FAA Sport Pilot Airplane (SPA)WeatherEasy
During an evening weather briefing, a pilot notices the temperature/dew point spread is narrowing rapidly as sunset approaches. What does this trend most likely indicate?
- AIncreasing convective activity and thunderstorm development
- BDecreasing relative humidity and improving visibility
- CIncreasing relative humidity, favoring fog or low cloud formation
- DAn approaching cold front with gusty winds
Show answer & explanationAnswer & explanation
Correct answer: C. Increasing relative humidity, favoring fog or low cloud formation
As the temperature/dew point spread narrows, relative humidity increases toward saturation. A rapidly closing spread in the evening, especially with clear skies and calm wind, is a classic precursor to fog or low stratus formation.
Why the other options are wrong
- A. Convective activity requires instability and moisture lift, not just a narrowing spread near sunset.
- B. A narrowing spread means humidity is increasing, not decreasing.
- D. Frontal passage is identified by wind shifts and pressure changes, not simply by dew point spread.
Temperature/Dew Point Spread
The difference between air temperature and dew point; a small or narrowing spread indicates high relative humidity and increases the likelihood of clouds or fog.
- Spread of 0°C means the air is saturated (100% RH)
- Rapidly narrowing spread near sunset often signals fog formation
- Wide spread indicates dry air and lower fog/cloud potential
Memory trick: 'Small spread, soggy air ahead'