FAA Instrument Flight Instructor (FII)WeatherHard
While climbing through 6,000 feet in visible moisture, a pilot notes the outside air temperature is -4°C and calculates the dewpoint at that altitude to be -6°C. Based on this temperature/dewpoint spread, what is the significance for icing potential?
- AIcing is unlikely because the spread is too large
- BIcing cannot occur because the temperature is below -0°C but above -20°C only in theory
- CIcing is likely because the small spread indicates high relative humidity near saturation
- DIcing is only a concern if the spread were exactly 0°C
Show answer & explanationAnswer & explanation
Correct answer: C. Icing is likely because the small spread indicates high relative humidity near saturation
A small temperature/dewpoint spread (here only 2°C) indicates the air is near saturation with high relative humidity, and combined with visible moisture and a temperature within the 0°C to -20°C range most favorable for supercooled water droplets, icing is likely. Icing potential increases as the spread narrows because it indicates the air is closer to its saturation point where cloud droplets exist.
Why the other options are wrong
- A. A small spread, not a large one, indicates conditions favorable for icing.
- B. Icing does occur in this temperature range; it is one of the most favorable ranges for structural icing.
- D. A spread near zero is favorable but doesn't need to be exactly 0°C; small spreads generally indicate high humidity.
Temperature/Dewpoint Spread and Icing
A small temperature/dewpoint spread indicates air near saturation; combined with visible moisture and temperatures between 0°C and -20°C, this signals high icing potential.
- Spread near 0 indicates high relative humidity/saturation
- Icing most likely between 0°C and -20°C
- Requires visible moisture (clouds, precipitation) to occur
Memory trick: Small spread, saturated air, icy pair—icing's near.