Microsoft Word - GEOG1306_Lab5_MoistureAtmosphericStability.docx
Name: Date:
Moisture and Atmospheric Stability
Lab Questions
***Please submit only this document at the end of the lab period***
INSTRUCTIONS: Please read the lab reader before beginning this exercise. The following lab questions will be based upon the material given in the lab reader. The course textbook and lecture notes will serve as useful resources for this exercise.
Part 1: Atmospheric Moisture
From the reading, we know that relative humidity is measured as:
We also know what the saturation vapor pressure (in millibars) is for every temperature (in degrees Celsius). Values of saturation vapor pressure are shown for a range of temperatures on the following tables:
Temperature (C)
Saturation Vapo
Pressure (mb)
Temperature (C)
Saturation Vapo
Pressure (mb)
40
73.8
10
12.3
35
56.2
5
8.7
30
42.2
0
6.1
25
31.7
-5
4.0
20
23.4
-10
2.6
15
17.0
-15
1.7
(
PHYSICAL GEOGRAPHY
LABORATORY
)
(
1
)
1. [10] The reading explained that relative humidity will change throughout the day even if the actual amount of water vapor in the air does not change. Let’s verify this with our equation and tables above. Calculate the relative humidity for every hour of an example summer day in Texas. Remember that the actual amount of water vapor is measured by vapor pressure. Complete the following table with your calculations.
Time
Temperature (C)
Vapor Pressure (mb)
Saturation Vapor Pressure
(mb)
Relative Humidity (%)
12:00 AM
(midnight)
27
23
3:00 AM
25
23
6:00 AM
20
23
9:00 AM
24
23
12:00 PM
(noon)
27
23
3:00 PM
36
23
6:00 PM
34
23
9:00 PM
31
23
2. [2.5] With a constant vapor pressure, describe how relative humidity changes throughout the day. When is it highest and lowest?
3. [2.5] Describe the relationship between temperature and saturation vapor pressure.
4. [2.5] What physical process is occu
ing at 6 AM? How do you know? (Use complete sentences.)
(
PHYSICAL GEOGRAPHY
LABORATORY
)
(
3
)
5. [2.5] Based on your understanding of relative humidity, vapor pressure, and saturation vapor pressure, when is it more likely to have large amounts of water vapor in the air: during a Texas winter or during a Texas summer? Why?
6. [2.5] If the average air temperature across Texas increases over the next decade (due to global warming) what can you say about the likely changes in water vapor in the atmosphere? Why?
Part 2: Adiabatic Processes
To calculate how an air parcel will change its temperature with height, you need to know whether the parcel is unsaturated or saturated, whether it is rising or sinking, and how far it moves. Here, we will calculate the changes of temperature in an air parcel as a result of adiabatic heating or cooling.
7. [2.5] What does it mean for an air parcel to be unsaturated? And to be saturated?
8. [2.5] What is the value of the lapse rate of an unsaturated air parcel as it moves up or down in the atmosphere?
9. [2.5] What is the value of the lapse rate of an saturated air parcel as it moves up or down in the atmosphere?
10. [15] Complete the following tables by computing the temperature of the air parcel by adiabatic processes.
Elevation (km)
Temperature of Unsaturated
Parcel (C)
Temperature of Saturated
Parcel (C)
0
25
25
1
2
3
4
5
6
(
PHYSICAL GEOGRAPHY
LABORATORY
)
(
10
)
Elevation (km)
Temperature of Unsaturated Parcel (C)
0
1
2
3
4
5
6
-20
Part 3: Cloud Formation from Orographic Lifting
On the following page, you will find a diagram illustrating air being forced to rise over a mountain and sink on the other side. Around an elevation of 1 km, condensation and cloud formation begins. By 2 km, precipitation occurs. Precipitation continues until the air peaks over the mountain top and begins to descend.
11. [2.5] At what height does the air parcel first reach saturation?
12. [5] The temperature at sea level on the windward side of the mountain is 21°C. Calculate the temperature of the air parcel as it is forced up the mountain. Write the temperature on the left side of each blank line on windward side of the diagram. Make sure you are using the co
ect lapse rate for each calculation. [You will eventually include the dew point on each line also, so make sure you have room to write something like “10°C , 4°C”, with the first value representing the parcel temperature and the second representing its dew point.]
13. [2.5] At what height did the air parcel cool to its dew point (i.e., what is the lifted condensation level)? At what temperature did the air parcel cool to its dew point?
14. [5] Complete the windward side of the diagram by calculating the dew point at each height. Write the dew point on the right side of the blank line. Separate the temperature and dew point by a comma (e.g., “10°C , 4°C”). Assume that below the lifted condensation level, the dew point is equal to the value at the lifted condensation level. Assume that above the lifted condensation level, the dew point is equal to the temperature when the air parcel is still in a cloud.
15. [2.5] Describe how the dew point has changed from the bottom to the top of the mountain on the windward side. What does that change mean for the amount of water vapor in the air? Where did the water go? (Use complete sentences.)
16. [5] Calculate the temperature of the air parcel as it is forced down the mountain. Write the temperature on the left side of each blank line on leeward side of the diagram. Make sure you are using the co
ect lapse rate for each calculation.
17. [2.5] What lapse rate did you use for your calculations on the leeward side of the mountain? Why?
18. [5] Complete the leeward side of the diagram by calculating the dew point at each height. Write the dew point on the right side of the blank line. Separate the temperature and dew point by a comma (e.g., “10°C , 4°C”). Assume that the amount of water vapor in the air does not change where there is no precipitation.
19. [2.5] What is the starting temperature and dew point at sea level before the air is forced over the mountain? What is the final temperature and dew point at sea level after the air is forced over the mountain? (Include co
ect units.)
Starting temperature: Starting dew point:
Final temperature: Final dew point:
20. [2.5] Using the values you calculated, describe how the air was changed as it was forced across this mountain range. (Use complete sentences.)
Part 4: Atmospheric Stability and Convection
In part 3, we looked at how clouds formed by forced lifting –– in this case, orographic lifting. In this section, we will examine how clouds can form by convection in an unstable or convectively unstable atmosphere. For this lab, we will assume that the environmental lapse rate (ELR) on the day we examine is 8°C in El Paso, TX.
21. [5] First, let’s calculate the temperature of the environment at different heights above El Paso. Use the environmental lapse rate to complete the following table:
Elevation (km)
Environmental (air) Temperature (C)
0
30
1
22
2
3
4
5
6
22. [2.5] Plot these temperature and elevation values on the graph below, connect the points by a line, and label the line “Environmental Lapse Rate”.
23. [7.5] Next, similar to what you did with the calculations on the windward side of the mountain, you will calculate and write the air temperature of an air parcel that is affected by adiabatic processes as it rises in the atmosphere. Assume that the air parcel begins unsaturated at the ground and then becomes saturated at 2 km in height. Recall the dry and moist adiabatic lapse rates from questions #8 and 9. In the last column, note whether you used the dry adiabatic lapse rate (DAR) or the moist adiabatic lapse rate (MAR) to calculate the change in temperature from one height to the next.
Elevation km
Parcel Temperature
°C
Rate (DAR or MAR)
0
30
1
20
DAR
2
3
4
5
6
24. [2.5] Plot these temperature and elevation values on the graph above, connect the points by line segments, and label the segments of the line “DAR” or “MAR”, as appropriate.
25. [2.5] Look at the two lines that represent the change in temperature of the air parcel with height and the change in the temperature of its su
ounding environment. Use what you learned in the reading to label the stable layer as “stable” and the unstable layer as “unstable”. Make sure it is clear what the depth of each layer is