GEOB
Read the
Lab 2 Tapestry page
, then work through the following questions on volcanism in Mount Rainier and the
Tahoma
watershed. After completing this lab, you should be able to:
· Summarize the tectonic processes (plates and directions of motion) that generate the Cascade volcanic arc
· Outline how lava flows have contributed to the Tahoma watershed topography
· Explain why lahars are the major geological hazard of Mt. Rainier
· Describe what hydrothermal alteration is and how it contributes to lahars
Q1
Using Fig. 1, describe in two or three sentences the tectonic processes responsible for the Cascade Volcanic Arc. Be sure to reference the relevant tectonic plates and their directions of motion.
Q2
Magma composition dictates the shape and eruptive style of volcanoes. The cascade volcanoes depicted in Fig. 1 are primarily of intermediate composition, so their eruptions tend to be explosive and lavas do not flow very readily: Select all of the evidence visible on the Cascade volcanoes in Fig. 1 that supports these statements.
Lightly-coloured parent material
Lava domes and calderas
Dark-coloured parent material
Summit glaciers separated by exposed ridges
Flanks dissected by fluvial channels
Relatively steep slopes
Q3
Judging from Fig. 2(b), approximately how thick are individual strata (in meters) on average at Willis Wall below “Gun Sight”, near the summit of Mount Rainier?
Q4
The exposed strata on Willis Wall below “Gun Sight” on Fig. 2(b) were deposited by lava flows in the period between 40 ka and 15 ka ago (remember that 1 ka = 1000 years)
[1]
. Given this time interval and any characteristics you can estimate from Fig. 2(b), what was the average rate of major lava flows occurring down the North flank of Rainier in this time period? You can assume each stratum was formed by one major lava flow. Report your answer as a number of lava flows per ka.
Q5
Figs. 3 and 4 indicate that the rock forming the ridges above Tahoma creek (Emerald Ridge and Success Cleaver) is younger than the bedrock in the valley bottom where Tahoma creek flows today. This scenario is surprising since the ridges at Tahoma watershed are formed by lava flows, and we expect flowing lava to take the path of steepest descent to the valley bottom — not to flow along the ridges. We expect the oldest rock to form the ridges.
Two competing hypotheses have been put forth to explain why ridges are younger than valleys at Tahoma watershed:
1. Rainier’s flank was initially a mixture of relatively hard and soft lava flows high above contemporary Tahoma creek. Glaciers preferentially eroded through Rainier’s softer lava flows, leaving the more resistant lava flows behind as the contemporary ridges
2. Lava flows from Rainier’s summit tracked through thin areas in much larger ancient glaciers, building ridges adjacent to deep valley-filling glacial ice which subsequently melted away
The next several questions will guide you toward the evidence to reject one of these hypotheses.
First, using the topographic and geological maps Figs. 3 and 4 (and Google Earth, if needed), estimate the elevation difference between Emerald Ridge (on the North Tahoma watershed margin) and Tahoma creek. Note that the contour interval in Fig. 4 is 50 m. Report your answer in meters.
Q6
Assuming the area where Tahoma creek flows today was once buried under lava flows of similar age and elevation as Emerald ridge on the Tahoma watershed boundary, how fast at minimum would have glaciers needed to incise downward to account for the modern topography of Tahoma watershed? Report the minimum required glacial incision rate for hypothesis #2 in mm/yr.
Q7
Glaciers in similar watersheds as Tahoma typically incise at rates between 0.05-0.5 mm/yr [1]. Given these values, which hypothesis (#1 or #2) seems more plausible to account for the relatively young Tahoma watershed ridges? Explain your reasoning in two to three sentences.
Q8
Lahars are most likely wherever steep gradients and hydrothermal alteration coincide. Given the Fig. 7 hydrothermal alteration map and the Fig. 4 topographic map, the next Rainier Lahar is most likely to flow predominately down which watershed?
