Is the PROB30 Realistic? A Sounding Worked Through by Hand
At 1200 local time the surface temperature at your aerodrome is 27 °C with a dewpoint of 19 °C. The 1200 sounding shows an environmental lapse rate of 8.5 °C/km from the surface to 1,500 m, 6.5 °C/km from 1,500 m to 3,000 m, and 8.0 °C/km above 3,000 m. There is no marked dry layer and the winds are light throughout. The aerodrome forecast issued at 0600 carries PROB30 TSRA with broken cumulonimbus for the afternoon, and the operations department has used it to justify extra fuel for a departure delay. Your sector is a short domestic hop and the aircraft has weather radar. The forecast maximum for the afternoon is 31 °C with the dewpoint unchanged. At the briefing you are asked whether the PROB30 is realistic, and whether an early-afternoon departure should expect embedded cumulonimbus on the route.
1. Using the sounding, estimate the lifted condensation level, the level of free convection and the equilibrium level, and state whether the PROB30 thunderstorm is supported.
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Use DALR ≈ 1 °C/100 m below the LCL and SALR ≈ 0.5 °C/100 m above it.
LCL. Spread = 27 − 19 = 8 °C; at roughly 125 m per °C of spread the LCL is about 1,000 m (3,300 ft) AGL.
Parcel temperature. Surface 27 °C; at 1,000 m, 27 − 9.8 = 17.2 °C. Environment at 1,000 m = 27 − 8.5 = 18.5 °C - the parcel is still 1.3 °C colder, so there is no free convection yet.
LFC. Continuing along the SALR: at 1,500 m the parcel is 17.2 − 2.5 = 14.7 °C; the environment is 27 − 12.75 = 14.25 °C. The parcel is now marginally warmer, so the LFC is about 1,450 m (≈4,750 ft).
EL. At 3,000 m the parcel is 14.7 − 7.5 = 7.2 °C; the environment is 4.5 °C. The gap of 2.7 °C closes at the rate of 3 °C/km (environment 8.0, parcel 5.0), so the EL is 3,000 + 900 = ≈3,900 m (about 12,800 ft).
Freezing level: environment 4.5 °C at 3,000 m with 8 °C/km gives 0 °C at about 3,560 m, i.e. ≈11,700 ft.
Verdict. Cloud tops near 12,800 ft, only about 1,100 ft above the freezing level, with a shallow warm-cloud depth and a small positive area. That supports towering cumulus and isolated, weak cumulonimbus with brief glazed icing at the top - not a widespread thunderstorm outbreak. A PROB30 is defensible, but the modest EL means the cell tops will not be high and the lightning threat is low.
2. The forecast maximum reaches 31 °C. Re-evaluate the risk, and say how the answer changes for a pilot departing at 1400.
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Heating changes the low levels far more than the upper levels. With T 31 °C and Td still 19 °C, the spread becomes 12 °C:
- LCL rises to about 12 × 125 = 1,500 m (≈5,000 ft) - higher cloud base, which is why surface heating alone does not lower the base.
- The parcel curve moves right: at the LCL the parcel is 31 − 14.7 = 16.3 °C against an environment of 14.25 °C at 1,500 m, so the parcel is already positively buoyant at the condensation level. The LFC effectively coincides with the LCL, and the layer from the surface is conditionally unstable with no negative area to overcome.
- The EL rises: at 3,000 m the parcel is 16.3 − 7.5 = 8.8 °C against 4.5 °C, a 4.3 °C excess closing at 3 °C/km, giving an EL near 4,400 m, i.e. ≈14,500 ft.
Consequences for a 1400 departure: deeper, more vigorous cells, tops above the freezing level by nearly 1,000 m, so glazed icing in the upper part of cells and a real (though still moderate) lightning and microburst potential; the base is higher, so the surface gust front and downburst risk is more relevant than low-level scud. With radar, the correct technique is to deviate around returns rather than to overfly, and to avoid the upwind flank and any cell with a hard core. The honest briefing line is: 'PROB30 is realistic but the tops are forecast to stay near 14,000 ft; the significant hazard is embedded CB in the 8,000–14,000 ft band during the afternoon, not a widespread storm outbreak.'
3. What would change in your assessment if the 1,500–3,000 m lapse rate had been 9.0 °C/km instead of 6.5 °C/km?
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A steeper mid-level lapse rate makes the environment cool faster with height than the saturated parcel (which cools at only about 5 °C/km), so the positive area grows rapidly and the equilibrium level is pushed much higher.
Repeat the arithmetic: at 3,000 m the environment would be 27 − 12.75 − 0.009 × 1,500 × 1,000 = 27 − 12.75 − 13.5 = 0.75 °C, while the parcel is still at 7.2 °C. The excess of 6.45 °C closes at (9.0 − 5.0) = 4 °C/km, so the EL moves to roughly 3,000 + 1,600 = 4,600 m - about 15,000 ft - and the freezing level drops to (0.75 °C at 3,000 m, 9 °C/km) ≈ 3,100 m, so a much greater part of the cloud is above 0 °C. That is a genuine deep-convection profile: large hail aloft, a proper anvil, and a lightning risk that no longer needs a PROB30 qualifier.
The wider lesson for the exam: the surface temperature and dewpoint decide whether the parcel starts and where cloud begins, but the mid-tropospheric lapse rate decides how far it goes. A sounding must be read top-down as well as bottom-up - a warm, moist boundary layer under a stable mid-troposphere gives you towering cumulus and showers, not thunderstorms, no matter how high the surface temperature.