity. In Section 2 we will learn about air pressure. In Section 3 we will introduce the
concept of adiabatic and environmental lapse rates, which describe the rate at which the temperature of
the environment or a rising parcel of humidified air changes with increased elevation. We will use this
information to calculate how much a parcel of air cools as it rises and at what elevation the parcel and
environment will have the same temperature, causing the parcel to stop.
Key Terms:
Cloud formation
Dew point temperature
Dry Adiabatic Rate (DAR)
Moist Adiabatic Rate (MAR)
Latent heat
Saturation
Latent heat of evaporation
Sling psychrometer
Relative humidity
Stability
Pressure Gradient Force (PGF)
Section 1: Air Pressure
Air pressure is the weight of all the air above you in the atmosphere. It’s pressing on all sides of you
equally with a force of approximately 14.7 pounds per square inch. In the following experiment, you’ll
experience and measure a change in air pressure.
Formulate a hypothesis on what you think will happen with pressure when you go up to the 7th floor from
the 1st floor. Will it increase or decrease?
I think it will decrease. I think this because I know from taking trips to Tahoe that air pressure
decreases as you go up in elevation—the air seems “thinner”
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1a) Get your group together and take a Kestrel weather tracker, and find the elevators in Butte Hall and go
to the bottom (1st) floor. Turn the Kestrel on, and push the “up” or “down” buttons until you see the
“baro” screen (barometric pressure). Have a friend record this number in the table below as you’re
waiting for the elevator on the first floor of the building. Now, get back in and take the elevator to the top
floor (7th) of the building, and watch the air pressure change as the elevator goes up. Get off the
elevator at the 7th floor, and check the air pressure. Wait a few seconds for the number to stabilize before
writing it in the following table.
Air Pressure, mb
(millibars)
DATA FROM FALL 2014
7th floor
1004.0
1st floor
1007.3
Pressure difference, in mb
(bottom floor – top floor)
3.2
1b) Take the stairs back down to the first floor. When you first enter the stairwell, record the air
pressure in the following table. Halfway down the stairs (4th or 3rd floors), stop to measure the wind
speed (use the arrow keys on the Kestrel weather tracker to find wind speed. Is the stairwell a windy
place? No (yes or no) If “yes” what is the speed? n/a . Measure the air pressure again at the bottom of
the stairwell (outside the glass doors), and record it in the table below.
A difference in air pressure between two different points creates a pressure gradient force (PGF). This
always points from high pressure to low pressure. This can produce wind, as air moves from an area of
high pressure to an area of low pressure.
Air Pressure, mb
(millibars)
Top of stairwell
(7th floor)
Bottom of stairwell
(1st floor, but walk
out the glass doors)
Draw an arrow between the two
labels below to indicate the
direction of the pressure gradient
force in the stairwell
Top of stairwell (7th floor)
1004.2
(Arrow pointing up)
1007.3
Bottom of stairwell (1st floor)
1c) Was your hypothesis supported or falsified? Explain why the air pressure on the top floor of the
building was lower than the air pressure on the bottom floor. Hint: this is also why the air pressure
decreases dramatically with altitude in the atmosphere.
My hypothesis was supported. Air pressure on the top floor was lower because of there are six
floors less “weight” of air molecules pushing down on them. Conversely, the 1st floor has the entire
weight of all the air molecules above them pushing down, thus the higher pressure on the first floor.
1d) The pressure gradient force (PGF) within the stairwell would’ve led you to expect strong winds to
blow up the stairs, from the high pressure on the bottom floor, to the low pressure on the top floor. But
our stairwells aren’t very windy, as you discovered, so there must be another force opposing the PGF
such that the two forces cancel each other out, leaving the stairwell wind-free. Explain what this other
force is. Hint: this force also opposes you when you walk up the stairs, but makes it feel easier to walk
down the stairs. The force of gravity is pushing down on the wind. This force is opposing the
pressure gradient force.
