FAA Instrument Rating Airplane (IRA)Weather and Weather ServicesEasy
A pilot obtains a surface observation showing a temperature of 20°C and a dew point of 10°C. Using the standard rule of thumb, approximately what height above ground level (AGL) should the pilot expect the cloud base to form?
- A4,000 feet AGL
- B2,000 feet AGL
- C3,000 feet AGL
- D5,000 feet AGL
Show answer & explanationAnswer & explanation
Correct answer: A. 4,000 feet AGL
The temperature/dew point spread rule states that cloud base height (AGL) can be approximated by dividing the spread by 2.5°C per 1,000 feet. Here, the spread is 20°C − 10°C = 10°C, so 10 ÷ 2.5 = 4, giving an estimated cloud base of 4,000 feet AGL.
Why the other options are wrong
- B. This would correspond to a spread of only 5°C, not 10°C.
- C. This would correspond to a spread of 7.5°C, not the 10°C spread given.
- D. This would require a 12.5°C spread, which is greater than given.
Cloud Base Estimation Rule
A rule of thumb used to estimate convective cloud base height by dividing the temperature/dew point spread (°C) by 2.5 and multiplying by 1,000 feet.
- Spread(°C) ÷ 2.5 = height in thousands of feet AGL
- In °F, divide spread by 4.4
- Only an approximation, not exact
Memory trick: Spread ÷ 2.5 = cloud base thousands of feet.