Skew-T / Log-P Diagram ::: The Paper Calculator (Part 1/3)

Referred hereafter primarily as "Skew-T's," and sometimes, 'soundings,' these diagrams / graphs play a significant role in meteorology. They are used to plot readings from weather balloons (called soundings) which allow the meteorologist to assess the entire state of the atmosphere (troposphere, primarily): from temperature, dew-point, and wind profiles to thunderstorm probability and severe-weather / winter-weather likelihood. Furthermore, you don't really need to have any real data on a blank graph. You can use it for theoretical purposes or basic calculations using limited info on the ground, hence me calling it, 'The paper calculator.'

Photo Courtesy: ShareMyJoys.WordPress.com

Looking at a plotted, or even a blank Skew-T will look very busy and confusing at first. Therefore, in my first post about them, I'll go over the many different lines and what they are used for. Then in my 2nd post I will briefly overview some simple calculations that can be made using them with little-to-no assistance of other tools. The 3rd part will focus on inferences that can be made and signals that are watched for. I'll also conclude with my experience of discovering Skew-T's and how they changed things for me in my education.

●   ●   ●

Small visual identifying lines; brief part-explanation of their usefulness

What do all of those lines mean?

Due to the significant number of intersecting lines, the skew-t looks very busy. It should, because it is. Each line-type has a specific and important role in display and interpretation of plotted data (actual or theoretical). Above is a small info-graphic identifying the lines you see. Below, I'll expound a little more on each line's usefulness.

Isotherms (Temperature Lines)

- These lines are responsible for the diagram name: "Skew-T's." It simply refers to how the temperature part of the graph consists of skewed (non-vertical; non-horizontal) lines. If you think about a regular grid, the isotherms would be like the vertical lines of the grid/graph.
- Despite the skewed nature, the rudimentary interpretation of changes in temperature with height is very little
- Dew-point temperatures (moisture parameters) are also plotted using these same guides
- Units are typically in °C. Not only is this the unit of choice, but it's much easier to convert from Celsius to Kelvin, another frequently used temperature scale, than from Fahrenheit to Celsius to Kelvin.

Isobars (Logarithmic Pressure Lines)
- These give the diagram the latter-part of their name, "Log-P." It models the vertical extent of the lower portion of the earth's atmosphere where virtually all sensible weather occurs: The troposphere. Near the bottom of the graph, ground data is found; near the top, upper-level data is found
- Furthermore, pressures decrease with height. This is due to mass (and thus, density) decreasing with height. The logarithmic nature is due to the gravity force keeping significant portions of mass closer to the surface. This is depicted (*) to make the graph a like-model of the atmosphere.

Dry Adiabat Lines
- As unsaturated air (we'll call it a parcel; you can think of it as a bubble) rises (or falls) it changes temperature adiabatically: As it ascends/descends, no heat-energy is exchanged between the atmosphere and the parcel.
- This notion is consistent for use, regardless if the theoretical parcel is forced upward or is freely-buoyant (convective).
- They are also very helpful in gauging the stability of the atmosphere

Saturated Adiabat Lines
- Discussed in a later post, there is a point that can be found on the graph where a theoretical rising parcel would reach saturation. You may know this as the point where condensation occurs; where clouds begin to form. It is at this point where the parcel temperatures still decrease with height, but at a much slower rate than a dry-parcel. This is why saturated (often referred to as moist, or pseudo) adiabats are needed.
- These lines are also used to find the wet-bulb temperature which can be used to determine approximately how cool the air will get if it does rain.

Mixing-Ratio Lines
- This ratio simply represents the moist-air/dry-air ratio.
- Once again, these will be discussed in a later post but are vital to the skew-t as the mixing-ratio of a theoretical rising parcel will not change during ascension to the saturation point.
- Higher dew-points will have higher mixing-ratios (more moisture)

So yes, despite the seemingly hectic nature of a skew-t, all lines you see play substantial roles in their employment..

Temperature, Dew-Point, and Wind-Profile

How Weather Balloons work from a Skew-T Perspective (sample skew-t retrieved from UWYO soundings site)

- The temperature profile is typically marked in red, but will always be on the right of the dew-point profile, which is traditionally marked in green. Dry and moist layers  are easily spotted in the sounding by assessing the spacing between the temperature and dew-point profiles. These are referred to as Dew-Point Depressions (DPD; Temperature MINUS Dew-Point). Layers are moist (dry) when DPD values are low (high).
- As the weather balloon ascends, it is also using GPS coordinates and time to calculate wind direction and speed at various levels of the atmosphere.
- Further inferences that can be analyzed will be discussed in the third-installment.

NEXT POST → Basic calculations

EDITING:
- 08 Jan 2018 - * At first, I resolved without sourcing that one reason skew-t's displayed logarithmic pressure was to accommodate approximate and uniform elevation changes, but I don't know that for sure.
- 25 Jan 2018 - * In process of further additions to the post
- 23 Feb 2018 - I decided against implementing a fourth part.