Questions: Thermodynamic Diagrams and Atmospheric Sounding Analysis

Question 1

On a skew-T log-P sounding, the temperature trace and dew point trace nearly coincide from 850 hPa to 700 hPa, then diverge sharply above 700 hPa. What does this pattern indicate about the atmosphere's vertical structure?

• A. Dry air near the surface with a moist elevated layer above 700 hPa — a typical capping inversion pattern
• B. A saturated (cloudy) layer from 850 to 700 hPa, with dry air above — the cloud tops are near 700 hPa
• C. A temperature inversion between 850 and 700 hPa that prevents any upward motion
• D. Increasing wind shear between 850 and 700 hPa that is tearing apart moisture structures

Answer: (B)

On a skew-T, temperature and dew point converging means the air is approaching saturation — the closer the two traces, the moister the air. When they touch, the relative humidity is 100% and the air is inside a cloud. Divergence above 700 hPa means dew point drops off much faster than temperature, indicating dry air with low relative humidity. A forecaster reading this sounding would identify a cloud layer from 850 to 700 hPa (where the traces coincide) with a clear, dry layer above. This structure is common in stratus or stratocumulus situations.

Question 2

Two soundings both show CAPE of 3000 J/kg. Sounding A has a straight hodograph; sounding B has a strongly curved clockwise hodograph with large changes in wind direction from the surface to 6 km. What does this difference imply for severe weather potential?

• A. Both present equal severe weather potential — CAPE determines updraft strength and storm intensity, making hodograph shape irrelevant
• B. Sounding A is more dangerous because straight hodographs indicate faster storm motion and larger hail
• C. Sounding B is more favorable for supercell thunderstorms with rotating updrafts, because curved hodographs indicate directional wind shear that promotes mesocyclone development
• D. Sounding B is less dangerous because the wind turning through many directions reduces effective vertical wind shear

Answer: (C)

CAPE quantifies the energy available for updrafts but says nothing about storm organization. The hodograph reveals wind shear structure. A straight hodograph means winds increase in speed with height but don't turn — this can support multicell storms and some severe weather. A curved (clockwise) hodograph means winds turn clockwise from the surface upward, which generates horizontal vorticity that can be tilted into the vertical by a storm's updraft, producing mesocyclone rotation and supercell thunderstorms. Large curved hodographs are one of the best discriminators between ordinary thunderstorms and tornadic supercells.

Question 3

On a skew-T log-P diagram, the area enclosed between a lifted parcel's moist adiabatic path and the environmental temperature sounding — where the parcel is warmer than the environment — represents CAPE (Convective Available Potential Energy).

• T. True
• F. False

Answer: True

CAPE is precisely this positive area on the skew-T. When the parcel's temperature exceeds the environmental temperature (parcel trace lies to the right of the environmental temperature trace on a standard skew-T), the parcel is positively buoyant — it accelerates upward. Integrating this buoyancy from the Level of Free Convection (LFC) to the Equilibrium Level (EL) gives CAPE in J/kg, which is proportional to the maximum potential updraft velocity. Larger CAPE means more energy available for thunderstorm updrafts.

Question 4

A large CAPE value on a sounding guarantees that severe thunderstorms will develop, regardless of other atmospheric conditions.

• T. True
• F. False

Answer: False

CAPE is necessary but not sufficient for severe thunderstorm development. Convective Inhibition (CIN) — the negative area on the skew-T below the LFC — represents an energy barrier that must be overcome to initiate convection. High CAPE with strong CIN means the atmosphere is loaded but 'capped'; storms won't develop without a triggering mechanism (frontal lift, surface heating, outflow boundaries). Moisture must also be sufficient throughout a deep layer, and — as the hodograph analysis shows — wind shear determines whether storms can organize into severe, rotating supercells rather than pulse storms.

Question 5

Explain how a forecaster uses the temperature trace, dew point trace, and adiabatic reference lines on a skew-T log-P diagram to assess whether the atmosphere can support deep moist convection.

Model answer: The forecaster lifts a surface parcel dry-adiabatically until its temperature matches the dew point — the Lifted Condensation Level (LCL), where cloud forms. Above the LCL, the parcel cools along the moist adiabat (more slowly, due to latent heat release). Wherever the parcel's moist adiabatic path is warmer than the environmental temperature sounding, the parcel is positively buoyant — this is CAPE. A large positive area from the Level of Free Convection (LFC) to the Equilibrium Level (EL) indicates the atmosphere can support deep convection; the CIN below the LFC shows how much lift is needed to trigger it.

The skew-T makes this analysis visual and immediate. The forecaster is essentially asking: 'If I took a parcel of near-surface air and lifted it, would it eventually become buoyant and accelerate on its own?' The intersection of the parcel's path with the environmental sounding marks the LFC (where buoyancy turns positive) and the EL (where it turns negative again). The shape and area of the positive region immediately communicates storm potential, while the shape of the CIN region communicates how strong a trigger is needed. This is why reading skew-T soundings is a core skill for operational weather forecasters.