What characteristic is most indicative of stable atmospheric conditions, which could lead to reduced visibility for a sUAS operator?

Stability is defined by how the atmosphere responds to a parcel of air displaced vertically. In stable air the displaced parcel is cooler and denser than its surroundings and returns to where it started, so vertical motion is resisted. The signature that follows is consistent: clouds form in flat layers rather than building vertically, because moisture spreads horizontally along the level where it condenses; precipitation is steady and light rather than showery; air is smooth; and pollutants and moisture accumulate near the surface because nothing mixes them away. That last consequence is the operational one for a remote pilot, since the visibility requirement is three statute miles and stable conditions erode surface visibility over hours while presenting as pleasant flying weather. The distractors all describe the unstable case — rapid vertical motion, a steep lapse rate, turbulence and building cumulus — which produces rough air and good visibility, the opposite pairing.

Stable air resists vertical motion, producing layered stratiform cloud, smooth conditions and the accumulation of haze and moisture near the surface that reduces visibility.

Stability describes resistance to vertical motion. Stable air produces stratiform cloud, steady precipitation, smooth conditions and poor surface visibility. Unstable air produces cumuliform cloud, showers, turbulence and good visibility.

Flat Clouds, Smooth Air, Bad Visibility. Stability spreads things sideways instead of mixing them up.

Lapse rate is the discriminator: a steep lapse rate means instability, a shallow one or an inversion means stability.

• •Associating smooth air with good visibility
• •Reversing the meaning of a steep lapse rate
• •Matching stratiform cloud to unstable conditions