ATS 351, Spring 2010
Lab #6
Stability & Skew-T – 48 points
1. (5 points) What is an adiabatic process? Why are the moist and dry adiabatic
rates of cooling different?
An adiabatic process is a process that takes place without a transfer of heat
between the system (such as an air parcel) and its surroundings.
The dry adiabatic lapse rate is about 10C/km. When an air parcel becomes
saturated, condensation occurs inside the parcel and latent heat is released.
Therefore, this heat release balances some of the cooling by expansion and the
parcel decreases temperature at a lesser rate. This rate is the moist adiabatic lapse
rate, which is about 6C/km.
2. (5 points) If the atmosphere is conditionally unstable, what condition is necessary
to bring on instability?
If unsaturated air could be lifted to a level where it becomes saturated,
instability would result.
Conditional instability means that the environmental lapse rate is between
the moist adiabatic lapse rate and dry adiabatic lapse rate. Therefore, an
unsaturated parcel will follow the dry adiabatic lapse rate and will be colder than
the environment. However, if that parcel becomes saturated, the parcel will now
decrease in temperature at a slower rate than the environment and will be warmer
= unstable.
3. (5 points) Explain the significance of the lifting condensation level. What is it
and how is it determined from a Skew-T diagram?
The lifting condensation level is the level at which a parcel of air, when
lifted dry adiabatically, would become saturated. Above the LCL, the parcel
cools at the moist adiabatic lapse rate.
The LCL is found on the skew T by:
1) Follow the saturation mixing ratio from the dew point (follow
line to lower pressures = up)
2) Following the dry adiabat line from the surface temperature
(follow line to lower pressures = up)
3) The point where these 2 lines cross is the LCL.
Using the skew-T below, please answer questions 4-9:
(3 points each)
4. What was the surface temperature in Birmingham, Alabama?
19 degrees Celsius
5. What was the surface dewpoint in Birmingham, Alabama?
10 degrees Celsius
6. What is the lifting condensation level (LCL) for this sounding?
850 mb
7. What kind of stability is present in the layer from 700 mb to 650 mb?
(absolutely unstable, absolutely stable, or neutral)
Absolutely stable
8. An air parcel following a moist adiabat from 850 mb to 750 mb would
experience what kind of stability? (unstable, stable, or neutral)
Unstable – parcel is in a conditionally unstable environment and since it is
saturated, parcel is unstable.
9. In what layer would clouds be located in the sounding? (give a range of
pressures)
850 – 700 mb
10. (15 points) Plot the following temperatures and dew point temperatures on
the Skew-T diagram provided. Do not forget to connect the points for each
profile.
The skew-T is constructed as follows:
 Pressure is represented by the horizontal blue lines. (mb)
 Temperature is represented by the diagonal red lines (C)
 Dry adiabats are the solid green lines
 Moist adiabats are the dashed green lines
 Mixing ratios are the diagonal dashed magenta line (g/kg)
Pressure [mb]
Temperature [ºC]
Dew Point Temp.
[ºC]
1000
24
17
950
22
16
900
19
14
850
15
10
800
10
8
750
6
4
700
2
-2
650
-2
-6
600
-4
-9
550
-8
-18
500
-11
-15
450
-14
-19
400
-18
-26
(1 point each) If an air parcel were to rise from the 1000 mb level:
a. What is the pressure of the Lifting Condensation Level?
About 900 mb
b. What is the temperature of the Lifting Condensation Level?
About 16C
c. What would be the temperature of the parcel if it rose to the 700 mb level?
About 6C
d. What would be the temperature of the parcel if it rose to the 500 mb level?
About -10C
e. What is the pressure at the Level of Free Convection?
About 825 mb
f. What is the pressure at the Equilibrium Level?
About 475 mb
g. Shade in the area of the Convective Inhibition (CIN) in blue.
On the scanned skew-T diagram
h. Shade in the area of the Convective Available Potential Energy (CAPE) in red.
On the scanned skew-T diagram
i. What is the lapse rate between 1000 and 900 mb (approximately)? Is this lapse
rate stable or unstable with respect to the dry adiabatic lapse rate?
T(1000)-T(900) = 24C-19C =
distance
~3000 ft.
STABLE.
5C * 3280 ft ~ 5.5C/km
3000ft
1 km