FAA Aircraft Dispatcher (ADX)MeteorologyEasy
A dispatcher reviews a surface observation showing a temperature of 20°C and dew point of 18°C shortly after sunset, with the temperature/dew point spread continuing to narrow as the evening progresses. What does this trend most likely indicate for the airport?
- AIncreasing likelihood of radiation fog as relative humidity approaches saturation
- BDecreasing relative humidity and improving visibility
- CRising cloud bases due to a drying atmosphere
- DReduced probability of any low-level cloud formation
Show answer & explanationAnswer & explanation
Correct answer: A. Increasing likelihood of radiation fog as relative humidity approaches saturation
As surface temperature cools after sunset while dew point remains relatively steady, the temperature/dew point spread narrows toward zero, indicating rising relative humidity. When the spread reaches zero (saturation) under clear, calm conditions, radiation fog is likely to form.
Why the other options are wrong
- B. Opposite of what a narrowing spread indicates.
- C. Narrowing spread relates to fog potential, not directly to cloud base height.
- D. Incorrect; a narrowing spread increases, not decreases, cloud/fog formation likelihood.
Temperature/Dew Point Spread
The difference between air temperature and dew point temperature; a small or narrowing spread indicates high relative humidity and increased potential for fog or low clouds.
- Spread of 0° indicates saturation (100% RH)
- Narrowing spread after sunset commonly signals radiation fog risk
- Wide spread indicates dry air and low fog potential
Memory trick: Spread shrinks, fog thinks about forming.