rm-skewt
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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.
http://www.airservicesaustralia.com/http://www.bom.gov.au/aviation/knowledge-centrehttp://www.bom.gov.au/aviation/knowledge-centrehttp://www.airservicesaustralia.com/
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