Pullayup
Tahoma
Nisqually
Mowich
GEOB
102 Our Changing Environment: Climate and Ecosystems Lab 2
1
Lab 2 – Lifting processes, precipitation, and extreme weather
Due: Friday, October 16th 2020 11:59 pm
Objectives:
● Explain the difference between specific and relative humidity and how these change with
temperature
● Explain the concept of adiabatic processes including the Dry Adiabatic Lapse Rate (DALR),
Moist Adiabatic Lapse Rate (MALR/SALR), and the Environmental Lapse Rate (ELR)
● Identify the difference between a stable and unstable air parcel
● Explain how air temperature and pressure change with height in the atmosphere
● Identify components and patterns observed in synoptic weather maps
Marks: 2
8
Introduction
Read the lab carefully. All answers should be filled out in Canvas. Please download this file, work on the
assignment, and then fill in the answers online. Express numerical answers to 1 decimal place, unless
otherwise indicated. Do not neglect to double-check your answers in Canvas before submitting the
assignment!
Part 1: Specific and Relative humidity [3 points]
Specific Humidity measures the vapour content of air by the mass of water vapour (grams) in a kilogram
of air. Figure 1 below represents the maximum specific humidity of air at different temperatures. It
shows that a kg of air can hold a maximum specific humidity of 26.5 g at 30 °C, 15 g at 20 °C, 7.5 at
10
°C and 4 g at 0 °C. If a kg of air at 30 °C has a specific humidity of 12 g, its relative humidity can be
calculated as follows:
12g / 26.5g = 0.453 x 100 = 45.3 %
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab
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2
Figure 1: Maximum specific humidity at different air temperatures (°C).
Q1. What is the relative humidity of an air parcel that is 30 °C and has a specific humidity of 15
g of water per kilogram air? [1]
Q2. What is the relative humidity of the same air parcel if it is cooled to 20 °C? [1]
Q3. What is another term that weather people would use to describe the air parcel in b, given its
relative humidity? [1]
PART 2: Adiabatic Processes [14 marks]
As a parcel of unsaturated (dry) air rises, it expands. The air molecules within it are spread over a wider
area, and this expansion process leads to the parcel cooling internally. The cooling brought about by
rising and expansion is an adiabatic process because there is no exchange of heat between the air parcel
(think of it as a “bubble” of air) and the surrounding air. If, on the other hand, a parcel of air sinks, it will
become compressed, occupying less volume. It warms as it sinks, and warming occurs at the same rate as
it cooled when rising. This is the Dry Adiabatic Lapse Rate (DALR) which is fixed at 10
°C/1000m).
If a rising air parcel is saturated (i.e., at 100% relative humidity) or if it reaches saturation at any point
during its rise, condensation must take place, as the air cannot cool (which it will do as it rises) and
continue to hold its vapour load. The condensation process releases latent heat into the parcel, reducing its
rate of cooling. The reduced rate is approximately 6 °C/1000 m of ascent and is called the Moist
Adiabatic Lapse Rate (MALR; sometimes called the Saturated Adiabatic Lapse Rate or SALR). The
environmental lapse rate (ELR) is the measured temperature change in the atmosphere above a surface. It
varies from place to place, and determines the temperature of the ambient atmosphere that an air parcel
will encounter at a given altitude.
You may wish to examine Figure 2 for to help visualize the journey of a rising air parcel with altitude.
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2
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Figure 2: An example diagram of air parcel being lifted adiabatically.
Stable Air [4 points]
An air parcel is considered to be stable when it tends to return to its original position in the atmosphere
after it has been set in vertical motion due to some sort of forcing mechanism, once that forcing has been
removed (i.e., it is an air parcel that, left alone, would tend not to move vertically). Air is stable when the
Environmental lapse rate (ELR) is less than the Dry Adiabatic Lapse rate (DALR). Note the ELR will
vary from place to place, while the DALR, the rate at which a parcel cools when lifted, is a constant for
dry (unsaturated) air. In the questions below, the “Steps” are simply for your own use in computing
answers. Further, be sure your numeric responses are mathematically supported: e.g., if the ambient
atmosphere started at a temperature of 20 °C, and the ELR was 9 °C/1000m, then if you ascend 2000 m,
you have lost 18 °C (2 x 9 °C) and reached an ambient temperature of 2 °C.
Step 1: Using graph A as a template, either print or save the figure 3 plot and annotate it. Draw a line
through point A on the chart of temperature vs. height to represent an ELR of 9 °C/1000m. Do not hand
in the graph – this is for your own use.
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2
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Graph A
Q4. If an air parcel is forced to rise from Point A, what would its rate of cooling be if it remains dry?
(You may assume that the parcel is the same temperature as the surrounding air at point A, 1000 m above
the ground.) [1]
Step 2: Draw a line on Graph A to show this rate of cooling with altitude.
