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The aerological diagram is used

by forecasters to view the current

vertical distribution of temperature

and moisture in the troposphere,

and to determine the atmospheresstability.

Stability is o interest because i the

atmosphere is unstable and moist,

then showers or thunderstorms may

develop, the severity dependent on

the magnitude and depth o instability

and moisture availability. Alternatively,

rain may occur when the atmosphere

is moist but stable, and a mechanism

exists to provide continuous lit.

There are a number o aerologicaldiagram types. They are designed to

represent the adiabatic processes

which air parcels undergo as they

ascend or descend through the

atmosphere. The Bureau uses the

Skew T - Log P aerological diagram.

Although an understanding o the

diagram is not a pre-requisite or pilots,

the Skew T - Log P is discussed here

because these diagrams are reely

available on the internet; they are used

extensively by glider pilots to calculate

thermal activity; and they are useul

or gaining a better understanding o

changing weather conditions.

The Skew T - Log P name reects the

act that temperature is plotted on the

horizontal axis with isotherms (lines

o constant temperature) skewed

rom the lower let to the upper righto the chart; and pressure is plotted

on the vertical axis with isobars (lines

o constant pressure) spaced using

a logarithmic scale. The diagram

also includes saturation mixing ratio

isopleths (lines o constant mass

o water vapour divided by mass o

dry air in a saturated air parcel), dry

adiabats and saturation adiabats. A line

showing the International Civil Aviation

Organization Standard Atmosphere is

also printed on the diagram.

Superimposed on to these lines is

a plot o the actual variation in air

temperature and dew-point with

height. The temperature, dew-point

and pressure data or these plots are

obtained rom radiosonde ights which

are released rom thirty eight Bureau

weather stations twice daily.

Additionally, wind direction and speed

are plotted on the right-hand side o

the diagram. This data is obtained romthe balloon ights which carry the

radiosonde through the atmosphere.

The diagram allows orecasters to

obtain a snapshot o the atmosphere

above a specifc location, rom

the surace to around the 100 hPa

level. They can then determine the

atmospheres stability by comparing

the environmental lapse rate (ELR),

given by the actual temperature profle,

with the dry adiabatic and saturation

adiabatic lapse rates (DALR and

SALR), given by the dry and saturation

adiabats printed on the diagram.

As an air parcel rises, it encounters

lower pressure. As a result it expands

and its temperature drops. I it is

known how much the temperature

changes, we can predict i the parcel

will be warmer or cooler than its

environment and thus the stability o

the parcel can be determined.

A rising air parcel will cool at a

rate dependent on whether it is

unsaturated or saturated, i.e. it will

cool at either the DALR or the SALR.

The dew-point lapse rate will ollow themixing ratio isopleths.

An air parcel will rise i its warmer,

and thereore less dense, than its

surrounding atmosphere. Such

an atmosphere is considered

to be unstable. I the air parcel

is cooler and more dense than

its surroundings, it will sink, i

it is allowed to do so. Such an

atmosphere is considered to be

stable. A parcel will compressas it descends and will generally

increase in temperature at the DALR,

quickly becoming warmer than the

surrounding air and hence becoming

stable. For more inormation on

stability, readers can reer to the

companion brochure in this series,

Vertical Stability of the Atmosphere.

A sample Skew T - Log P is shown

overlea. It plots a sounding rom a

radiosonde released at Perth Airport at

2300 on 02/07/2001 UTC. The Bureaus

diagrams normally also display the plot

rom the previous sounding so as to

give an idea o how the atmosphere

is changing. The plot on the right-hand

side shows the variation in wind with

height at the time o the sounding.

The wind stas point to the direction

rom which the wind is blowing (with

north at the top o the page). The solid

triangular barbs represent a wind

speed o 50 knots; the single line 10

knots; and the hal line 5 knots, e.g.the wind at 400 hPa is rom the west-

northwest at 65 knots. Note the light

wind in the surace inversion layer

compared to the westerly at 20 knots

above the inversion.

AVIATION REFERENCE MATERIAL

The Skew T Log PAerological Diagram

Bureau of Meteorology > Weather Services > Aviation

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

issued by the Bureau o Meteorology. Airservices fight brieng services are available atwww.airservicesaustralia.com. Telephone contact details or elaborative briengs are

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

available online through their website.

Other brochures produced by the Bureau o Meteorologys aviation weather services program

can be ound at www.bom.gov.au/aviation/knowledge-centre.

Commonwealth o Australia, 11 May 2012

A Skew T - Log P aerological diagramdepicting the vertical temperature,dew-point and wind structure overPerth on the 3 July 2001.

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