2
Section 2: Air Pressure on a Weather Map
To understand atmospheric circulations, you must be able to understand how variables (temperature,
pressure, winds, humidity, clouds) are changing in time and how they are changing with respect to one
another. The weather map is a tool that aids this understanding. Various kinds of maps, or charts, are
used to graphically depict these variables. A good map allows you to quickly identify patterns. For
example, a weather map of forecasted high temperatures typically available in newspapers indicates the
location of warm and cold regions of the country. From these maps you can quickly gauge the predicted
high temperature for your town.
Maps depicting weather conditions are drawn based on simultaneous observations made at many places
throughout the world. Accurate portrayal of these observations is the key to a correct interpretation of the
data. Meteorologists and geographers use a technique called contour analysis to visually explain the
information the data is providing. Contouring data represents an elementary step in data analysis. The
ability to correctly and confidently analyze data is critical to interpreting conditions.
In this section, you will develop a pressure map from the data reported by 26 different weather stations in
the Western United States. The blank map with weather station locations is attached to the back of this
lab.
Using the following pressure (mb) readings from the cities below and a set of three colored pencils (Blue,
Red and Green) construct:
a) An isobar contour map using the 1028, 1024, 1020, 1016, 1012, 1008, 1004, and 1000 isobars.
b) Label any highs or lows, which may exist. High = Blue; Low = Red
c) On the same map, place green arrows at convenient locations to indicate probable wind
directions.
The map provided is blank but you Lab TA will provide a map on the screen providing the locations of
these cities in order to make your isobars.
City
Seattle, WA
Portland, OR
Spokane, WA
San Francisco, CA
Los Angeles, CA
San Diego, CA
Las Vegas, NV
Boise, ID
Great Falls, MT
Billings, MT
Salt Lake City, UT
Phoenix, AZ
El Paso, TX
Pressure (mb)
1024
1029
1019
1019
1011
1010
1009
1014
1012
1000
1007
1015
1021
DATA
City
Albuquerque, NM
Denver, CO
Cheyenne, WY
Rapid City, SD
Bismark, ND
Omaha, NB
Des Moines, IA
Kansas City, MO
Wichita, KS
Tulsa, OK
Dallas, TX
San Antonio, TX
Houston, TX
Pressure (mb)
1018
1017
1015
1011
1013
1015
1017
1018
1022
1023
1026
1024
1025
1) Preparation – finding patterns
A. Search for spatial continuity on the pressure map by labeling each point with its appropriate
mb reading.
B. Locate regions of high and low values.
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C. Review data to determine isopleths (contour) spacing.
2) Drawing the map
A. Use a pencil!
B. Draw smooth lines.
C. Interpolate between given values to correctly place an isobar.
D. Isobars cannot touch or cross.
E. Isobars cannot branch or fork.
F. Label the isobars at the end of the line drawn.
The Lab TA will show this video: http://www.youtube.com/watch?v=XtWlAwSAPNE
Section 3: Humidity
Evaporation occurs when liquid water heats up, and changes from a liquid to a gaseous state. Relative
humidity affects such processes as evaporation – the higher the relative humidity the slower the
evaporation. Your lab instructor will provide you with a tool called a sling psychrometer that takes this
into account and uses two thermometers – one dry and the other with a wet cloth over the bulb – to
measure relative humidity and the dew point temperature. The dry bulb measures the air temperature.
Because evaporation can take a while, we rapidly twirl the sling psychrometer to speed up the process,
and the wet bulb thermometer is cooled due to the latent heat of evaporation that is required to
evaporate the water. We can use the difference in temperature between the dry bulb and wet bulb
thermometers to calculate the wet bulb depression and relative humidity.
Step 1: Your Lab TA will leed you to a shaded place outside to conduct this experiment. Be sure to keep
both of the sling psychrometer’s thermometers out of direct sunlight at all times. Confirm that they’re
both measuring approximately the same temperature.
Step 2: Your Lab TA will pour a bit of water on the thermometer with the cloth (the wet bulb). Don’t let
any water touch the other thermometer (the dry bulb). When you wet the wick of the thermometer and
leave it for a few minutes, will the temperature be the same? Hypothesize what will happen to the
temperature and explain why.
I think the temperature of the wet bulb will be the same. The water temperature and the air
temperature are the same, so there is no reason for anything to change.