Q5. Is the rising parcel of air cooler or warmer than the ambient atmosphere at the same altitude above
(not at) Point A? You can assume the parcel was at the ambient air temperature when it started out. [1]
Q6. What is the difference in temperature between the rising air parcel and the ambient atmosphere at an
altitude of 4000 m? [1]
Q7. If the impelling force were to be removed, would the air parcel tend to return to its original position
(See possible answers in Canvas; Multiple choice question) [1]
Unstable Air [5 points]
Unstable air is air that, once set in vertical motion, will tend to continue to rise in the atmosphere with
increasing velocity. Air is considered unstable when the ELR is greater than the DALR (i.e., ELR > 10
°C/1000m).
Step 1: Draw a line through Point A on Graph B to represent an ELR of 13°C/1000m. Do not hand in the
graph – this is for your reference only.
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2
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Graph B
A parcel of “dry” air at Point A (1000 m above the ground), the same temperature as surrounding air, is
forced upward.
Step 2: Draw a line on Graph B to represent the parcel’s rate of temperature change with height. Start the
line at point A, and extend upward to 5000m.
Q8. What will be the temperature of the rising air parcel at an altitude of 3000m? What will be the
ambient air temperature at this altitude? [2]
Q9. Assume that at 3000 m, the parcel reaches dew point (saturation). What will be its temperature at
4000 m? [1]
Q10. Will this rising parcel of air, once set in motion, continue to rise past this point (4000)? For how
long? Why/Why not? [Hint: Think back to our lesson on the different layers of the atmosphere. What
happens to the temperature profile at the top of the Troposphere?] [2]
Orographic lifting [5 points]
It is a spring day in southern B.C., with temperatures of 12 ºC. Westerly winds push air off the ocean
towards the coastal mountains, which are 1700 m high. The dew point temperature of the air mass is 6 ºC.
Determine what happens as the air is forced to rise by answering the questions below. A temperature-height
graph (Graph C) is provided at the end of the question in case you wish to work on the answers graphically.
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2
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Graph C
Q11. At what elevation does condensation begin on the western slope of the range? [1]
Q12. What will the temperature of the air layer be as it passes over the summit of the range (i.e. at 1700
m)? You may wish to explain your steps. [1]
Q13. What will the temperature of the air layer be after it has descended to the surface (i.e. at 0 m) over an
interior plateau? Assume that all condensed moisture fell as rain on the windward slope (i.e. the descending
air is now dry). [1]
Q14. Do you think it will be raining or snowing at the top of the Whistler-Blackcomb (at 1800 m elevation),
the ski area north of Vancouver? Be sure to explain your steps. [2]
PART 3: Calculating lapse rates [6] (Tang 2020)
Questions 15 is material from Tang, L, 2020, Lab 03: Atmospheric Structure and Pressure Systems In: Laboratory
Manual for Introduction to Physical Geography, First British Columbia Edition licensed under a Creative
Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.
In this part of the exercise, you will obtain temperature data and plot them on a graph to produce a
sounding (a vertical profile of the atmosphere, which represents atmospheric conditions). In canvas please
submit your sounding, with full annotations, as outlined in Q15.
Step 1: Go to the earth.null school website
(https://earth.nullschool.net/#current/wind/surface/level/overlay=temp/orthographic=-
123.81,49.18,3000/loc=-123.100,49.250), which provides an animated map of global weather conditions.
https://creativecommons.org/licenses/by-nc-sa/4.0/
https://creativecommons.org/licenses/by-nc-sa/4.0/
https://earth.nullschool.net/#current/wind/surface/level/overlay=temp/orthographic=-123.81,49.18,3000/loc=-123.100,49.250
https://earth.nullschool.net/#current/wind/surface/level/overlay=temp/orthographic=-123.81,49.18,3000/loc=-123.100,49.250
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2
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The green circle on the map is the approximate location of Vancouver (see below. At the bottom left of
the map, you will see the coordinates of Vancouver, the wind direction (expressed as azimuth), wind
speed, and air temperature at the surface of the location.
Step 2: Click on the label “earth” at the bottom left and the menu will expand. You have the option of
choosing many different types of data (e.g. wind, temperature, relative humidity, etc.), source (e.g. GFS,
NCEP, etc.), height (e.g. surface, 1000 hPa, etc.), and more. For the purpose of this exercise, we will only
focus on temperature data at different heights. For example, in the screenshot below, the temperature in
Vancouver at the surface is 15.2°C, on June 17th, 2020 at 17:00 local time.
