Unlocking the upper atmosphere: an introduction to the SkewT



  • Yesterday I tweaked up my old Windy SkewT plugin and thought it long past time to give the Windy community a rundown of what this crazy-looking diagram actually means, and why it's the most data-rich diagram in all of meteorology – if you know how to use it.

    What's a SkewT, and why do I care?

    One day during my forecaster training, our instructor handed each of us an odd-looking printout. It looked like this:

    skewT.png

    "WTF is this!?" was my first thought. Truth be told, it terrified me. And I'll be honest: a SkewT is not for the faint of heart. The general public will rarely, if ever, become acquainted with it. Those who understand those straight and curvy lines will usually fall into the categories of meteorologists, pilots, balloonists, gliders, and storm chasers. However, on a different occasion during training, I asked my instructor: "If you had only one tool available on the weather-forecasting bench, what would it be?". "A SkewT", she replied.

    Here's what a SkewT can tell you about, with no other information to hand: Convective cloud, upper-level wind, layer cloud, thunderstorm development potential, wind shear, upper-level instability, temperature, dewpoint, humidity, tropospheric depth, potential instability, conditional instability, fog point, adiabatic processes, airmass type... I'll be honest, the list goes on and on.

    Interested yet?

    Okay, let's dig in...

    A weather balloon has attached to it a piece of tech called a radiosonde. A radiosonde is essentially just a sensor that reads the temperature and humidity of the air as the balloon ascends, and GPS allows it to deduce how it is moving (giving us wind data). By the time the balloon has entered the stratosphere (about 20 km up), it's expanded to about the size of a double-decker bus – then the balloon pops.

    The temperature, dew point temperature, and winds get plotted on the radiosonde like so:

    valentia_skewt.png

    With a little practice, you can deduce a lot from this one diagram. For instance, the first thing I would notice is how dry the air suddenly becomes at around 825 hPa (roughly 5000 ft). This is a clear sign of high pressure, an anticyclone. I would, therefore, expect to see light winds low down, which I do. Below 5000 ft, there is more moisture, and the sounding is a little more unstable. Using just this, I could probably take a guess at the exact time of morning when fair weather cumulus will start popping up in the sky.

    The air temperature at the surface in this sounding is at around 20°C. Note the 45° line extending toward the top right from there – this is an isotherm. The reason it's skewed in this way is to make the temperature curve seem more vertical and also to separate out the dewpoint curve from the temperature curve (for readability). This is from where the SkewT gets its name.

    SkewT is shorthand for 'SkewT–LogP diagram'. The LogP part relates to pressure, which is plotted on a logarithmic scale. This makes sense, as pressure decreases exponentially with height in the atmosphere.

    The dotted lines are isohumes – lines of equal humidity.

    And then we have the adiabats. Now, our understanding of atmospheric thermodynamics is actually quite sophisticated. We know a lot about how changes to fields like temperature or pressure can affect other fields, like humidity or entropy. This is where the real magic of the SkewT happens. A parcel of air lifted to a different pressure will cool, but conserve its entropy. This allows us to do useful things like diagnosing thunderstorm potential before the cumulonimbus clouds even start to form. You can also diagnose the expected time of fog onset, predict a föhn effect, or identify an area of potential instability (potential instability can wreak havoc in rainfall totals). The cool thing is that you can do all of this from one balloon ascent, there's no computer model involved. There's a dry adiabat and a moist adiabat, and these are the remaining lines on the SkewT, but that's all I can really say for now.

    Windy plugins, and how to use windy-plugin-skewt

    Windy opened up their platform to outside developers a few years ago. Just click on the menu and then Install Windy Plugin to try them out. There's a nice list to choose from, and you'll find the SkewT under the SkewT diagram. You can also try Sounding, which is a related plugin.

    When the plugin is loaded, simply open Windy's picker by clicking on any point on the map. Since Windy is a model visualization web app, the data plotted is model data. I also host my own complementary web app at skewt.org, which uses real-time radiosonde data from balloon launches across the world, rather than model data.

    So that's it for now! I hope I've piqued your interest a little into the wonderful world of thermodynamic diagrams. As always, please send me a message on the Windy community (username johnckealy) with any questions or comments.



  • @Korina @pavelneuman

    I wrote a new article today. Please take a look and let me know if it's publishable, as per the usual :)

    p.s. @Korina, I had to put my proposed rainfall extrapolation project on hold for now, I've got a new job as a web developer that needs my attention in the short term :(



  • Hello @johnckealy, thank you for your work and for explaining the article! I've published it :)



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