Step 3: Whirl the sling psychrometer for 60 seconds. As soon as you stop, quickly read off the
temperatures of both thermometers (read the web bulb thermometer first), and record each in the table
below for your group.
Step 4: Calculate the wet bulb depression. This is simply the dry bulb temperature minus the wet bulb
temperature. Use this, along with the table included in the last pages of most of our psychrometer
instruction manuals, to find the relative humidity, and record that in the table below.
Step 5: After this lab, share data with two other groups, add your relative humidity measurements
together, and divide by 3 to calculate an average relative humidity.
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Your
measurement
Dry bulb
Temperature
Wet bulb
Temperature
(° C)
(° C)
Wet-bulb
depression (° C)
(dry bulb – wet bulb)
Relative Humidity
(your instructor provides
a RH table)
Answers will vary
Class average
47% (FALL 2014)
3a) There are many variables we aren’t taking into account in the sling psychrometer experiment that can
make our relative humidity calculations inaccurate (such as impurities in the water). Describe a physical
mechanism that you think would influence the results, and explain how this mechanism might have
decreased the accuracy of the relative humidity you calculated. There are dozens of possible factors, so
be creative! Explain why finding the average relative humidity of the class might provide a more reliable
estimate than any of the individual measurements. Was your hypothesis supported or falsified? If it was
falsified, come up with an explanation for your observations.
Results from the class may vary based on location of the person spinning (on grass or on concrete),
how high the sling psychrometer is being held above the ground, speed of the spin, or perhaps how wet
the cloth was. Finding the class average helps to nullify some of these errors.
My hypothesis was falsified. I did not take into account the evaporation of the water and effect that
would have on the temperature.
3b) Explain how the sling psychrometer works. Why is the wet bulb colder than the dry bulb? Be sure to
mention latent heat and evaporation. In the diagram below, draw arrows to show the flow of heat energy
involved in the process of latent heat of evaporation. Where does this energy go?
The sling psychrometer works by measuring the extent of evaporation of water from the cloth. It
takes energy to evaporate water, this energy comes from the thermometer and the air directly
adjacent to the thermometer. This transfer of energy registers as a drop in temperature in the wetbulb thermometer. The energy becomes part of the latent heat of the evaporated water (water
vapor).
Energy goes
into the
evaporated
water.
3c) If the relative humidity were 100%, the air would be saturated with respect to water. Any evaporation
from the wet bulb into the air would be almost exactly balanced by condensation from water vapor in the
atmosphere back onto the wet surface (this is the definition of saturation). What would be the wet-bulb
depression in this case?
In this case the wet-bulb depression would be zero. No water would be able to evaporate, so there
would be no temperature change.
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Your Lab TA will explain to you what “dew point” means and what happens when a parcel of air is cooled
to its dew point.
When a parcel of air is cooled to “dew point” that means the air is no longer warm enough to hold
the water in gas (vapor) form. Thus the water condenses into liquid water drops. This is seen when
drops of water form on grass on a cool morning.
3d) Examine the graph below and explain how relative humidity (solid line) generally changes throughout a
day in relation to air temperature (dashed line). Why does this happen? Why does dew sometimes form
just before sunrise?
As the temperature of the air increases, it has a greater ability to hold water as vapor. As you can see
from the graph, at noon the air has warmed such that it is at “50% capacity of holding water vapor”
a.k.a. 50% humidity. When the day is hottest, humidity is at its lowest—the graph shows an inverse
relationship. Dew forms just before sunrise because the air is no longer warm enough to hold the
water in gas (vapor) form. Humidity has reached 100%.
Temperature
Relative Humidity (RH) %
100
0
Sunrise
Noon
6
Sunset
Section 4: Atmospheric Stability: Adiabatic Processes
Parcels expand as they are lifted because the air pressure decreases with altitude. This causes the parcel’s
temperature to decrease adiabatically. Adiabatic describes the warming and cooling rates for a parcel of
expanding or compressing air.
