AVIATION REFERENCE MATERIAL The Skew T Log P …

1 AVIATION REFERENCE MATERIAL . The skew T Log P. Aerological Diagram Bureau of Meteorology AVIATION Weather Services The skew T - Log P name reflects the A rising air parcel will cool at a fact that temperature is plotted on the rate dependent on whether it is horizontal axis with isotherms skewed unsaturated or saturated, it will from the lower left to the upper right cool at either the DALR or the SALR. of the chart; and pressure is plotted on The dew-point lapse rate will follow the the vertical axis with isobars spaced mixing ratios. using a logarithmic scale. The diagram also includes saturation mixing ratios An air parcel will rise if it's warmer, (lines of constant mass of water and therefore less dense, than its vapour divided by mass of dry air in a surrounding atmosphere. Such an saturated air parcel), dry adiabats and atmosphere is considered to be saturation adiabats. unstable. If the air parcel is cooler and more dense than its surroundings, Superimposed on to these lines is it will sink, if it is allowed to do so.

2 A plot of the actual variation in air Such an atmosphere is considered temperature and dewpoint with to be stable. For more information height. The temperature, dewpoint on stability, readers can refer to our and pressure data for these plots are brochure Vertical Stability of the obtained from radiosonde flights which Atmosphere. The aerological diagram is used are released from thirty eight Bureau by forecasters to view the current A sample diagram is shown overleaf. weather stations daily. vertical distribution of temperature It plots a sounding from a radiosonde and moisture in the troposphere, Wind direction and speed are plotted released at Perth Airport at 2300 on and to determine the atmosphere's on the right-hand side of the diagram. 02/07/2001 UTC (Bureau diagrams stability. This data is obtained from the balloon normally also display the plot from flights which carry the radiosonde the previous sounding).

3 The Lifting Stability is of interest because if the through the atmosphere. Condensation Level (LCL) shown is atmosphere is unstable and moist, the level at which cloud will form if it is then showers or thunderstorms may Additionally, indices calculated from lifted to that level, assuming a surface develop, the severity dependent on the the radiosonde data are displayed at air temperature of , and dew- magnitude and depth of instability and the top-right of the image. These are point of is reached. The LCL is the moisture availability. Alternatively, explained on the following page. produced by an automated algorithm rain may occur when the atmosphere which uses (unlike the classic method), The diagram allows forecasters to is moist but stable, and a mechanism for the dry adiabat isopleth, the obtain a snapshot of the atmosphere exists to provide continuous lift. estimated maximum afternoon surface above a specific location, from There are a number of aerological the surface to around the 100 hPa air temperature; and for the mixing diagram types.

4 The Bureau uses the level. They can then determine the ratio, the estimated surface dew-point skew T - Log P aerological diagram. atmosphere's stability by comparing temperature at that time. The LCL. the actual temperature profile with the occurs where the dry adiabat isopleth Although an understanding of the dry and saturation adiabatic lapse rates crosses the mixing ratio. diagram is not a pre-requisite for pilots, (DALR and SALR), given by the dry the skew T - Log P is discussed here and saturation adiabats printed on the The plot on the right shows the because these diagrams are freely diagram. variation in wind with height at the available on the internet; they are used time of the sounding. Wind staffs point extensively by glider pilots to calculate As an air parcel rises, it encounters to the direction from which the wind thermal activity; and they are useful lower pressure. As a result it expands is blowing (with north at the top of for gaining a better understanding of and its temperature drops.)

5 If it is the page). The solid barbs represent changing weather conditions. known how much the temperature a speed of 50KT; the single line 10KT;. changes, we can predict if the parcel and the half line 5KT. Note the light will be warmer or cooler than its wind in the surface inversion layer environment and thus the stability of compared to the westerly at 20KT. the parcel can be determined. above the inversion. A skew T - Log P aerological diagram depicting the vertical temperature, dew-point and wind structure over Perth on 3 July 2001. KEY. PW = precipitable water. The amount of attempts to predict the atmospheric Tlcl temperature at the lifting rainfall which would be generated if all of conditions at the time of maximum condensation level. The lifting the water vapour in the atmosphere was surface temperature ( maximum condensation level is the level at which precipitated to the surface.

6 Instability). the parcel becomes saturated when TT = total totals. A measure of Ps surface pressure of the parcel. lifted dry-adiabatically. atmospheric stability based on the Ts surface temperature of the parcel. LI lifted index. A measure of temperature difference between atmospheric stability, generated by 850hPa and 500hPa. A value of 44 or Ds surface dew point of the parcel. subtracting the temperature of the higher indicates sufficient instability for Plcl pressure at the lifting parcel when lifted to 500hPa from thunderstorms to develop. condensation level. The lifting the observed temperature at 500hPa. The following parcel data is automatically condensation level is the level at which Negative values indicate an unstable generated using an algorithm which the parcel becomes saturated when atmosphere. lifted dry-adiabatically. Airservices Australia is the official distributor of AVIATION forecasts, warnings and observations issued by the Bureau of Meteorology.

7 Airservices' flight briefing services are available at Telephone contact details for elaborative briefings are contained in Airservices' Aeronautical Information Publication Australia (AIP), which is available online through their website. Other brochures produced by the Bureau of Meteorology's AVIATION weather services program can be found at Commonwealth of Australia, 6 June 2015.