Atmospheric Soundings – An Introduction (Part One)

Atmospheric soundings are an essential part to forecasting – whether you forecast professionally, casually out of self-interest or for storm chasing purposes.  However atmospheric soundings are easy to learn but difficult to master.  This series of modules will guide you step by step from basic concepts to far more advanced methods including looking at case study examples.

In this introduction to atmospheric soundings module you’ll learn:

  • How to read an atmospheric sounding
  • What all the lines mean and represent
  • How to plot an atmospheric sounding
  • Some basic fundamentals behind the atmospheric physics of how soundings work

If you already have a sound grasp of the basics of atmospheric soundings – you may wish to refer to the follow-on modules that discuss more advanced concepts (these will be added progressively over time).

The most common way to interpret the atmosphere is to look at 2D charts – examples of these are charts such as the mean sea level pressure charts, or winds and temperatures at various levels such as below.

While these are excellent at identifying broadscale features, we can only look at one level at a time.  Atmospheric soundings, on the other hand, provide a vertical slide of the atmosphere at a single point on the map.  This allows us to see temperatures, winds, dew point etc in the upper atmosphere at a single point.  Soundings become daunting and unnecessarily complicated for many – but look far more complicated than they really are.

Here is an example of an atmospheric sounding.  You can see two red lines through the centre of the chart – very simply:

  • The red line on the right is the temperature line
  • The red line on the left is the dew point line

The actual temperature and dew point can be read on the chart by using the diagonal lines that originated from the bottom of the chart, and move towards the right as you move up the chart.

There are intervals every 10C and the scale can be read on the bottom.

If we wanted to determine where the temperature and dew point were both 0C, we can read it off this chart which highlights where they both reach 0C.

However, in order for this to be meaningful to forecasters, we need to know how high these temperatures are in the atmosphere.

When we discuss levels in the upper atmosphere, we often use the term “pressure height” rather than the physical height in metres.

The pressure decreases the higher you ascend into the atmosphere because there is less air pushing down on you.  As an example, the 500mb height is a common pressure height used – simply the 500mb height is the height that you need to ascend into the atmosphere to reach a pressure of 500mb.   Some common pressure heights that are used are:

  • 925mb
  • 850mb
  • 700mb
  • 500mb
  • 300mb

The pressure heights can be read on the left hand side on the sounding as shown below – and this includes some of the major, more significant pressure heights that are used for meteorological applications.

The reason pressure height is used rather than the physical height in metres is because the pressure is not constant in the upper atmosphere!  Just as there are low and high pressure systems at the surface there are low and high pressure systems in the upper atmosphere – and this means that while one day you might reach the 500mb height level by ascending to 5700m, another day you might reach the 500mb height level by ascending to only 5600m.  Cold air is denser than warm air, so tends to correlate with lower pressure heights than warmer air.  This is also why low pressure heights tend to correlate with more unsettled weather conditions.

Pressure heights are very easy to read on the soundings – they’re the values on the left hand side.  In the below example you can see the pressure start at 1000mb at the bottom, then rise to 900, 800, 700 etc as you ascend into the atmosphere.  Notice that the gaps between pressure markings become larger as the pressure falls?  This is because air is compressed at the surface so a small change in elevation can equate to a large change in pressure.

Now that we understand pressure heights – we can start to establish something more meaningful reading our sounding.  Remember the 0C temperature and dew point levels?  We can go back to the sounding and see what level in the atmosphere these occurred at!

We can see that the temperature falls to 0C at around 600mb, while the dew point falls to 0C above 700mb but below 800mb (probably around 740-750mb to be exact).

What if we wanted to see temperatures throughout the atmosphere?  Again we can follow the same process, look for the pressure height and draw a horizontal line across until you reach the temperature line drawn on the sounding, then match that up with the diagonal temperature lines.  Below you can see how we can easily read the temperature at different heights.  You can even see that the scale goes much colder than -30C!  There’s a temperature scale on top of the soundings too, which means that you can still read the temperatures to values well below -30C by using the scale on top of the sounding.

Let’s look at another example – a much colder one.  In this example, we can see that the dew point and temperature are much closer together!  In fact, overall the temperature and dew point are much closer together – this tells us something about the atmosphere.  When the temperature and dew point are far apart (like in the previous example), it means that the humidity is low and the atmosphere is dry.  In this example, the temperature and dew point are closer together which shows higher humidity.

Let’s look at one final example – take a look at the temperature and dew point lines around the 500mb level – they both merge into one before splitting apart!  This means that the dew point is exactly the same as the temperature – in other words, humidity is 100% (or the atmosphere at these levels is saturated).

Below we’ve broken up the atmosphere into sections showing where humidity is high, where the atmosphere is saturated (often where cloud might develop), and where the atmosphere is dry.