The Afternoon Cumulus Build-Up
It is 1300 local time in late summer. The 0600 sounding showed a surface temperature of 24°C, a dew point of 18°C, and a dry adiabatic lapse rate in the lowest 3,000 ft. The environmental lapse rate above 3,000 ft is 7.5°C per 1,000 ft up to 12,000 ft, then becomes 5°C per 1,000 ft. Surface heating has raised the temperature to 33°C by 1300. The lifting condensation level (LCL) is approximately 125 m per °C of surface dew-point spread. You plan a 2-hour local flight at 4,500 ft AMSL in a light aircraft with no weather radar and no de-icing.
1. Estimate the LCL and the level of free convection (LFC) and decide whether convective cloud will form and whether it will grow vertically.
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LCL: Surface dew-point spread = 33 − 18 = 15°C. LCL ≈ 125 × 15 ≈ 1,875 m ≈ 6,150 ft. Because the air is well mixed, the dew point near the surface has likely fallen as the temperature rose, so the true spread may be larger and the LCL higher - treat 6,150 ft as a minimum.
LFC: A parcel rising dry-adiabatically from 33°C cools at 3°C/1,000 ft. At 3,000 ft it is 33 − 9 = 24°C. The environment at 3,000 ft, starting from 24°C at the surface and cooling at 7.5°C/1,000 ft, is 24 − 22.5 = 1.5°C. The parcel (24°C) is far warmer than the environment (1.5°C), so it is already buoyant below 3,000 ft. The parcel remains buoyant through the 7.5°C/1,000 ft layer and through the 5°C/1,000 ft layer above 12,000 ft, because 5°C/1,000 ft is still steeper than the saturated adiabatic rate. So the LFC is essentially at the LCL, and the parcel is freely buoyant above it.
Decision: Deep convection is likely, with cloud bases near 6,000 ft and tops potentially well above 12,000 ft. At 4,500 ft you would be below cloud base, which is good for visibility but bad for the anvil and gust-front outflow aloft.
2. Given your planned 4,500 ft flight, list the specific hazards you should expect and the go/no-go reasoning.
Read the worked answer
At 4,500 ft you are below the convective cloud base (≈6,000 ft), so you will not be in cloud, but you are in the inflow region and under the developing cells. Hazards:
- Downbursts and gust fronts: mature cells can send outflow of 30–50 kt at the surface, causing sudden airspeed loss and wind shear on approach and departure.
- Turbulence: strong thermals and cumulus updraughts/downdraughts below cloud base; expect moderate, locally severe, turbulence.
- Hail and lightning: hail can fall well outside the radar echo, and lightning is a direct strike and electrical hazard.
- Visibility: dust and rain shafts reduce visibility below cloud.
- Icing: not applicable below cloud base in warm air, but if you climb into the cloud you would encounter heavy rain and possible icing in the upper parts.
Decision: No-go for a 2-hour local flight in this airmass without radar. The convection is forecast to be deep, the aircraft has no weather avoidance equipment, and the outflow hazard is greatest exactly at your planned level. Either fly early morning before heating, or stay on the ground.