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Skew-T Log-P · Aviation weather ground school

Why the air
goes up

A sounding is just a vertical strip of atmosphere plotted on a rigged grid. Once a student can lift a parcel on that grid, "unstable air" stops being a vocabulary word and becomes something they can measure.

Sounding

Parcel readout

Cloud base (LCL)—
Free convection (LFC)—
Equilibrium (EL)—
Freezing level—
CAPE—
CIN (the cap)—

—

Show on chart

Temperature

The right-hand solid trace. Where it leans left with height quickly, the atmosphere is losing heat fast — the setup for instability.

Dewpoint

The left-hand solid trace. The gap between the two traces is how dry that layer is. Touching traces mean cloud.

Dry adiabat

The track an unsaturated parcel follows when lifted: about 3 °C per 1,000 ft. Same number your students already use for temperature aloft.

Moist adiabat

The track once the parcel is saturated. Only about 1.2–1.5 °C per 1,000 ft, because condensing water gives latent heat back to the parcel.

Mixing ratio

Grams of water vapor per kilogram of dry air. A parcel keeps its value while it is lifted unsaturated, which is what fixes cloud base.

Positive area

Parcel warmer than its surroundings, so it accelerates upward on its own. Its size is CAPE — the fuel available to a thunderstorm.

Rules of thumb worth saying out loud

Glossary

Everything a student needs to read the chart out loud. Levels are places on the sounding; energies are areas between the parcel and the environment.

Sounding
The vertical profile a radiosonde measures on its way up — temperature, dewpoint, and wind against pressure. Most sites release balloons twice a day, at 00Z and 12Z.
Skew-T Log-P
The chart. Pressure climbs the side on a logarithmic scale so vertical spacing is close to real altitude, and isotherms are tilted about 45° right so a sounding spreads across the page instead of collapsing into a narrow band.
Parcel
An imagined blob of air treated as though it neither mixes with nor exchanges heat with its surroundings. Every level and every area on the chart describes what happens to one when it is lifted.
Lapse rate
How quickly temperature falls with height. Comparing two of them — the parcel's and the environment's — is the entire stability question.
ELR · environmental lapse rate
What the balloon actually measured. This is the red temperature trace, and its steepness is what you are judging everything against.
DALR · dry adiabatic lapse rate
About 3 °C per 1,000 ft. The rate an unsaturated parcel cools purely from expanding as it rises. Fixed by physics, so it never changes.
SALR · saturated adiabatic lapse rate
Roughly 1.2–1.5 °C per 1,000 ft in warm low levels, steepening toward the dry rate up high where little vapor is left. Slower than dry because condensing water releases latent heat back into the parcel.
Mixing ratio
Grams of water vapor per kilogram of dry air. A parcel carries its value along unchanged while it is lifted unsaturated, which is exactly why cloud base is predictable.
Dewpoint spread
Temperature minus dewpoint. On the chart it is the horizontal gap between the two traces — wide means dry, zero means saturated.
LCL · lifting condensation level
Where a lifted parcel first saturates, and therefore cloud base. Found where the parcel's dry adiabat crosses its mixing ratio line.
CCL · convective condensation level
Cloud base when surface heating rather than mechanical lift does the work — where the surface mixing ratio line meets the temperature trace. This is the one that matches your afternoon cumulus.
LFC · level of free convection
The level above which the parcel is warmer than the air around it and keeps rising with no further help. Below it, something has to do the lifting.
EL · equilibrium level
Where the rising parcel cools back to the environment's temperature and stops accelerating. Practically, the anvil. Momentum carries overshooting tops a few thousand feet higher.
CAPE · J/kg
Convective available potential energy — the positive area between LFC and EL, the fuel available to an updraft. Under 1,000 means showers; over 2,500 means storms you do not go near. Theoretical peak updraft in m/s is roughly the square root of twice the CAPE.
CIN · J/kg
Convective inhibition — the negative area below the LFC, reported as a negative number. It is the work required to shove a parcel up to where it can go on its own, and why storms wait until afternoon.
Cap
The warm layer producing that inhibition. A weak cap erodes with heating and lets storms fire; a strong one holds all day, then breaks late and violently because nothing has released the energy piling up underneath.
Inversion
Any layer where temperature increases with height. It is stable, it stops vertical motion cold, and it traps haze, smoke, and moisture beneath it. Expect a distinct change in ride quality crossing one.
Conditional instability
An ELR between the saturated and dry rates: stable while the parcel is dry, unstable once it saturates. The most common real-world setup and the reason moisture matters as much as heat.
Absolute instability
ELR steeper than the dry adiabatic rate. Air overturns on its own whether or not it is saturated. Usually a shallow, superheated surface layer — the source of low-level thermals and afternoon turbulence.
Absolute stability
ELR shallower than the saturated rate. Nothing rises on its own in any condition. Smooth air, stratiform cloud if there is moisture, and poor visibility if there is not.
Freezing level
Where the temperature trace crosses 0 °C. In cloud above it, plan on airframe ice; in a storm, this is roughly where the hail growth zone begins.
Entrainment
Dry environmental air pulled into the updraft from the sides. It evaporates cloud droplets, chills the air, and gives you the downdraft — the mechanism behind microbursts and gust fronts.