GEOB

GEOB 102 Our Changing Environment: Climate and Ecosystems Lab 2

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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: 28

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 2

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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.

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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.

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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.

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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.

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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.

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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.

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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

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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]

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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]

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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]

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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

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Appendix A: Adiabatic Cooling and Heating

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’.

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