Weather hazards for drone pilots: stability, fog, thunderstorms and wind
Stable and unstable air, fog and frost, thunderstorm stages, microbursts and mountain wind, explained for the Part 107 knowledge test.
Checked against the rules in force on 2026-10-01Small drones are light and fly low, right where wind, turbulence and fog are at their worst. The Part 107 test checks that you can predict these hazards from a few clues.
Stable or unstable air
Stability is the air's resistance to vertical movement. Cool, dry air tends to be stable; warm, moist air tends to be unstable.
- Unstable air: cumulus clouds, showery rain, turbulence, usually good visibility.
- Stable air: stratus clouds and fog, steady rain, smooth air, haze and poor visibility.
A temperature inversion, where temperature rises with height, traps haze and fog near the ground. Surface inversions form on clear, cool, calm nights.
Dew point, fog and frost
When air cools to its dew point it becomes saturated, and fog, dew or clouds form. A small temperature and dew point spread is a warning sign for fog. Below freezing, the moisture deposits as frost, which disrupts airflow over propellers and wings. Remove all frost before flight.
Thunderstorms
A thunderstorm needs moisture, unstable air and a lifting force. It grows through three stages:
- Cumulus: strong updrafts build the cloud.
- Mature: rain falls from the cloud base, meaning downdrafts have begun. Every hazard is at its worst: lightning, hail, turbulence and gusty surface winds.
- Dissipating: downdrafts dominate and the storm dies.
A microburst is a short-lived, concentrated downdraft, about 1 mile across and lasting 5 to 15 minutes. An aircraft flying through meets a headwind, then a strong downdraft, then a tailwind. Land well before a storm reaches your site; gust fronts arrive ahead of the rain.
Wind near obstacles and terrain
Buildings, trees and hills break up the wind into gusts that change speed and direction. On a mountain, air flows smoothly up the windward side and tumbles down the leeward side, where turbulence and downdrafts can push an aircraft into the slope. Standing lenticular clouds over mountains signal strong mountain-wave turbulence.
Ceiling and visibility
Part 107 needs 3 statute miles of visibility and clearance of 500 feet below clouds. A low ceiling caps how high you can fly even when everything else is legal.
Sources: FAA-G-8082-22 ch. 3b · FAA-H-8083-25C ch. 12
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Free sample questions with a debrief for each answer.
Which conditions are typical of a stable air mass?
At dawn, the temperature is 10 °C and the dew point is 9 °C, with calm wind. What should you expect?
You plan to inspect a warehouse roof on a windy day. What hazard should you expect near the building?
You fly along a lakeshore on a sunny afternoon. Which wind should you expect?
Which conditions favour radiation fog?
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