Mountain Cloud Activity
Use the Iowa State University simulation at the link below to help you
work through this activity.
http://www.iastate.edu/~abc/java/mtnsim/mtnsim.html
Part 1 Objective: Learn what happens to air temperature, vapor
pressure, and dew point temperature as an air parcel is force up and
over a mountain range.
This animation assumes wind is blowing from left to right over the
mountain. Parcel Temperature is the air temperature at the base of the
mountain before it ascends. To change dew point temperature you must use
the slider for Water Vapor Pressure- you will see values for dew point
temperature change as you move the Water Vapor Pressure slider. Adjust the
wind speed to be fairly slow so you can watch how the values in the black
boxes below the graphs vary. Try varying Parcel Temperature and Vapor
Pressure to learn how the animation works. You must click on New Trial
after each run. This resets values in the boxes. Click on Clear Graphs after
each run so you can better see your results. Slope is Lapse Rate. When you
get a cloud to form, pay close attention to the change in slope (Lapse Rate)you will see the difference between Dry Adiabatic Lapse Rate (shown as a
slope of -10 C/km) and Moist Adiabatic Lapse Rate (slope of -6 C/km).
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When you are comfortable using the animation you will probably want to
speed up the wind speed.
Part 1 Questions
1. Starting with a Parcel Temperature of 25 C, determine the dew point
temperature by trial and error at which a cloud will form at 1 km - 1.1
km (i.e., the altitude of cloud base is between 1 km and 1.1 km). What
is the starting dew point temperature needed?
__________ degrees C
2. What is the slope (Lapse Rate in degrees C/km) as the air parcel rises
up the windward side of the mountain before a cloud forms?
________________ degrees C/km
(Rerun the simulation if you didn’t catch it the first time.)
3. What is the slope (Lapse Rate in degrees C/km) as the air parcel rises
up the windward side of the mountain after a cloud forms?
4. ________________ degrees C/km
(Rerun the simulation if you didn’t catch it the first time.)
5. Which slope value above represents the Dry Adiabatic Lapse Rate?
_________
6. Which slope value above represents the Moist Adiabatic Lapse Rate?
_________
7. What is the slope (Lapse Rate in degrees C/km) as the air parcel
descends down the leeward side of the mountain back to the ground?
________________ degrees C/km
(Rerun the simulation if you didn’t catch it the first time.)
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8. What is the final Parcel Temperature at the base of the mountain at the
end of the trip over the mountain?
_________ degrees C
9. What is the final dew point temperature at the base of the mountain at
the end of the trip over the mountain?
_________ degrees C
10.Is the air warmer or cooler after it completes its trip over the
mountain? Why?
_____________________
11.What is the source of heat energy that resulted in the observed
temperature change?
______________________
12.Does the air have more or less moisture in it after it completes its trip
over the mountain?
______________________
13.Where does this moisture go to or come from?
_______________________________________________________
14.Now again use a starting Parcel Temperature of 25 degrees C but this
time determine the dew point temperature that results in a cloud
forming at 1.5 km. What is the dew point temperature?
______________ degrees C
15.Using your answers from questions 1 and 15, compute the difference
between starting air temperature (referred to in this exercise as Parcel
Temperature) and dew point temperature for each trial, and enter them
below:
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Question 1: Starting TAIR – starting TDEW POINT = _____ C
Cloud formed at 1 km
Question 2: Starting TAIR – starting TDEW POINT = _____ C
Cloud formed at 1.5 km
Judging from the results above, does a larger difference between air
temperature and dew point temperature result in higher or lower cloud
bases?
___________________________
Part 2 Objective: Learn how to determine where a cloud will
form.
Using the simulator, for each initial temperature in the table below
adjust the dew point temperature of the air so that a cloud will
form at 1 km (i.e., the altitude of cloud base is about1 km).
Complete the table below. Remember you control dew point
temperature using the Water Vapor Pressure slider.
Part 2 Questions
1. Determine Dew Point Temperatures required to form a cloud
at1.0 km for given Air Temperatures (ie Parcel
Temperatures) in the table below:
Air Temperature
(in degrees C)
30
25
20
15
10
Dew Point Temperature
(in degrees C)
4
2. Look carefully in the table above at the relationship between
air temperature and dew point temperature that resulted in
clouds forming at 1 km. Describe the relationship between air
temperature and dew point temperature that results in a cloud
base of 1.0 km:
For a cloud to form at 1 km, TAIR – TDEW POINT = _____ degrees C
3. What air temperature will give a cloud base of 1.0 km when
the dew point temperature is 4 C?
________ degrees C
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