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Ascending air = cooling = expansion
Descending air = heating = compression
We will concentrate in this section on ascending air at two different rates: dry air (RH < 100%) and
moist air (RH = 100%)
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Dry Adiabatic Rate (DAR): 10 oC per 1000 m
Moist Adiabatic Rate (MAR): 4 oC per 1000 m (ranges 4°-10° C, depending on H2O content)
For all of the exercises in this section, use a ruler or straightedge to draw straight, neat lines (this is
very important), and assume the following:
4a) When a parcel of air is dry (that is, the air is not saturated) and rises from the surface, its temperature
decreases at DAR. For example, if the temperature of the air at sea level (0 m altitude) is 20° C and it is
rising, its temperature is 10° C when it reaches the altitude of 1000 m.
Assume a parcel of dry air has a temperature of 26° C at sea level. It begins to rise, cooling at the DAR.
In the table below, calculate the change in the temperature of this parcel dry air with different elevations.
Elevation (m)
Temperature (°C)
2000
6
1500
11
1000
16
500
21
0
26
4b) A parcel of air over the oceans is nearly saturated. When a parcel of saturated air rises, its
temperature decreases at MAR. The rate of the temperature decrease is slower due to the release of
latent heat. If the temperature of the air at sea level (0 m altitude) is 20°C and it is rising, its temperature
is 16° C when it reaches the altitude of 1000 m. Compare this value to 4a.
The temperature change at MAR is slower than DAR.
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Assume a parcel of saturated air has a temperature of 26 °C at sea level. It begins to rise, cooling at the
MAR. In the table below, calculate the change in the temperature of this parcel with different elevations.
Elevation (m)
Temperature (°C)
2000
18
1500
20
1000
22
500
24
0
26
4c) Using a ruler or straightedge, plot your values that you calculated in the tables above in the graph
below. Extend the linear relationship beyond the last value that you have in your tables. You now can
read the temperature of rising air parcels at different elevations.
4000
3500
3000
2500
2000
1500
1000
500
0
-20
-10
0
10
20
Temperature (C)
30
40
4d) Different from DAR and MAR, the snapshot of the atmosphere’s vertical temperature profile (called
the Environmental Temperature Lapse Rate or ETLR) is not a straight line. It is influenced by many
factors, and it changes over days and seasons, and can vary greatly with location. The ETLR has a
profound influence on cloud formation and weather, as we’re about to discover.
A simplified ETLR is draw in the graph below. Copy and plot the line of DAR that you drew in
the graph in 4c in the graph below.
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4000
3500
ETLR
3000
2500
2000
1500
1000
500
0
-20
-10
0
10
20
Temperature (C)
30
40
4e) Notice that your line of DAR intersects the line of ETLR. This is the elevation that a parcel of rising
dry air can reach. A parcel of air rises as long as it is warmer than its surrounding. At the elevation
where the two lines (DAR and ELR intersect), the temperature of the rising air parcel and its surrounding
(environment) are the same. What is this elevation? About 800 meters
4f) Go back and look at the table in 4a. Assume that this parcel of rising dry air has a dew point
temperature of 11° C. At what elevation, do you observe that the temperature of the rising air parcel
reaches the dew point temperature? The parcel of air is at dew point temperature at 1500 meters
4g) The answer in 4f is where saturation of air (RH = 100%) takes place. That is, the relative humidity of
this rising air is 100% when air temperature = dew point temperature. If this air is still rising (that is, it is
still warmer than its surrounding temperature – ETLR), the rate of temperature decrease takes place with
MAR. Based on your answer for 4e, complete the table below. Note: this table looks slightly different
from the one for 4c.
Elevation (m)
Temperature (°C)
2000
15
1500
17
1000
19
500
21
0
26
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Synthesis: Putting it All Together
4h) Clouds are optically opaque because they’re composed of suspended liquid water droplets. What is the
relative humidity of the beginning of a cloud base (the lowest part of the cloud)? Does it represent dew
point temperature?
Relative humidity at the lowest part of the cloud is 100%. This does represent dew point
temperature.
4i) Using everything you learned in this lab, explain why clouds can’t form in sinking air.
If the air is sinking, it is also warming. Therefore, as the air sinks it is increasing its ability to hold
water in gaseous form (water vapor). Clouds will not form; rather, they will dissipate.
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