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Vertical motion of air is

an important driver of

weather. Sometimes rising

air is made visible by the

development of clouds or

by the rising dust in dust

devils. Violent vertical motion

can be seen in tornadoes.

At other times rising air

may occur in the absence of

any visual clue. Subsiding

air is normally relatively

gentle and associated with

clear conditions (except in

association with mountain

waves and downburstactivity from convective

clouds).

Vertical motion in the atmosphere is

largely responsible or turbulence and

cloud ormation.

The strength o vertical motion is

mostly determined by the vertical

stability o the atmosphere. A stable

atmosphere will tend to resist vertical

motion, while an unstable atmosphere

will assist it. When the atmosphere

neither resists nor assists vertical

motion it is said to have neutralstability.

Adiabatic Processes and

Lapse Rates

To explain the stability or instability o

the atmosphere, it is useul to consider

what happens to an imaginary parcel

o air displaced vertically rom one

level to another. The parcel will expand

when it moves to lower pressure

(higher altitude) and contract when

it moves to higher pressure (lower

altitude).

During displacement it is assumed

the parcel undergoes an adiabatic

temperature change, i.e. no heat rom

the external environment is added

or subtracted. Adiabatic heating is

demonstrated when using a bicycle

pump. Compression heats the air and

thus the outer casing o the pump. The

reverse occurs when air escapes rom

a tyre. It cools due to rapid expansion.

Accurate measurements o heating

or cooling during these processes

are difcult because some mixing

normally occurs with air outside the

parcel, and heat may also be lost or

gained through radiation. However,

assuming adiabatic processes is useul

in explaining how the atmosphere

behaves.

The rate o change o temperature with

height or a vertically displaced parcel

o air is termed the adiabatic lapse

rate. Two dierent rates apply, the dry

adiabatic lapse rate (DALR) and the

saturated adiabatic lapse rate (SALR).

The DALR is the rate at which the

temperature o an unsaturated (dry)

air parcel changes as it ascends or

descends through the atmosphere.The

DALR is approximately 3C per 1000

eet.

The SALR is the rate at which the

temperature o a parcel o air saturated

with water vapour changes as the

parcel ascends or descends through

the atmosphere. The SALR is oten

taken as 1.5C per 1000 eet, although

the actual fgure varies according to

the amount o water vapour present

and also the temperature (higher

temperature air can contain more

water vapour).

The SALR is less than the DALR

because as a parcel o saturated air

ascends and cools, water vapour

condenses into water droplets,releasing latent heat into the parcel,

which slows the cooling and thus

maintains the air parcels bouyancy.

Conversely, i a saturated parcel

descends and warms, water droplets

will evaporate causing heat (latent heat

o evaporaration) to be absorbed rom

the parcel, thus reducing the rate o

warming (generally termed evaporative

cooling).

Determining StabilityAir rises i it is warmer than its

surroundings. Technically air doesnt

rise because it is warmer; it rises

because warm air is less dense than

cold air, and thus more buoyant.

The vertical temperature profle (also

known as the temperature lapse rate)

o the atmosphere changes as dierent

air masses dominate a region. I the

temperature profle is known, the

rate o change o temperature with

height can be determined and thus

the stability o the atmosphere at that

point in time.

AVIATION REFERENCE MATERIAL

Vertical Stability of the Atmosphere

Bureau of Meteorology Weather Services Aviation

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Airservices Australia is the ofcial distributor o aviation orecasts, warnings and observations

issued by the Bureau o Meteorology. Flight briefng services are available at:www.airservicesaustralia.com. Telephone contact details or elaborative briefngs are

contained in Airservices Aeronautical Inormation Publication Australia (AIP), which is

available online through their website.

The Australian Bureau o Meteorology provides a wide range o observations, synoptic charts,

satellite and radar images, orecast and warnings via the internet at www.bom.gov.au.

Commonwealth o Australia, 26 September 2011

The atmosphere is considered

to be stable, unstable, neutral or

conditionally unstable as follows:

ifaliftedparcelofairiscooler,

and thereore denser, than the

surrounding atmosphere, the

parcel will tend to sink once the

liting mechanism ceases. Such an

environment is defned as being

STABLE;

ifaliftedparceliswarmerand

less dense than the surrounding

atmosphere, the lited parcel will

continue to rise once the liting

mechanism ceases. In this case the

environment is defned as being

UNSTABLE;

ifaliftedparcelisthesame

temperature as the surrounding

air, the conditions are said to be

NEUTRAL;

insomesituationstheatmosphere

is stable or unsaturated parcels o

air but unstable i saturated. This is

called CONDITIONAL INSTABILITY.

Referring to the diagram above,

stable, unstable, neutral and

conditionally unstable layers in

the air mass can be determined by

comparing the temperature lapse

rates (orange line) of the air mass

with the DALR and the SALR:

anyparcel(dryorsaturated)forced

to rise (by, or example, convection)

and cool at the DALR or SALR,

between A and B will remain cooler

than the environmental temperature

and would thereore sink once

orcing had ceased. The layer is said

to be stable;

aparcelforcedtoriseandcoolat

the DALR through the neutrally

stable layer between B and C will

continue to rise only i orcing

continues, because the parcel would

be neither warmer or cooler than the

environment;

anysaturatedparcelrisingthrough

the conditionally unstable layer rom

C to D at the SALR would remain

warmer than the environment and

thus continue to rise and cool at

the SALR. On the other hand, an

unsaturated parcel would cool at

the DALR, remain cooler than the

environment and sink once any

orcing had been removed;

likethelayerfromAtoB,all

parcels, saturated or unsaturated

rising between D and E would only

continue to rise i they were orced

upward, since the layer is stable.

In general, when the lapse rate of

the air mass:

isbetweentheDALRandtheSALR,

the atmosphere is considered to be

conditionally unstable;

issteeperthantheSALR,the

atmosphere is considered to be

absolutely stable;

isthesameastheDALR,the

atmosphere is considered to be

neutrally stable;

islesssteepthantheDALR,the

atmosphere is considered to be

absolutely unstable.

A temperature/altitude graph, with

the lapse rate (the rate o change o

temperature) o the air mass displayed by

the orange line.

In the layer A to B the temperature

increases with height (a temperasture

inversion), as it does in the layer

immediately above D.

Elsewhere the temperature decreases with

height.

The DALR and SALR are depicted as white

dashed lines.

The yellow dashed line depicts the

displacement o a saturated air parcel

rising rom C to E along the SALR. It stops

rising at E because its temperature at this

point is the same as the environmental

temperature and thus the air parcel loses

its bouyancy.

Actual lapse rate

Neutral stabilityfor dry air parcels

Stable for both dry andsaturated air parcels

Conditionallyunstable

Stable for both dry andsaturated air parcels

Unstable for a saturatedparcel but stable forunsaturated

DALR

20000

10000

Height(feet)

0

0 10 20 30

Temperature (C)

SALR

E

D

A

B

C