Atmospheric Stability (weather and climate)

stable air parcel

Resists upward displacement or when disturbed tends to return to starting place

Stable air tends to ___ movement

resist

unstable air tends to ___ and ___ movement

rise, encourage

unstable air parcel 

continues to rise until it reaches an altitude where it can no longer rise 

What does atmospheric stability depend on?

density of the air

Moist air is

less dense than dry air

Adiabatic process

as an air parcel rises it expands and cools, as it sinks it compresses and heats up. *no heat exchange with the surrounding environment 

What causes an air parcel to rise?

convection, convergence, frontal, and orographic uplift

Convergent lift

results when air flows toward an area of low pressure

Convectional lift

when air is stimulated by local surface hating and rises

Orographic uplift

occurs when air is force over a barrier like a mountain range 

Windward side

side facing the wind that gets the rain

Leeward side

side facing away from the wind, very dry rain-shadowed area

Rain shadow

leeward side of the mountain that receives anomalously low precipitation because of its motion relative to mountains

Frontal lifting 

occurs as air is displaced upward along the leading edges of contrasting air masses 

Lapse rate

rate at which atmospheric temperature decreases with altitude 

Environmental lapse rate (ELR)

actual lapse rate at a particular time and place

Dry adiabatic lapse rate (DALR)

Rate at which the temperature of a dry (unsaturated) air parcel changes as it rises or sinks

As a parcel rises…

Volume expands and temperature falls 

Moist adiabatic lapse rate (MALR)

rate at which temperature of a moist (saturated) air parcel changes as it rises or sinks

Why is the MALR slower that the DALR?

latent heat release: condensation releases eat and partially offsets cooling. This varies because of the amount of condensation.

More condensation =

More latent heat release (LHR) and smaller MALR

Lifting condensation level (LCL)

altitude at which a dry air parcel cools to its dew point temperature, achieve saturation, and undergo condensation.

Relationship of the DALR and MALR to the ELR at any given time and place determines what?

The stability of the atmosphere over an area.

Temperature relationships in the lower atmosphere produce three condtions:

Unstable, conditionally unstable, and stable 

Absolute stability (DALR>ELR)

air parcel has a lower temperature that the surrounding environment. Air parcel cools faster than surrounding environment and tends to resist lifting unless forced, if forcing stops, air parcel sinks.

Absolutely unstable (DALR<ELR)

ELR is greater than the DALR. Air parcel cools slower than surrounding environment and is warmer than environment. Needed for thunderstorms. 

Conditionally unstable (DALR>ELR>MALR>

lower part of the atmosphere is stable, upper part it unstable. Air parcel goes from being cooler to warmer. Clouds form when air is unstable. ELR in between DALR and MALR the atmosphere is neither unstable nor stable. If air parcel is unsaturated it will resist upward movement unless forced. If it becomes saturated at the LCL it will then be unstable and rise.