Stealth And Detection In Aerial Terrain
How far a character can see in the air depends on the presence or absence of clouds. Creatures can usually see 5d8×100 feet if the sky is completely clear, with minimal clouds (or other aerial objects) blocking their views. Clouds generally provide enough concealment to hide within (though the hiding creature might have difficulty seeing out from its hiding place).
Source
Core Rulebook pg. 396
