This is the first of a series I’m planning to write on various resources that are useful for people interested in weather observation. In this article I’ll discuss the Aerological Diagram, also known as a sounding chart or SKEW-T diagram.
When you watch the weather on the TV all the reports are based on surface observations and forecasts. By surface we mean the air within around 10-20m of the ground. This presents us with a largely 2 dimensional view of the weather. The atmosphere is nearly 100km thick so this is a very limited view of what’s going on with the weather. The purpose of the sounding chart is to allow us to understand what’s going on throughout the major part of the atmosphere in the vertical direction.
Aerological diagrams or soundings are usually produced by sending a balloon with an instrument package, know as a radiosonde, into the sky. In Australia there are 34 locations where these are launched regularly. While they can carry any instruments, for weather forecasting purposes they typically carry a temperature and humidity sensor as well as a radar target. As the balloon ascends it radios back the temperature and dew point values. The radar tracks the motion of the balloon to determine wind speed and direction. These are then turned into a chart.
The data is plotted on a chart type called a Skew-T diagram. This chart is quite different to a standard graph paper chart. It’s designed specifically to present the thermal cross-section of the atmosphere in a useful way.
One of the best explanations of how to use the sounding charts can be found on Anthony Cornelius’s Downunder Chase website. In Part 2 of his Weather Education section he has written 5 documents on understanding it and using it. I strongly recommend anyone interested in how weather is forecast have a read of his entire guide.
If you don’t have time to read it all right now, I’ll give you a quick dirty guide to the sounding chart. The red line on the right represents the most recent temperature profile with altitude increasing as you go up the chart. The red line on the left represents the most recent dew point profile. The closer the lines are together the more moist the atmosphere is at that altitude. Previous readings from the most recent prior sounding flight are also shown as blue lines.
The scale on the far left shows altitude in millibars, another words the air pressure at the height. BOM soundings also include a scale in Flight Level(FL500 is 50000 feet) and kilometres. On the right hand side wind speeds and directions are expressed by means of wind barbs.
Once you learn to read the Aerological Diagram it becomes easy to note significant weather scenarios. Things like temperature inversions and jetstreams stand out very easily. Learning to read the SKEW-T Chart is a valuable skill for all weather enthusiasts.
Bureau of Meteorology Aerological Diagrams can be found under the Aviation Observations option in the Aviation Weather section of their website. Here is a direct link to the map showing sounding locations. Simply click on a location to view the chart.