Skew-T / Log-P Diagram ::: The Paper Calculator (Part 3/3) ::: Inferences and Signals

In this segment, I'll briefly go over some information about inferences and signals forecasters look for while analyzing a sounding. This post is rather in-depth, so I'd recommend taking it in one section of a time. Please feel free to follow along (previous post has a link to a skew-t).

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*** Click Here to read Part 2/3 in this Skew-T series ***

Change in Moisture Content

- In meteorology, we're very interested in how variables change. This comparison could be to another variable or time. As I mentioned in a previous post, higher dew-points yield higher mixing-ratios. But the change does not happen equally. As dew-point increases, mixing-ratios increase at an ever-increasing pace. This shows up nicely when simply analyzing the mixing-ratio quantities on a Skew-T.
- To forecasters, dew-points near the surface in the upper-60's and 70's (20-25 °C) draw more concern because of their potential contribution to the heat-index and importance in forecasting severe weather due to flagging very-moist lower-levels.

Stability

- Stability, especially in the lower-levels, plays a big part in formation of clouds, and its assessment factors the forecast of adverse weather.
- In the first example, we'll look at a temperature profile, focusing on a hypothetical layer. In general, the environment becomes more unstable over time if the top of the layer cools at a faster rate than the bottom of the layer. The opposite is true for a layer becoming more stable.

- Instability-increases occur from heating of the day, warm advection in the lower portion of the layer, and/or cold-advection in the top portion of the layer. Air that becomes more unstable increases the amount of energy in the atmosphere that can feed severe thunderstorms.
- A layer can become more stable from radiational cooling (occurs at night), cold-advection in the lower-portion of the layer, and/or warm-advection in the upper-portion of the layer. Most weather associated with more-stable conditions vary from clear and pleasant to cloudy and rainy.

Assessing Layer Instability (*)

- The above illustrates how to identify areas of instability on a sounding. If the temperature profile cools more rapidly than the dry-adiabatic lapse rate (DALR), that layer is absolutely unstable. Skew-T's and soundings offer simplicity as we just have to compare the environmental lapse rate (ELR; the layer's lapse rate) to the dry-adiabat guides. These conditions are met if the temperature profile rises left of a dry-adiabat guide.
- Any layer whose ELR rises between the DALR and the relative Moist-Adiabatic Lapse-Rate (MALR) is considered conditionally unstable. The other case is handled in the following section.

Inversions

An example of an inversion occurring in the lower levels

- In general, temperatures decrease with height in the troposphere. Inversions occur when the contrary, or inverse occurs, with temperatures increasing with height. In the graphic posted above, they are considered absolute stable layers. These are easy to find on a Skew-T. Locate the nearest isotherm. If the profile warms with height (ascends to the right of an isotherm), you've found an inversion.
- Inversions can prevent air from below from rising above. Inversions are commonly observed close to the surface as dusk sets in and near-surface air yields to radiational cooling. Inversions are a key signal in cold-air damming events. As referenced in the stability section, less severe weather is associated with near-surface inversions occurring but can be a key factor in determining if sleet/freezing rain will occur.

Heat + No Wind + Inversion = Bad Air Quality

- Inversions can have some negative impacts. They are key culprits for bad air-quality days, especially in the summer-time. Often accompanied by oppressive heat and weak winds, they end up trapping pollution near the surface, preventing their dispersal which yields higher concentrations in the air we breathe and interact in.
- Furthermore, they can act as a sturdy lifting mechanism for moisture, leading to heavy rain where flooding may become a concern (See post on Cold-Air Damming)
- Moreover, with conditions just right, severe weather can actually benefit from capping inversions, holding convection at bay until surface-heating becomes most intense during the day.

CAPE (Convective Available Potential Energy)
- Assessing the threat of severe weather is a primary function of analyzing Skew-T's or soundings. CAPE identifies the layer in the atmosphere where a theoretical parcel (or bubble) would be positively buoyant; where ascension would be favored. It is also amongst the first parameters (including its shape) that forecasters analyze. In combination with other variables,like wind shear (how the winds are changing with height), forecasted heating, forcing mechanisms (fronts), and available moisture, severe-weather threats can become keenly evident.

How to Identify the parcel-path and CAPE/CIN Areas (sounding courtesy of UWYO)

- To find the CAPE visually, you would follow the steps outlined in my second post and find the LCL. This is the point where condensation is reached and a parcel would thus be releasing latent-heat. This leads to the parcel still cooling as it ascends, but just at a slower rate. So then you ascend along the moist-adiabats until the parcel profile ultimately becomes cooler than the environmental temperature (commonly will occur near the top of the troposphere; this is where thunderstorm-anvils form and diverge from the parent storm). Areas between the parcel-path and temperature where the parcel-path is warmer (to the right) of the temperature profile are areas of CAPE: where the parcel would be positively-buoyant (upward acceleration).
- This process also reveals 'anti-CAPE,' known as convective inhibition (CIN): where negatively-buoyant air exists (downward acceleration) and would be favored. Simply put, this is the area of the graph between the parcel-path and temperature where the parcel is cooler (more-dense) than the environment.
- It's quite common for CAPE and CIN to exist in the same sounding. Furthermore, there can be virtually unlimited CIN and no CAPE, or CAPE with no CIN.
- As touched upon a few paragraphs up, another way to understand CAPE is thinking of it as upward acceleration. Yes, energy is a key word in the acronym. Here in the United States, assessments will mostly list the units to reflect that: Joules (SI unit of energy) per kilogram (J/kg). But I've seen European countries use the m²/s² units for CAPE, where acceleration is directly implied. Don't fear. They mean the exact same thing. Below, I 'spell-out' the conversion from J/kg to m²/s², showing they are exactly identical. As you observe other severe-weather indices, you'll notice that m²/s² are frequently encountered/reported units.

How CAPE's energy-based units are also the same as areal acceleration

- CAPE values are generally calculated via computer software, but visually, the CAPE or CIN area can give general ideas to the forecaster about what is available aloft.

Wind & Shear Changes w/Height

Simple Guide to Wind Barbs

- As mentioned in the first post, wind speeds and directions (wind vectors) are able to be calculated from weather balloon data. Knowing how wind vectors change with height is vital in assessing the probable severe-weather type and storm-longevity.
- The directional changes within layers can also imply the type of temperature advection that is favored in that layer.

Diagram showing inferential use of directional changes with height

- As you ascend in a layer, if barb-ends are swinging counter-clockwise (example on right) with height, they are said to be 'backing with height,' which is generally associated with cold-air advection (CAA).
- If they are swinging clockwise (example on left), they are 'veering with height,' generally associated with warm-air advection (WAA).

Thanks for powering through this series with me. I learned Skew-T's at a critical point in my undergraduate studies in my Atmospheric Thermodynamics course: the 2nd weather course in the major. I had delayed college by over 9 years, so I was playing catch-up on a lot of math. I was faced with taking this class which included knowing derivation AND integral rules, but I barely even knew what a derivative was (I was taking Calculus 1 concurrently). Luckily, it was also a lot of algebra. I had to 'wing-it' with the advanced math stuff. I felt unsure about my prospects to continue in the major. Then we learned about skew-t diagrams. It changed everything for me! The understanding of them came quickly. It gave me a morale boost that I needed. I know I went over a lot in this series, but these graphs are amazing to me due to how much information can be extracted from them, even without any data plotted. This is why I call it, the 'Paper Calculator.'

* Source: Tsonis, A. "An Introduction to Atmospheric Thermodynamics." Pg 148.