FAA Private Pilot Airplane (PAR)Weather Theory and ServicesHard
A pilot is planning a flight through an area where the surface temperature is 15°C and the dew point is 10°C. Assuming the air is lifted and cools at the dry adiabatic lapse rate of 3°C per 1,000 feet, what is the approximate height of the cloud bases?
- A2,000 feet AGL
- B1,000 feet AGL
- C3,000 feet AGL
- D4,000 feet AGL
Show answer & explanationAnswer & explanation
Correct answer: B. 1,000 feet AGL
The formula for estimating cloud bases (in feet AGL) is (Surface Temperature - Dew Point) / 2.5 * 1,000. So, (15°C - 10°C) / 2.5 * 1,000 = 5 / 2.5 * 1,000 = 2 * 1,000 = 2,000 feet AGL. The dry adiabatic lapse rate of 3°C per 1,000 feet is a red herring in this specific calculation but relevant to the overall process of cloud formation. The question asks for the approximate height of the cloud bases, for which the temperature/dew point spread is the primary factor.
Why the other options are wrong
- A. This is the correct calculation: (15°C - 10°C) / 2.5 * 1,000 = 2,000 feet AGL.
- C. This is incorrect. A common mistake might be to use a different divisor or miscalculate.
- D. This is incorrect. This would imply a larger temperature/dew point spread.
Cloud Base Estimation
The approximate height of cloud bases can be estimated by using the surface temperature and dew point spread.
- Formula: (Surface Temperature - Dew Point) / 2.5 * 1,000 feet AGL.
- This estimates the lifting condensation level (LCL).
- The '2.5' factor is a simplified approximation for the difference between the dry adiabatic lapse rate and the dew point lapse rate.
Memory trick: Cloud Base: Temperature Minus Dew Point, then Divide by Two Point Five, Times a Thousand.