FAA Private Pilot Helicopter (PRH)WeatherMedium
A pilot is reviewing a weather briefing and sees that the temperature/dew point spread at the destination airport is forecast to decrease from 8°C to 1°C over the next several hours during the overnight period with clear skies and light winds. What should the pilot expect as a result?
- ANo change in visibility since dew point has no effect on visibility
- BFormation of a strong temperature inversion aloft with no surface effects
- CAn increasing likelihood of fog or low clouds forming as the spread approaches zero
- DRapidly improving visibility as the air becomes drier
Show answer & explanationAnswer & explanation
Correct answer: C. An increasing likelihood of fog or low clouds forming as the spread approaches zero
As the temperature/dew point spread narrows, the air is approaching saturation. When temperature and dew point converge to within a few degrees, especially overnight with clear skies and calm winds, condensation is likely to occur near the surface, forming fog or low stratus clouds.
Why the other options are wrong
- A. Dew point spread is directly related to relative humidity and is a key indicator of fog/cloud potential.
- B. While inversions can support fog formation, the direct concern here is the narrowing spread near the surface indicating fog risk, not an aloft-only inversion with no surface effect.
- D. A narrowing spread indicates increasing moisture relative to temperature, which favors decreasing visibility, not improving.
Temperature/Dew Point Spread
The difference between temperature and dew point indicates how close the air is to saturation; a narrow spread indicates high relative humidity and fog potential.
- Spread of 4°F/2°C or less often indicates fog or cloud formation likely
- Spread narrows overnight as temperature drops toward the dew point
- Clear skies and calm winds favor radiational cooling that narrows the spread
- When spread reaches zero, the air is saturated (100% relative humidity)
Memory trick: Narrow spread = Near saturation = Nebulous (foggy) conditions