Bureau of Meteorology > Weather Services > Aviation

REFERENCE MATERIAL
The Aerological Diagram 
‘SkewT - LogP (F160)’

The Australian Bureau of Meteorology mainly uses the 'SkewT - LogP' aerological diagram to determine stability. 

Although an understanding of the aerological diagram is not a pre-requisite for pilots, it is discussed here because such diagrams are freely available over the internet and are used extensively by glider pilots to calculate thermal activity. The diagrams are also useful for a better understanding of the changes that occur in the atmosphere on a daily basis. 

The diagram's name reflects the parameters associated with the vertical and horizontal axes. Atmospheric pressure (hPa) is plotted along the vertical axis using a logarithmic scale. This represents the variation of atmospheric pressure with height observed in the earth's atmosphere. The pressure scale is along the left-hand side of the diagram. Temperature (0°C) is shown along the horizontal axis, increasing from left to right. The lines of constant temperature are 'skewed' from the lower left to the upper right of the chart. 

The diagram is used for determining the stability of the atmosphere above a specific location by comparing the actual lapse rate with the DALR and SALR lines (thin dashed lines discussed later) on the chart. The stability is of interest in relation to rising air parcels, because if the atmosphere is unstable and moist, showers or thunderstorms may be a consequence, depending on the magnitude and depth of instability and moisture availability. Rain occurs when the atmosphere is moist but stable, and there is a mechanism for continuous lift. 

Since the pressure and volume of the rising parcel will change according to the lower pressure encountered higher in the atmosphere, the temperature and dew-point will also change. If it is known how much these parameters change we can predict if the parcel will be warmer or cooler than the actual lapse rate and thus the stability of the parcel can be determined. For this reason dashed lines of dry adiabats, moist adiabats and mixing ratio are displayed on the diagram. A parcel will rise parallel to one of the lines depending on whether it is unsaturated or saturated. The dew-point will follow the mixing ratio lines (dew-point decreases about 0.6°C per thousand feet) to the level of saturation. Thus:

•	dry adiabat lines depict the DALR. They represent the path for an unsaturated parcel of air; 
•	moist adiabat lines depict the SALR. They represent the path for a saturated parcel of air; 
•	mixing ratio lines depict the dew-point rate of change (and the path) for an unsaturated air parcel (Dew-point temperature is the temperature a parcel of air would have to be cooled to, for it to become saturated). 

A parcel will generally subside (sink) along a dry adiabat, undergoing compressional warming and quickly becoming warmer than the surrounding air and hence becoming stable. 

(above) An aerological diagram depicting the vertical temperature lapse rate, dew-point and wind structure over Perth on the 2 July 2001. 

The figure above shows two separate sets of temperature and dew-point temperature plots. The red plot is for 02/07/2001 at 2300 UTC and the blue plot is for 02/07/2001 at 1100 UTC. The two plots show changes in temperature and moisture, and from this can be inferred changes in the stability during a twelve-hour period. The solid grey curves marked on the chart represent the changing temperature and dewpoint characteristics of a surface air "parcel" as it is lifted. In this case, a "what-if" scenario is shown, where the temperature of the surface parcel has been projected to be around 19C before lifting. The barbs on the right-hand side show the variation in wind direction and speed (knots) with height for the time of the latest plot. Solid triangular barbs represent a wind speed of 50 knots; a single line represents 10 knots while a half line represents 5 knots. So at 400 hPa the wind is 65 knots while at 200 hPa it is 155 knots. The wind direction is plotted to the compass, i.e. at the surface the wind is a northeasterly while at 600 hPa it is a westerly. Note the light and variable wind in the surface inversion layer (northeast at 5 knots) compared to above the inversion (westerly at 20 knots) indicative of significant wind shear of approximately 25 knots at the top of the inversion. Also note that winds increase with height to a maximum of 155 knots near 200 hPa.

Airservices Australia is the official distributor of aviation forecasts, warnings and observations issued by the Bureau of Meteorology. Flight briefing services are available at: www.airservicesaustralia.com. Telephone contact details for elaborative briefings are contained in Airservices’ Aeronautical Information Publication Australia (AIP), which is available online through their website.

The Australian Bureau of Meteorology provides a wide range of observations, synoptic charts, satellite and radar images, forecast and warnings via the internet at www.bom.gov.au. 

©Commonwealth of Australia, 14 February 2011