Severe Weather Discussion > General Weather - all topics not current severe weather.

Understanding Aerological Diagrams / Soundings

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Michael Bath:
This thread has been started to help members read and understand aerological diagrams - or soundings - the atmospheric measurements taken by weather balloons. These provide a cross section of the temperature, dew point, wind speed and wind direction from the surface to the troposphere.

Some of this information will be pretty basic to many forum readers. To others it may be new. I encourage anyone to ask questions and also provide your own examples and answers. Use the image attach function as necessary.

Soundings are often included in the severe thunderstorm threads as they can provide a very clear picture of atmospheric conditions associated with the event. They are also extremely useful in non-alpine snow events too.

You can then try and identity similar features or patterns on other days to help with forecasting or nowcasting.

The diagrams are available here:
http://www.bom.gov.au/products/reg/IDS65024/
(enter bomw0007" for userid and "aviation" for password)

Archived soundings are available from this website:
http://soundings.bsch.au.com/index.html




The ground is at the bottom of the chart. Working your way up the left margin is the height in hectopascals (hPa). The equivalent height in km and feet is also shown slightly to the right of the hPa scale.

The temperature scale in degrees Celsius is marked with diagonal lines connecting equal values in the bottom, top and right hand side margins.

There are two red trace lines. The one on the right is temperature, the one on the left is the dew point. You can follow the diagonal temperature scale lines and where they cross the trace it is the temperature at that height (hPa). In this example above, the 500hPa temperature is about minus 12C.  The closer the left and right lines are together the moister the atmosphere is at that level, and of course the further apart they are the drier it is.

There are also two blue trace lines. These show you the values from the previous sounding for comparison.

The time the weather balloon was released is shown at the bottom of the diagram in red. The previous sounding time/date is in blue.

Along the right hand side of the diagram are wind speed and direction barbs. They point towards the way the wind is heading but are referred to by the direction the wind is coming from. In this example all the winds in the top half of the sounding are from the west. One line on the end of barb equals 10 knots, a half barb is 5 knows, and a solid triangle barb is 50 knots. You add all the barbs together to get the totals winds speed in knots. At 200hPa, the wind is a 100knot westerly (jetstream). At 700hPa there is a 20 knot SW wind.

You now have one chart that gives you a complete picture of the temperature, moisture, wind speed and direction.

Next we have to look at some of the other lines and variables marked, then how to work out good conditions for thunderstorms, snow, rain, heat or other weather.

Dave Nelson:
Thanks Michael for following up on my suggestion  :)

 Even the new bit of info like .... "The closer the left and right lines are together the moister the atmosphere
 is at that level, and of course the further apart they are the drier it is."  .....    added to my knowledge,  something I
never knew before

As time goes by and you add to this teaching session looking at identifying things like   Windshear, CAP's,
  how moisture at a particular level determines if you would get low or high based storm cells,  etc,  will be great

cheers
Dave N

Harley Pearman:
Michael

Thank you. I am still learning how to use these. I have one question to ask. On many of these sounding charts, I see a grey line curving usually to the left. I do not understand what the grey curve means or what it is telling me. Can you please explain how the grey curve line function.

I see this grey curve on the standard NCEP Soundings or NOMAD soundings on the Stormcasts charts. Thank you.

Harley Pearman

Jason(pato):
Gday Harley,

That grey line you refer to is known as the Theoretical Air Parcel Plot line (TAPP). Its generally understood that the area between this grey line and the temp trace line indicates the amount of Convective Available Potential Energy (CAPE). So if there is a greater area between the TAPP and temp trace lines then the atmosphere is more unstable but a smaller or no area between those two lines would generally mean the atmosphere is more stable. There are many other variables that come into play though, and this area should not always be taken as gospel that storms are on the cards. Hope this helps, I'm still learning about all the little nuances that Aerological Diagrams (or Skew-T diagrams) throw up which either help or hinder a potential storm day. Looking forward to more input from our more experienced weather watchers, and feel free to add anything that might expand on my brief explanation.

Cheers Jason

Michael Bath:
All thunderstorms require low level moisture, instability and a trigger. Severe thunderstorms also require decent wind shear - speed and/or directional change of the winds with height.

The sounding immediately shows the moisture profile. The better thunderstorm setups will have plenty of moisture in the lowest few km of the atmosphere with dry air aloft. But not too much low level moisture - you want to see the temperature and dew point lines with a few degrees of separation like in the first diagram posted above. If there is too much moisture it will be cloudy and/or raining. The temp/DP lines will show very close together or the same values in those cases.

You can also see the complete wind shear profile at a glance. Wind speeds will typically increase the higher up you go. The better thunderstorm setups will have winds backing (anticlockwise) with height in the lower few km. For example N-NE at the surface, N at 925hPa, NW at 850hpa and W by 700hPa is a typical scenario along the east coast. This turning helps establish good updraft/downdraft separation and hopefully a supercell. Of course it is not that simple.

Instability has been touched on by some comments already. We are now going to look at working out instability on analysis soundings retrieved from the BSCH archives. The data for these is actually archived BoM data stored at the University of Wyoming (US). It is presented in much the same way as the BoM diagrams but also include some extra variables. The forecast soundings available from BSCH are drawn in the same way but are based on the GFS forecast of conditions, not the analysis of actual conditions.





As mentioned by Jason, the Theoretical Air Parcel Plot (TAPP) is a representation of how a parcel of air may rise from near the surface. Where the grey line is to the right of the temperature trace it is unstable. You will see that the TAPP starts at the bottom of the diagram as an inverted "V". Those two lines meet at the Lifted Condensation Level (LCL) and the single line continues upwards following the alignment of the light green dashed curve (shown on the diagram from the surface to 200hPa). The left grey line is a forecast surface dew point (DS), the value of which is also shown amongst the variables on the top right of the diagram. The right grey line is a forecast surface temperature (TS). These two lines will follow the angles of the other two light green dashed lines.

The LCL will be where cloud bases will initially form - ie. the first cumulus clouds. As more and more convection occurs the air will mix - and cloud bases will develop lower down.

How do we know what the right values for DS and TS should be? There are a few calculations used with the BoM using one that differs from BSCH. With the BSCH website, you can also put in your own values. Eg. Look at observations from towns near the sounding location and enter in the actual temp and dew point.

Numerous thunderstorm indices can be determined from the calculations. The most common are Lifted Index (LI) and Convective Available Potential Energy (CAPE). The high the CAPE and lower the LI the better.





The CAP (or inversion) is a relatively warm layer of air that may delay or completely suppress the formation of thunderstorms. On the sounding it can be seen where the temperature stays the same or often increases with height. You will regularly see this showing above 900hPa. The first sounding posted in this thread has a CAP starting just below 900hPa. The 2nd sounding has none and the one just above this paragraph has quite a strong cap around 900hPa.

Inversions will also occur higher up - the most obvious one is at the tropopause where thunderstorm anvils occur. You may also see inversions in the mid levels. The 2nd sounding shown in this thread has an inversion at 550hPa. These do not necessarily stop thunderstorms but may slow updraft strength.


Lastly (for now), the trigger. This is one part of thunderstorm forecasting that you cannot see on a single sounding. Thunderstorm triggers can be from a surface trough or low pressure system, an upper trough or upper low, cold front, surface convergence, or just surface heating during the course of the afternoon.  None of these can be seen on the sounding, however you can use more than one sounding and compare upper level temperatures to help determine if an upper trough may be approaching.

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