OEAS-220T_Lab 5_Stability_F2023-2
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OEAS 220T – Introduction to Meteorology – Fall 2023
Lab/Recitation Assignment 5: Atmospheric Stability
Assignment Due: Friday, October 20, 2023 (note:
late work will not be accepted
)
Part 1 - Air Mass Ascent
In the figure below, suppose a parcel of air with a temperature of 80°F and a dew point
temperature of 71°F is lifted up and over a 6000-foot high mountain.
The dry adiabatic rate is
5.5°F per 1000 ft
, and the cooling rate for dew point temperature is around
l°F per 1000 ft
).
a).
In the table below, calculate the air temperature and dew point temperature of the rising air
parcel
, until you reach the point of condensation
, then stop there (use what you know about air
temperature and dew point temperature to decide when this point is reached).
Also keep in mind
that the
dew point can
never be higher than the air temperature
.
Therefore, if you calculate an
air temperature of 66°F and a dew point of 68°F, the dew point must actually be at 66°F.
Height (ft)
Temperature (°F)
Dew point (°F)
0
80
71
1000
2000
3000
4000
5000
6000
b).
At what height would condensation begin and clouds start to form (= condensation level)?
c).
What is the air temperature and dew-point temperature at this condensation level?
1d).
At the condensation level, a cloud forms and the parcel continues its upward journey.
The
air temperature and the dew point temperature now both decrease at the same rate, called the
moist adiabatic rate (assume a moist adiabatic rate of
3.0°F per 1000 ft
).
Continue filling in the
table above with air temperature and dew point temperature until you reach a height of 6000 ft.
Part 2 - Air Mass Descent
a).
Suppose that clouds build up in the air parcel until it reaches the top of the mountain, and that
the air mass then loses some moisture as rain: it is now
unsaturated
.
Use the dry adiabatic
warming rate of
5.5°F per 1000 ft
for air temperature and a
1°F per 1000 ft
increase for dew
point temperature for the descending air parcel.
Calculate the temperature of the air parcel as it
descends the mountain.
Remember that the air parcel is now descending, so it will be
warming,
not
cooling, and also dew point temperature will be increasing.
Height (ft)
Temperature (°F)
Dew point (°F)
6000
5000
4000
3000
2000
1000
0
b).
How much warmer is the sinking air on the eastern (right) side of the mountain at 0 feet of
elevation compared to the air when it was at the same level (0 feet) on the western (left) side of
the mountain?
c).
Why is the air warmer at ground level on the eastern side compared to the western side?
(hint: consider the different rates of cooling/warming of moist versus dry air)
d).
Why is the dew point temperature at 0 feet on the eastern side lower than at 0 feet on the
western side? (hint: consider how much water vapor is in the air parcel on either side)
e).
If the environmental lapse rate of the surrounding air mass is
3.6°F per 1000 ft
, is the
atmosphere absolutely stable, absolutely unstable, or conditionally unstable, and why?
(hint: refer to pages 13-16 in the Lecture 9 summary, and compare the environmental lapse rate
to the dry and moist adiabatic rates)
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