Unstable Atmosphere:
- Air forced upwards will continue to rise
- As air rises, Temperature decreases and RH increases
if RH=100%
=> condensation and cloud formation
=> latent heat released
=> possibility of storm formation
Clicker Question
Set Frequency to "BB"
Which type of clouds are more likely to form in unstable conditions?
(A) stratus clouds (layered)
(B) cumulus clouds (vertically developed)
(C) fog (cloud in contact with surface)
1. WHAT CAUSES ATMOSPHERE TO BECOME
MORE/LESS UNSTABLE?
2. WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE?
1. WHAT CAUSES ATMOSPHERE TO BECOME
MORE/LESS UNSTABLE?
ONE WAY IS THROUGH MIXING =>
Rising Air Cools
Sinking Air Warms Height
initial profile of
environment air
expands
and cools
compresses
and warms
Temperature
Height
Conditionally
Unstable
ΓM
(6°C/km)
ΓDRY
(10°C/km)
Stable
Unstable
Temperature
1. WHAT CAUSES ATMOSPHERE TO BECOME
MORE/LESS UNSTABLE?
ONE WAY IS THROUGH MIXING =>
Rising Air Cools
Sinking Air Warms Height
initial profile of
environment air
expands
and cools
compresses
and warms
Temperature
1. WHAT CAUSES ATMOSPHERE TO BECOME
MORE/LESS UNSTABLE?
ONE WAY IS THROUGH MIXING =>
Rising Air Cools
Sinking Air Warms Height
initial profile of
environment air
final profile of environment air
expands
and cools
compresses
and warms
Temperature
Another way the atmosphere can become more unstable:
The large-scale lifting an entire layer of air
- here a large-scale layer of air is much larger than a parcel
- layer may be 100's of kilometers across
- the lifting of a layer typically happens with a low pressure system (more later in course)
Height
1000 m
Temperature
- Original layer (in blue) is very stable
Height
1500 m
dry adiabatic lapse rate (10C/km)
1000 m
Temperature
- Original layer (in blue) is very stable
- As layer rises, it expands
- As air rises => cools at dry adiabatic lapse rate (10°C/km) (assume no condensation)
- Top of layer rises 500 meters more than bottom of layer
=> Top of layer cools more than bottom of layer
- Result => environment lapse rate within layer more likely unstable
This occurs when low pressure over area. Opposite happens with high pressure system.
Low Pressure => Rising Air => Increasing INSTABILITY
High Pressure => Sinking Air => Increasing STABILITY
Height
SPECIAL CASE: CONVECTIVE INSTABILITY
1500 m
dry adiabatic lapse rate (10C/km)
RH << 100%
moist adiabatic lapse rate (6C/km)
RH=100%
1000 m
Temperature
- Bottom of layer moist (RH=100%), top of layer dry (RH<<100%)
- Bottom of layer cools at moist adiabatic lapse rate (6°C/km)
- Top of layer cools at dry adiabatic lapse rate (10°C/km)
=> Now even more unstable than before!!
CONVECTIVE INSTABILITY
2. WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE?
2.  WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE?
A. Surface heating leading to Convection
2.  WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE?
A.  Surface heating leading to Convection
B.  Topography (Mountains) = Orographic Uplift
2.  WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE?
A.  Surface heating leading to Convection
B.  Topography (Mountains) = Orographic Uplift
C.  Ascent due to Convergence
2.  WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE?
A.  Surface heating leading to Convection
B.  Topography (Mountains) = Orographic Uplift
C.  Ascent due to Convergence
D.  Uplift along Weather Fronts
2.  WHAT CAUSES AN AIR PARCEL TO INITIALLY RISE?
A.  Surface heating leading to Convection
B.  Topography (Mountains) = Orographic Uplift
C.  Ascent due to Convergence
D.  Uplift along Weather Fronts
Set Frequency to "AB"
Clicker Question
Wind
Windward Side
Mountain
Leeward Side
Which side of the mountain would generally receive
more precipitation?
(A) Windward
(B) Leeward
(C) Both sides would receive equal amounts
Prevailing
Wind
Prevailing
Wind
Collision - Coalescence Process (Warm process)
Clicker Question
Set Frequency to "AB"
In general, which cloud would you expect to produce
larger rain drops through the collision-coalescence process?
(A) cumulus cloud (B) stratus cloud
(C) cirrus cloud