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2
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Step 3: Obtain temperature data at the different elevations starting at 1000 hPa using the “Height” option
in the menu. Note: Hectopascals (hPa) are a unit of atmospheric pressure that decreases with height.
Record the data in an Excel spreadsheet. When you’re finished, your spreadsheet should like something
similar to this:
Step 4: Before you plot the data, reverse the columns so that temperature is the left column and pressure
is the right column. Then select all the data including the title. Use the “Insert” tab in the Excel window to
insert a Chart. Choose a scatter chart. A chart should appear on your spreadsheet. Hopefully, it will show
a straight line with pressure values on the vertical axis and temperature values on the horizontal axis. But
we want the pressure values to be the highest at the bottom, to correspond to the high pressures at the
bottom of the atmosphere. To do this, double click on the numbers in the vertical axis. This should cause
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2
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a “Format Axis” box to appear. In the Axis Options section, near the bottom, click on the check box for
“Values in reverse order”.
Step 5: To complete this graph, label the axes (with units), and add a figure caption below describing the
figure (e.g. Figure 1. Sounding in Vancouver, June 17, 2020 at 1700 PDT) . The resulting graph shows
the vertical temperature variation of the lower atmosphere, also known as a sounding.
Step 6: Using the sounding you plotted in the previous exercise, label where the troposphere, tropopause,
and stratosphere are located. You can add textboxes on the sounding for labelling purposes. Use arrows to
show the extent of the troposphere and stratosphere.
Step 7: Using the data collected, we can calculate the lapse rate of the troposphere (the ELR). To do that,
first take the difference in temperature between the surface (or 1000 hPa) and the tropopause. For
simplicity sake, we assume the tropopause to be at 12 km above sea level (in actuality, the height of the
tropopause varies with latitude and time of year). We then divide the difference in temperature by the
height of the tropopause. For example:
15.2 𝐶 − (−55.4 𝐶)
12 𝑘𝑚
= 5.88 𝐶/𝑘𝑚
The lapse rate is therefore 5.88°C/km, meaning the temperature drops by about 6°C for each kilometre we
go up in the troposphere.
You should show your calculation and answer next to the sounding (use a textbox, for example).
Q15. Upload your sounding to canvas annotated with layers of the atmosphere (step 6) and the ELR of
the troposphere (step 7) [6]
PART 4: Synoptic Weather Maps [5 marks]
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2
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Background: While solar radiation provides the energy for all Earth systems, it is not received evenly on
Earth’s surface. Earth’s atmospheric and ocean circulation systems redistribute energy and heat from areas
of surplus to areas of deficit, producing global weather patterns. To forecast weather, data from many
stations must be synthesized and mapped. Synoptic weather maps provide a ‘synopsis’ of current
conditions. The word synoptic means “view together” or “view at a common point”. The size of weather
patterns on synoptic maps range from about 1,000 to 2,500 km across. When different parameters of the
earth’s atmosphere are viewed together at the synoptic scale then large-scale weather patterns emerge,
such as cyclones and their associated fronts.
Synoptic maps combine information taken simultaneously several times a day from observing stations,
ships and buoys. In Canada, there are some 2,800 such stations. Weather information at a particular site is
transmitted to the Meteorological Service of Canada, a division of Environment Canada, where computers
analyze and plot the data for that particular station onto station models. A station model uses a standard
layout and set of symbols to describe weather conditions at a particular location (See
Appendix
B, inset
on ‘Station model’ for a simplified example). Given the large number of stations collecting data, only a
small number are ever recorded on a map.
Pressure Systems and Their Relationships With Wind
Question 16 is material from Tang, L, 2020, Lab 03: Atmospheric Structure and Pressure Systems In: Laboratory
Manual for Introduction to Physical Geography, First British Columbia Edition licensed under a Creative
Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.
Atmospheric pressure is the weight of air exerted on a surface, whereas wind is a direct result of the
difference in atmospheric pressure (aka pressure gradient) between to places. As such, if we know the
spatial distribution of pressure systems over a certain area, we can identify the wind patterns in that same
area.
Another common method to look at pressure systems is to look at an actual weather map. The following
is a synoptic weather map produced by Environment Canada. You will notice the isobars drawn on the
map, as well as the areas of high pressure (“H”) and low pressure (“L”).
