Atomspheric Stability

Atmospheric Stability
• The resistance of the atmosphere to
vertical motion.
• Stable air resists vertical motion
• Unstable air encourages vertical motion
↓ pressure
↑ volume
= cooling
↑ pressure
↓ volume
= warming
Dry adiabatic lapse rate
(unsaturated air)
• Lapse rate – change in temperature
with elevation
• Adiabatic process: - No heat is gained
or lost by mixing with the surrounding
air (“parcels of air”)
• Dry adiabatic lapse rate = 5.5°F/1000
Moist-adiabatic Rate
(saturated air)
• Air rises and cools – if dew point reached,
condensation occurs
• Energy released with condensation → warming
• Lapse rate reduced to 3°F/1,000 ft
• Parcel becomes warmer than surrounding air –
continues to rise (free convection)
• Eventually reaches colder air aloft – cools and
stabilizes at new level
SOUNDING – Atmospheric Temperature
Moist adiabatic lapse rate
- May become unstable if air becomes
warmer than surrounding area and
continues to rise
Dry adiabatic lapse rate (stable)
- resists vertical motion
- parcel will remain in position
Super adiabatic lapse rate
- Lapse rate > dry-adiabatic rate
- Example: Strong heating of ground surface
- Warm air below cool air
How is air lifted in the atmosphere?
• Thermal lifting (convection)
• Orographic lifing
• Frontal lifting
What is the stability of the air?
What is the stability of the air?
• Read the material independently (2 min)
• One person explains the material – the other
person listens (3 min)
• The “listener” adds to the explanation any
information that they believe would help to
improve their understanding of it. (2 min)
• Both people decide on the best explanation that
they both agree with. (2 min)
• Both should be prepared to share the
information with the entire class.
Night Inversion
Night inversion
• Air near Earth’s surface cools at night
(radiation, conduction, convection)
• Cold air flows downward
• Layer of warm air above = thermal belt
• Inversions strongest when atmospheric
conditions = stable, calm
– Clouds & water vapor reduce outgoing radiation
– Wind causes air layers to mix
• Surface heating during day – cause inversion
to break = severe fire conditions
Marine inversion
Subsidence Inversion
Subsidence & mountain waves
(& foehn winds)
Signs of a Stable Atmosphere
Signs of an Unstable Atmosphere
Night Inversion
Marine inversion
Subsidence & mountain waves (& foehn winds)
Signs of a Stable Atmosphere
Signs of an Unstable Atmosphere