FAA Airline Transport Pilot (ATM)Meteorology and Weather ServicesHard
At 2100 local time, the temperature is 20°C and the dew point is 14°C. Skies are clear with calm winds, and the dew point spread has been narrowing at a steady rate of 2°C per hour since sunset. Assuming this trend continues, approximately how many hours after 2100 should the pilot expect fog to begin forming?
- A4 hours
- B1 hour
- C3 hours
- D2 hours
Show answer & explanationAnswer & explanation
Correct answer: C. 3 hours
The current dew point spread is 20°C − 14°C = 6°C. Fog tends to form when the spread approaches 0°C (temperature and dew point converge). At a narrowing rate of 2°C per hour, it will take 6 ÷ 2 = 3 hours for the spread to reach zero, so fog should be expected approximately 3 hours after 2100 (around 0000 local).
Why the other options are wrong
- A. 4 hours would overshoot convergence, closing 8°C when only 6°C exists.
- B. 1 hour would only close a 2°C spread, not the actual 6°C spread present.
- D. 2 hours closes only 4°C of the 6°C spread, leaving a 2°C gap remaining.
Dew Point Spread and Fog Formation
The temperature/dew point spread narrows as the air cools at night; fog is likely to form when temperature and dew point converge (spread approaches 0°C) under clear, calm conditions.
- Radiation fog favored by clear skies, calm wind, and narrow/narrowing spread
- Spread = temperature minus dew point
- Rate of narrowing can be used to estimate time until fog onset
Memory trick: 'Spread hits zero, fog hits the field' — track convergence rate to predict fog