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Fog when temperature and dew point are close

Remote Pilot (Part 107) practice question 22 of 25 on Weather / Effects on Performance. ACS element UA.III.B.K1i · difficulty: standard. Want to answer it in a timed quiz? Use the Remote Pilot (Part 107) practice mode.

Question bank updated 2026-09-22

QUESTIONDuring an early-morning preflight briefing, a remote pilot sees that the air temperature and the dew point are very close together. What should the pilot expect if the air reaches saturation?

  1. AClear skies, because the air is too dry for condensation
  2. BThunderstorms, because the air is unstable
  3. CFog or low clouds are likely to form
Show the answer and explanation

Correct answer: C. Fog or low clouds are likely to form

  • A — incorrect A small temperature/dew point spread means the air is close to saturation, not dry.
  • B — incorrect A close temperature/dew point spread by itself indicates saturation and fog potential; instability depends on the combination of warmth and moisture, and is not implied here.
  • C — correct The study guide states that if air reaches the saturation point while temperature and dew point are close together, fog, low clouds and precipitation are highly likely to form.

BACKGROUND

The dew point is the temperature at which the air can hold no more moisture. When the air temperature is reduced to the dew point, the air is completely saturated and moisture begins to condense as fog, dew, frost, clouds, rain or snow. If air reaches saturation while temperature and dew point are close together, fog, low clouds and precipitation are highly likely. Air can reach saturation in four ways: warm air moving over a cold surface, cold and warm air mixing, air cooling at night through contact with the ground, and air being lifted or forced upward.

STUDY TIP

Watch the temperature/dew point spread in the METAR; a small spread near dawn is the classic setup for fog that can ground a flight.

KEY POINTS

  • Dew point is the temperature at which air becomes saturated.
  • A small temperature/dew point spread makes fog and low clouds highly likely.
  • Air reaches saturation by cooling over a cold surface, mixing, nighttime cooling or lifting.

SOURCES

  • remote_pilot_study_guide.txt ch3b "Temperature/Dew Point Relationship" / "Methods by Which Air Reaches the Saturation Point"
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SkyDrill is not affiliated with the FAA. This is an original practice item based on 14 CFR part 107, 14 CFR part 89, and the FAA Remote Pilot – Small UAS Study Guide (FAA-G-8082-22). Regulations and handbooks are revised; confirm details against the current edition.