Q16 (Tang 2020). Below is a synoptic map (Fig. 3) accessed via
https://weather.gc.ca/data/analysis/jac18_100.gif. You may wish to follow the link to zoom in on the map
and get a clearer image. This map represents conditions at 11am PST on the 10th of August. You may
notice that the time on the maps says 18Z, which is referring to time in ZULU/UTC (f.k.a. Greenwich
mean time in Britain). This is 11am local time (if you are on the west coast of Canada!).
Save the map below (or from the website above) and draw arrows on the map (Fig. 3) to indicate the wind
directions based on where the high and low pressures are located. You should draw 3-4 arrows for each
pressure system. Upload this to canvas. [2]
https://creativecommons.org/licenses/by-nc-sa/4.0/
https://creativecommons.org/licenses/by-nc-sa/4.0/
https://weather.gc.ca/data/analysis/jac18_100.gif
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2
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Fig 3: Synoptic weather map, 10th August 2020. Source: Environment Canada
Q17. Observe the surface weather map (Fig. 4) of the United States below from the 4th of January 2018
(accessed from https://www.wpc.ncep.noaa.gov/dailywxmap/index_20180104.html). Take note of the
low pressure system on the east coast. What kind of air masses are shown and where are they likely to
have originated from? [2]
Q18. What is the name of this weather system? [1]
https://www.wpc.ncep.noaa.gov/dailywxmap/index_20180104.html
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2
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Fig 4: Synoptic weather map, 4th January, 2018. Source: NOAA.
References:
Tang, L, 2020, Lab 03: Atmospheric Structure and Pressure Systems In: Laboratory Manual for
Introduction to Physical Geography, First British Columbia Edition licensed under a Creative Commons
Attribution-NonCommercial-ShareAlike 4.0 International License
Appendix
https://creativecommons.org/licenses/by-nc-sa/4.0/
https://creativecommons.org/licenses/by-nc-sa/4.0/
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2
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As an air parcel is lifted, it may cool to its dew point temperature, at which point the water vapour within
the parcel will condense. If the air parcel continues to rise, condensation will continue to occur, resulting
in cloud formation and perhaps precipitation. Likewise, if rising air never cools to its dew point, no
condensation will take place and no clouds will form.
To determine vertical atmospheric stability, meteorologists use parcel theory, which describes the
behavior of an idealized air parcel as it travels through the atmosphere. Parcel theory requires the use of
lapse rates describing the average decrease in temperature with height through the atmosphere:
● When air is unsaturated (dry), temperature decreases at a rate of 10 ºC/1000m (or 1 ºC /100m).
This is the Dry Adiabatic Lapse Rate (DALR).
● When air is saturated, the temperature decreases at a rate of 6 ºC/1000 m (or 0.6 ºC /100m). This
is the Moist Adiabatic Lapse Rate (MALR) (sometimes called the Saturated Adiabatic Lapse
Rate, SALR)
● The actual temperature profile of the atmosphere, measured by weather balloons, is the
Environmental Lapse Rate (ELR). ELR varies in both time and space.
To use parcel theory:
1. Plot the ELR on a height vs. temperature graph, given a particular starting temperature. If the starting
temperature you are given is at some altitude (e.g., point A on Graph A), simply extend the line down
through it to the ground level, assuming the same rate of temperature change above and below that point.
2. Plot the DALR or SALR, depending on whether the atmosphere is unsaturated or saturated.
2a. If the atmosphere is unsaturated, the DALR extends to a height at which the temperature
reaches the dew point. Beyond this height the atmosphere becomes saturated and the MALR
should then be drawn (extending from the DALR).
2b. The DALR/MALR is the “temperature-height” line along which the idealized air parcel will
move.
3. At every height, compare the temperature of the parcel (Tparcel) to the temperature of the
environment (Tenv):
3a. If Tparcel > Tenv, the air is unstable and the parcel will rise. If/when the parcel continues to
rise along the MALR, clouds will form, and will continue to form to the height where the parcel
no longer rises.
3b. If Tparcel < Tenv, the air is stable and the parcel will sink/not rise.
Appendix B: Example of a synoptic map showing weather station models and what values represent, as
well as a cross-section of the weather experienced along a transect running through the weather system at
A-A’.
GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2
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- Appendix A: Adiabatic Cooling and Heating